MCB 231
BASIC TECHNIQUES IN MICROBIOLOGY
Culturing microorganisms; preparation of media; isolation of pure cultures using streaking and pour plates; subculturing; differential staining; direct and indirect enumeration; identification using colonial and cellular morphology and biochemical procedures; serological techniques; antibiotic sensitivity discs and agar-well methods; anaerobic cultivation; and preservation by agar slants, subculture, refrigeration, deep freezing, lyophilisation and liquid nitrogen.
Aseptic technique; culture medium; inoculation; pure culture; streak plate; pour plate; spread plate; subculture; serial dilution; colony-forming unit; staining; biochemical test; serology; susceptibility test; anaerobic jar; preservation; lyophilisation; cryopreservation.
Laboratory Foundations and Aseptic Practice
1.1 Purpose of practical microbiology
Microbiology laboratory work aims to recover, cultivate, observe, differentiate, enumerate, identify and preserve microorganisms while preventing unwanted contamination.
1.2 Aseptic technique
Aseptic technique is the disciplined handling of cultures, media and equipment so that unwanted organisms are not introduced into the material or released into the working environment.
1.3 Labelling and traceability
Every culture and medium should be labelled sufficiently to connect it with the sample, date and procedure. Traceability prevents specimen mix-up and supports reliable interpretation.
1.4 Controls
Positive and negative controls help show whether reagents and procedures behaved as expected. A result without appropriate controls may be difficult to interpret.
1.5 Waste management
Used cultures and contaminated materials require approved decontamination and disposal procedures. Laboratory safety and scientific validity are inseparable.
Chapter Summary
- Purpose of practical microbiology: Microbiology laboratory work aims to recover, cultivate, observe, differentiate, enumerate, identify and preserve microorganisms while preventing unwanted contamination.
- Aseptic technique: Aseptic technique is the disciplined handling of cultures, media and equipment so that unwanted organisms are not introduced into the material or released into the working environment.
- Labelling and traceability: Every culture and medium should be labelled sufficiently to connect it with the sample, date and procedure.
- Controls: Positive and negative controls help show whether reagents and procedures behaved as expected.
- Waste management: Used cultures and contaminated materials require approved decontamination and disposal procedures.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to laboratory foundations and aseptic practice.
Culture Media and Preparation
2.1 Culture medium
A culture medium provides nutrients and physicochemical conditions suitable for microbial growth. It may be liquid, semisolid or solid.
2.2 Basal media
General-purpose media support organisms without demanding highly specialised nutrients and can be used for routine cultivation.
2.3 Enriched media
Enriched media contain additional nutrients that support organisms with more demanding growth requirements.
2.4 Selective and differential media
Selective media suppress some organisms while permitting others to grow; differential media reveal visible differences among organisms based on particular reactions.
2.5 Preparation principles
Accurate weighing, dissolution, pH adjustment where required, dispensing, sterilisation and appropriate storage determine whether a prepared medium performs correctly.
2.6 Quality assessment
Prepared media should be checked for contamination, expected appearance and ability to support or differentiate organisms as intended.
Chapter Summary
- Culture medium: A culture medium provides nutrients and physicochemical conditions suitable for microbial growth.
- Basal media: General-purpose media support organisms without demanding highly specialised nutrients and can be used for routine cultivation.
- Enriched media: Enriched media contain additional nutrients that support organisms with more demanding growth requirements.
- Selective and differential media: Selective media suppress some organisms while permitting others to grow; differential media reveal visible differences among organisms based on particular reactions.
- Preparation principles: Accurate weighing, dissolution, pH adjustment where required, dispensing, sterilisation and appropriate storage determine whether a prepared medium performs correctly.
- Quality assessment: Prepared media should be checked for contamination, expected appearance and ability to support or differentiate organisms as intended.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to culture media and preparation.
Culturing Microorganisms
3.1 Inoculation
Inoculation introduces a microbial sample into or onto a suitable culture medium under controlled conditions.
3.2 Broth culture
Liquid media permit dispersed growth, sediment, surface films or other patterns depending on the organism and conditions.
3.3 Agar culture
Solid media permit development of discrete colonies and make colony morphology and isolation easier to observe.
3.4 Incubation
Temperature, atmosphere and time should match the purpose of the experiment and biological requirements of the organism.
3.5 Culture observation
Record growth objectively: turbidity, pigment, colony number, colony form or other relevant observations. Avoid assigning identity before sufficient evidence is obtained.
Chapter Summary
- Inoculation: Inoculation introduces a microbial sample into or onto a suitable culture medium under controlled conditions.
- Broth culture: Liquid media permit dispersed growth, sediment, surface films or other patterns depending on the organism and conditions.
- Agar culture: Solid media permit development of discrete colonies and make colony morphology and isolation easier to observe.
- Incubation: Temperature, atmosphere and time should match the purpose of the experiment and biological requirements of the organism.
- Culture observation: Record growth objectively: turbidity, pigment, colony number, colony form or other relevant observations.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to culturing microorganisms.
Isolation from Soil
4.1 Soil as a microbial habitat
Soil contains diverse bacteria and fungi associated with mineral particles, organic matter, roots, water films and pore spaces.
4.2 Sampling principle
A soil sample should represent the location and question being investigated. Sterile containers and careful handling reduce contamination introduced after collection.
4.3 Suspension and dilution
A measured quantity of soil can be dispersed in sterile diluent, followed by serial dilution to reduce microbial concentration to a countable or isolatable level.
4.4 Plating
Appropriate dilutions are inoculated onto suitable media. Distinct colonies are then selected according to the objectives of the exercise.
4.5 Interpretation
Cultivation reveals organisms able to grow under the chosen conditions; it does not recover every organism originally present in soil.
Chapter Summary
- Soil as a microbial habitat: Soil contains diverse bacteria and fungi associated with mineral particles, organic matter, roots, water films and pore spaces.
- Sampling principle: A soil sample should represent the location and question being investigated.
- Suspension and dilution: A measured quantity of soil can be dispersed in sterile diluent, followed by serial dilution to reduce microbial concentration to a countable or isolatable level.
- Plating: Appropriate dilutions are inoculated onto suitable media.
- Interpretation: Cultivation reveals organisms able to grow under the chosen conditions; it does not recover every organism originally present in soil.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to isolation from soil.
Isolation from Water
5.1 Water sampling
Water samples should be collected in suitable sterile containers and processed using procedures appropriate to the source and objective.
5.2 Direct plating
When microbial density is sufficient, measured volumes or dilutions can be inoculated onto appropriate media.
5.3 Concentration
When organisms are sparse, a concentration method can increase the probability of detection before cultivation.
5.4 Colony recovery
Colonies differing in observable characters may be selected and purified for further characterisation.
5.5 Limitations
Culture results depend on medium, incubation atmosphere, temperature, sample handling and the physiological state of organisms.
Chapter Summary
- Water sampling: Water samples should be collected in suitable sterile containers and processed using procedures appropriate to the source and objective.
- Direct plating: When microbial density is sufficient, measured volumes or dilutions can be inoculated onto appropriate media.
- Concentration: When organisms are sparse, a concentration method can increase the probability of detection before cultivation.
- Colony recovery: Colonies differing in observable characters may be selected and purified for further characterisation.
- Limitations: Culture results depend on medium, incubation atmosphere, temperature, sample handling and the physiological state of organisms.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to isolation from water.
Isolation from Food
6.1 Food as a microbial substrate
Foods differ greatly in water activity, pH, nutrient composition, processing history and storage, all of which influence microbial populations.
6.2 Representative sampling
The analytical portion should represent the food material being investigated. Aseptic handling prevents addition of organisms during processing.
6.3 Homogenisation
A measured food portion is dispersed in sterile diluent to produce a suspension from which further dilutions can be prepared.
6.4 Plating and isolation
Suitable dilutions are cultured on media selected for the target group. Distinct colonies can then be subcultured for purification.
6.5 Interpretation
Recovery of a microorganism demonstrates culturability under the test conditions and must be interpreted in relation to sampling and method limitations.
Chapter Summary
- Food as a microbial substrate: Foods differ greatly in water activity, pH, nutrient composition, processing history and storage, all of which influence microbial populations.
- Representative sampling: The analytical portion should represent the food material being investigated.
- Homogenisation: A measured food portion is dispersed in sterile diluent to produce a suspension from which further dilutions can be prepared.
- Plating and isolation: Suitable dilutions are cultured on media selected for the target group.
- Interpretation: Recovery of a microorganism demonstrates culturability under the test conditions and must be interpreted in relation to sampling and method limitations.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to isolation from food.
Isolation from Air
7.1 Airborne microorganisms
Air transports bacterial cells, fungal spores and microorganism-containing droplets or particles originating from soil, water, people, animals and surfaces.
7.2 Settle plate principle
An exposed agar surface can collect particles by gravitational settling, providing a simple qualitative or semi-quantitative demonstration.
7.3 Active sampling
Air-sampling devices draw known or controlled volumes of air onto or through collection systems, allowing more standardised assessment.
7.4 Colony observation
After appropriate incubation, colonies are counted or described according to the purpose of the exercise.
7.5 Limitations
Passive settling does not sample a defined air volume and favours particles that settle readily; results therefore require cautious interpretation.
Chapter Summary
- Airborne microorganisms: Air transports bacterial cells, fungal spores and microorganism-containing droplets or particles originating from soil, water, people, animals and surfaces.
- Settle plate principle: An exposed agar surface can collect particles by gravitational settling, providing a simple qualitative or semi-quantitative demonstration.
- Active sampling: Air-sampling devices draw known or controlled volumes of air onto or through collection systems, allowing more standardised assessment.
- Colony observation: After appropriate incubation, colonies are counted or described according to the purpose of the exercise.
- Limitations: Passive settling does not sample a defined air volume and favours particles that settle readily; results therefore require cautious interpretation.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to isolation from air.
Obtaining Pure Cultures: Streak Plate
8.1 Pure culture
A pure culture contains one microbial type suitable for consistent morphological and physiological study.
8.2 Streaking principle
The streak-plate method progressively reduces cell density across an agar surface so that individual cells or small clusters become spatially separated.
8.3 Quadrant approach
Successive streak regions are made using appropriate aseptic technique, with decreasing inoculum carried into later sectors.
8.4 Colony selection
A well-isolated colony with the desired morphology is transferred to fresh medium rather than assuming that an entire crowded plate is pure.
8.5 Verification
Purity should be checked by consistent colony appearance, microscopy and other appropriate tests.
Label the base of the agar plate; use aseptic technique to obtain a small inoculum; streak the first region; reduce the carried inoculum before streaking subsequent regions; incubate under the specified conditions; select a well-isolated colony; transfer it to fresh medium; then verify purity by colony appearance and microscopy.
Chapter Summary
- Pure culture: A pure culture contains one microbial type suitable for consistent morphological and physiological study.
- Streaking principle: The streak-plate method progressively reduces cell density across an agar surface so that individual cells or small clusters become spatially separated.
- Quadrant approach: Successive streak regions are made using appropriate aseptic technique, with decreasing inoculum carried into later sectors.
- Colony selection: A well-isolated colony with the desired morphology is transferred to fresh medium rather than assuming that an entire crowded plate is pure.
- Verification: Purity should be checked by consistent colony appearance, microscopy and other appropriate tests.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to obtaining pure cultures: streak plate.
Pour Plate, Spread Plate and Subculturing
9.1 Pour plate
A measured inoculum is mixed with molten agar at a suitable temperature. Colonies may develop within the agar and on its surface.
9.2 Spread plate
A measured liquid inoculum is distributed over the surface of solid agar. Surface colonies are accessible for counting and selection.
9.3 Method choice
Pour and spread plates differ in oxygen exposure, colony position and practical handling. Method selection should match the experimental objective.
9.4 Subculture
Subculturing transfers growth from an existing culture to fresh medium to maintain, purify or test an organism.
9.5 Contamination checks
Unexpected colony types, mixed microscopic morphology or inconsistent reactions may indicate contamination and require re-isolation.
Chapter Summary
- Pour plate: A measured inoculum is mixed with molten agar at a suitable temperature.
- Spread plate: A measured liquid inoculum is distributed over the surface of solid agar.
- Method choice: Pour and spread plates differ in oxygen exposure, colony position and practical handling.
- Subculture: Subculturing transfers growth from an existing culture to fresh medium to maintain, purify or test an organism.
- Contamination checks: Unexpected colony types, mixed microscopic morphology or inconsistent reactions may indicate contamination and require re-isolation.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to pour plate, spread plate and subculturing.
Microscopy and Smear Preparation
10.1 Microscope use
Begin with the lowest appropriate objective, focus and centre the specimen, then move to higher magnification as required.
10.2 Smear
A microbial smear is a thin film of cells placed on a clean slide for staining and microscopic examination.
10.3 Air drying
A smear should be allowed to dry before fixation so that heating does not cause violent boiling or distortion.
10.4 Fixation
Appropriate fixation attaches cells to the slide and can preserve morphology for staining, although excessive heating may distort cells.
10.5 Observation
Record cell shape, arrangement, staining response and other visible structures without claiming characteristics that cannot be resolved by the method.
Chapter Summary
- Microscope use: Begin with the lowest appropriate objective, focus and centre the specimen, then move to higher magnification as required.
- Smear: A microbial smear is a thin film of cells placed on a clean slide for staining and microscopic examination.
- Air drying: A smear should be allowed to dry before fixation so that heating does not cause violent boiling or distortion.
- Fixation: Appropriate fixation attaches cells to the slide and can preserve morphology for staining, although excessive heating may distort cells.
- Observation: Record cell shape, arrangement, staining response and other visible structures without claiming characteristics that cannot be resolved by the method.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to microscopy and smear preparation.
Simple and Differential Staining
11.1 Simple stain
A simple stain uses a single dye to increase contrast and demonstrate cell morphology and arrangement.
11.2 Differential staining
Differential stains use multiple reagents to distinguish organisms or structures according to differences in cellular properties.
11.3 Gram stain
The Gram stain separates bacteria into Gram-positive and Gram-negative staining responses through a sequence involving primary stain, mordant, decolourisation and counterstaining.
11.4 Critical step
Decolourisation is particularly sensitive to timing and smear thickness; over- or under-decolourisation can produce misleading results.
11.5 Interpretation
Staining response is one identification character and should be combined with culture and biochemical evidence.
Chapter Summary
- Simple stain: A simple stain uses a single dye to increase contrast and demonstrate cell morphology and arrangement.
- Differential staining: Differential stains use multiple reagents to distinguish organisms or structures according to differences in cellular properties.
- Gram stain: The Gram stain separates bacteria into Gram-positive and Gram-negative staining responses through a sequence involving primary stain, mordant, decolourisation and counterstaining.
- Critical step: Decolourisation is particularly sensitive to timing and smear thickness; over- or under-decolourisation can produce misleading results.
- Interpretation: Staining response is one identification character and should be combined with culture and biochemical evidence.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to simple and differential staining.
Special Staining Techniques
12.1 Endospore staining
Endospore methods demonstrate resistant intracellular structures produced by certain bacteria and distinguish spores from vegetative cells.
12.2 Capsule demonstration
Capsule methods aim to reveal extracellular material surrounding cells, often by creating contrast between the background, cell and unstained capsule.
12.3 Acid-fast staining
Acid-fast staining detects cells whose wax-rich envelopes retain primary stain despite acid-alcohol treatment.
12.4 Fungal preparations
Fungal microscopy focuses on hyphae, septation and reproductive structures that may assist identification.
12.5 Technical caution
Culture age, fixation, reagent condition and procedure timing influence special stains and must be considered when interpreting atypical results.
Chapter Summary
- Endospore staining: Endospore methods demonstrate resistant intracellular structures produced by certain bacteria and distinguish spores from vegetative cells.
- Capsule demonstration: Capsule methods aim to reveal extracellular material surrounding cells, often by creating contrast between the background, cell and unstained capsule.
- Acid-fast staining: Acid-fast staining detects cells whose wax-rich envelopes retain primary stain despite acid-alcohol treatment.
- Fungal preparations: Fungal microscopy focuses on hyphae, septation and reproductive structures that may assist identification.
- Technical caution: Culture age, fixation, reagent condition and procedure timing influence special stains and must be considered when interpreting atypical results.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to special staining techniques.
Enumeration: Direct Methods
13.1 Enumeration
Microbial enumeration estimates or determines the number of cells or viable units in a sample.
13.2 Microscopic counting
Cells in a known chamber volume can be counted directly under a microscope and converted to concentration.
13.3 Strengths
Direct counts can be rapid and do not require cells to form colonies.
13.4 Limitations
Ordinary direct microscopy may not distinguish living from dead cells and can be difficult at low cell densities.
13.5 Calculation principle
When a known volume is counted, cell concentration equals the number counted divided by the volume represented, with any dilution factor incorporated correctly.
Chapter Summary
- Enumeration: Microbial enumeration estimates or determines the number of cells or viable units in a sample.
- Microscopic counting: Cells in a known chamber volume can be counted directly under a microscope and converted to concentration.
- Strengths: Direct counts can be rapid and do not require cells to form colonies.
- Limitations: Ordinary direct microscopy may not distinguish living from dead cells and can be difficult at low cell densities.
- Calculation principle: When a known volume is counted, cell concentration equals the number counted divided by the volume represented, with any dilution factor incorporated correctly.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to enumeration: direct methods.
Enumeration: Viable and Indirect Methods
14.1 Viable plate count
A viable count estimates colony-forming units from colonies produced by culturable cells or cell clusters.
14.2 Countable dilution
Dilution reduces cell concentration so that discrete colonies can be counted reliably. The chosen dilution must be recorded.
14.3 CFU calculation
The count is related to plated volume and dilution to estimate colony-forming units per unit volume or mass of the original sample.
14.4 Turbidity
Optical density provides an indirect estimate of biomass or cell concentration after calibration against an appropriate reference.
14.5 Dry mass and other measures
Biomass may also be estimated by dry mass, packed cell volume or biochemical components, depending on the organism and purpose.
14.6 Interpretive caution
A colony does not always arise from exactly one cell, so viable counts are expressed as colony-forming units rather than absolute individual-cell counts.
Chapter Summary
- Viable plate count: A viable count estimates colony-forming units from colonies produced by culturable cells or cell clusters.
- Countable dilution: Dilution reduces cell concentration so that discrete colonies can be counted reliably.
- CFU calculation: The count is related to plated volume and dilution to estimate colony-forming units per unit volume or mass of the original sample.
- Turbidity: Optical density provides an indirect estimate of biomass or cell concentration after calibration against an appropriate reference.
- Dry mass and other measures: Biomass may also be estimated by dry mass, packed cell volume or biochemical components, depending on the organism and purpose.
- Interpretive caution: A colony does not always arise from exactly one cell, so viable counts are expressed as colony-forming units rather than absolute individual-cell counts.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to enumeration: viable and indirect methods.
Worked Enumeration Problems
15.1 Serial dilution
A serial dilution is a stepwise reduction in sample concentration by transferring a measured portion into a known volume of diluent.
15.2 Dilution factor
For a 1:10 dilution, the dilution is 10^-1 and the reciprocal dilution factor is 10. Repeating the step multiplies dilution effects.
15.3 CFU per millilitre
For a suitable plate, CFU/mL can be calculated from colony count divided by the product of dilution and plated volume in millilitres.
15.4 CFU per gram
For solid samples, the original sample-to-diluent preparation must also be considered so that the final answer refers to the original mass.
15.5 Significant interpretation
Calculations should report units, dilution used and assumptions; numerical precision should not exceed the reliability of the experimental method.
Question: A plate inoculated with 0.1 mL of a 10-5 dilution gives 86 colonies. Estimate the viable count in the original suspension.
Given: Colonies = 86; dilution = 10-5; plated volume = 0.1 mL.
Required: CFU/mL of original sample.
Formula: CFU/mL = colonies ÷ (dilution × volume plated).
Solution: 86 ÷ (10-5 × 0.1) = 86 ÷ 10-6 = 8.6 × 107.
Answer: 8.6 × 107 CFU/mL, assuming the selected plate and procedure satisfy the laboratory's counting criteria.
Chapter Summary
- Serial dilution: A serial dilution is a stepwise reduction in sample concentration by transferring a measured portion into a known volume of diluent.
- Dilution factor: For a 1:10 dilution, the dilution is 10^-1 and the reciprocal dilution factor is 10.
- CFU per millilitre: For a suitable plate, CFU/mL can be calculated from colony count divided by the product of dilution and plated volume in millilitres.
- CFU per gram: For solid samples, the original sample-to-diluent preparation must also be considered so that the final answer refers to the original mass.
- Significant interpretation: Calculations should report units, dilution used and assumptions; numerical precision should not exceed the reliability of the experimental method.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to worked enumeration problems.
Colonial and Cellular Characterisation
16.1 Colony form
Describe overall colony shape such as circular, irregular or filamentous without using vague terms.
16.2 Margin and elevation
Margins may be entire, undulate, lobate or otherwise patterned, while elevation describes vertical profile.
16.3 Surface and pigmentation
Texture, shininess, opacity and pigment production provide additional characters.
16.4 Cell morphology
Microscopy establishes cell shape, arrangement and staining properties.
16.5 Combined evidence
Colonial and cellular morphology together narrow possibilities but generally do not provide definitive species identification.
Chapter Summary
- Colony form: Describe overall colony shape such as circular, irregular or filamentous without using vague terms.
- Margin and elevation: Margins may be entire, undulate, lobate or otherwise patterned, while elevation describes vertical profile.
- Surface and pigmentation: Texture, shininess, opacity and pigment production provide additional characters.
- Cell morphology: Microscopy establishes cell shape, arrangement and staining properties.
- Combined evidence: Colonial and cellular morphology together narrow possibilities but generally do not provide definitive species identification.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to colonial and cellular characterisation.
Biochemical Identification
17.1 Principle
Biochemical tests detect metabolic enzymes, substrate utilisation or characteristic products.
17.2 Carbohydrate reactions
Fermentation or utilisation tests can detect acid, gas or other changes produced from specified substrates.
17.3 Enzyme tests
Catalase, oxidase and related tests detect particular enzyme activities and can help separate groups.
17.4 Hydrolysis tests
Microorganisms may be assessed for their ability to degrade macromolecules such as starch, proteins or lipids.
17.5 Test panels
Identification commonly depends on a pattern of multiple reactions rather than a single positive test.
17.6 Quality control
Known control organisms or expected control reactions help verify that reagents and conditions function correctly.
Chapter Summary
- Principle: Biochemical tests detect metabolic enzymes, substrate utilisation or characteristic products.
- Carbohydrate reactions: Fermentation or utilisation tests can detect acid, gas or other changes produced from specified substrates.
- Enzyme tests: Catalase, oxidase and related tests detect particular enzyme activities and can help separate groups.
- Hydrolysis tests: Microorganisms may be assessed for their ability to degrade macromolecules such as starch, proteins or lipids.
- Test panels: Identification commonly depends on a pattern of multiple reactions rather than a single positive test.
- Quality control: Known control organisms or expected control reactions help verify that reagents and conditions function correctly.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to biochemical identification.
Serological Techniques in Identification
18.1 Serological principle
Serological identification uses specific binding between microbial antigens and antibodies.
18.2 Agglutination
Antibody-mediated clumping of particulate antigens can provide a visible reaction.
18.3 Specificity
A positive reaction indicates recognition under test conditions but must be interpreted with controls and awareness of possible cross-reactions.
18.4 Controls
Positive and negative controls distinguish valid reactions from reagent failure or nonspecific effects.
18.5 Role in identification
Serology supplements morphology and biochemical evidence and is particularly useful when characteristic antigens are known.
Chapter Summary
- Serological principle: Serological identification uses specific binding between microbial antigens and antibodies.
- Agglutination: Antibody-mediated clumping of particulate antigens can provide a visible reaction.
- Specificity: A positive reaction indicates recognition under test conditions but must be interpreted with controls and awareness of possible cross-reactions.
- Controls: Positive and negative controls distinguish valid reactions from reagent failure or nonspecific effects.
- Role in identification: Serology supplements morphology and biochemical evidence and is particularly useful when characteristic antigens are known.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to serological techniques in identification.
Antibiotic Sensitivity Disc Testing
19.1 Purpose
Antibiotic susceptibility testing evaluates inhibition of a bacterial isolate by defined antimicrobial agents under standardised conditions.
19.2 Standardised inoculum
The density of the inoculum must be controlled because excessive or insufficient growth can alter inhibition-zone sizes.
19.3 Disc diffusion
Antibiotic-impregnated discs are placed on an inoculated agar surface, and antimicrobial diffusion establishes concentration gradients.
19.4 Zone measurement
After specified incubation, inhibition-zone diameters are measured accurately.
19.5 Interpretation
Zone diameters are compared with validated interpretive standards appropriate to the organism, drug and method. Visual size alone is not enough.
19.6 Sources of error
Medium depth, inoculum density, disc potency, incubation conditions and measurement errors can affect results.
Chapter Summary
- Purpose: Antibiotic susceptibility testing evaluates inhibition of a bacterial isolate by defined antimicrobial agents under standardised conditions.
- Standardised inoculum: The density of the inoculum must be controlled because excessive or insufficient growth can alter inhibition-zone sizes.
- Disc diffusion: Antibiotic-impregnated discs are placed on an inoculated agar surface, and antimicrobial diffusion establishes concentration gradients.
- Zone measurement: After specified incubation, inhibition-zone diameters are measured accurately.
- Interpretation: Zone diameters are compared with validated interpretive standards appropriate to the organism, drug and method.
- Sources of error: Medium depth, inoculum density, disc potency, incubation conditions and measurement errors can affect results.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to antibiotic sensitivity disc testing.
Agar-Well Diffusion Method
20.1 Principle
In agar-well diffusion, a test substance placed in a well diffuses through inoculated agar and may produce a zone of growth inhibition.
20.2 Preparation
A uniform microbial lawn, consistent agar depth and wells of comparable dimensions are needed for meaningful comparison.
20.3 Application
The method can screen antimicrobial activity of preparations under defined laboratory conditions.
20.4 Measurement
Zones are measured consistently after incubation, with suitable controls included.
20.5 Limitations
Zone size depends not only on antimicrobial activity but also on diffusion, solubility and experimental conditions; it should not automatically be equated with clinical susceptibility.
Chapter Summary
- Principle: In agar-well diffusion, a test substance placed in a well diffuses through inoculated agar and may produce a zone of growth inhibition.
- Preparation: A uniform microbial lawn, consistent agar depth and wells of comparable dimensions are needed for meaningful comparison.
- Application: The method can screen antimicrobial activity of preparations under defined laboratory conditions.
- Measurement: Zones are measured consistently after incubation, with suitable controls included.
- Limitations: Zone size depends not only on antimicrobial activity but also on diffusion, solubility and experimental conditions; it should not automatically be equated with clinical susceptibility.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to agar-well diffusion method.
Cultivation of Anaerobic Microorganisms
21.1 Anaerobiosis
Anaerobic cultivation creates conditions with little or no molecular oxygen for organisms that require or tolerate such environments.
21.2 Anaerobic jar
An anaerobic jar encloses inoculated media while a chemical or gas-generating system removes or reduces oxygen.
21.3 Indicator
A redox indicator can help demonstrate whether sufficiently reduced conditions were achieved.
21.4 Culture handling
Media choice, inoculation, atmosphere and incubation must be coordinated because oxygen exposure can inhibit strict anaerobes.
21.5 Interpretation
Failure of growth may reflect unsuitable medium or atmosphere rather than absence of viable anaerobic organisms.
Chapter Summary
- Anaerobiosis: Anaerobic cultivation creates conditions with little or no molecular oxygen for organisms that require or tolerate such environments.
- Anaerobic jar: An anaerobic jar encloses inoculated media while a chemical or gas-generating system removes or reduces oxygen.
- Indicator: A redox indicator can help demonstrate whether sufficiently reduced conditions were achieved.
- Culture handling: Media choice, inoculation, atmosphere and incubation must be coordinated because oxygen exposure can inhibit strict anaerobes.
- Interpretation: Failure of growth may reflect unsuitable medium or atmosphere rather than absence of viable anaerobic organisms.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to cultivation of anaerobic microorganisms.
Preservation by Agar Slants and Subculture
22.1 Short-term maintenance
Cultures can be maintained on agar slants under appropriate refrigeration for limited periods.
22.2 Periodic subculture
Transfer to fresh medium restores nutrients and viable growth but repeated subculture carries contamination and genetic or phenotypic change risks.
22.3 Labelling
Preserved cultures require strain identity, date and storage information.
22.4 Purity checks
Cultures should be examined before and after maintenance to detect contamination.
22.5 Appropriate use
Slants and repeated subculture are convenient for teaching and routine work but are less suitable for very long-term preservation.
Chapter Summary
- Short-term maintenance: Cultures can be maintained on agar slants under appropriate refrigeration for limited periods.
- Periodic subculture: Transfer to fresh medium restores nutrients and viable growth but repeated subculture carries contamination and genetic or phenotypic change risks.
- Labelling: Preserved cultures require strain identity, date and storage information.
- Purity checks: Cultures should be examined before and after maintenance to detect contamination.
- Appropriate use: Slants and repeated subculture are convenient for teaching and routine work but are less suitable for very long-term preservation.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to preservation by agar slants and subculture.
Preservation by Refrigeration and Deep Freezing
23.1 Refrigeration
Low temperature slows metabolism and growth, extending survival of many cultures for short or intermediate periods.
23.2 Freezing
Deep freezing greatly reduces metabolic activity and can preserve cultures for longer periods.
23.3 Cryoprotection
Suitable cryoprotective agents can reduce damage caused by ice formation and osmotic stress during freezing and thawing.
23.4 Aliquoting
Separate stored portions reduce repeated freeze-thaw cycles and limit contamination of the master stock.
23.5 Recovery
Preserved cultures should be revived on suitable media and checked for purity and expected characteristics.
Chapter Summary
- Refrigeration: Low temperature slows metabolism and growth, extending survival of many cultures for short or intermediate periods.
- Freezing: Deep freezing greatly reduces metabolic activity and can preserve cultures for longer periods.
- Cryoprotection: Suitable cryoprotective agents can reduce damage caused by ice formation and osmotic stress during freezing and thawing.
- Aliquoting: Separate stored portions reduce repeated freeze-thaw cycles and limit contamination of the master stock.
- Recovery: Preserved cultures should be revived on suitable media and checked for purity and expected characteristics.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to preservation by refrigeration and deep freezing.
Lyophilisation and Liquid-Nitrogen Storage
24.1 Lyophilisation
Freeze-drying removes water from a frozen culture under reduced pressure, producing a stable dried preparation for organisms that tolerate the process.
24.2 Advantages
Properly prepared lyophilised cultures can remain stable for long periods and are convenient for transport and storage.
24.3 Limitations
Not all organisms survive drying equally well, and careful revival procedures are required.
24.4 Liquid nitrogen
Very low-temperature storage greatly reduces biochemical reactions and can preserve suitable microbial cultures for long periods.
24.5 Culture collections
Long-term preservation protects reference strains from repeated subculture and helps maintain reproducibility in research and teaching.
| Method | General use | Main consideration |
|---|---|---|
| Agar slant/refrigeration | Short-term maintenance | Metabolism continues slowly |
| Repeated subculture | Routine maintenance | Contamination and change can accumulate |
| Deep freezing | Longer-term storage | Freezing injury and freeze-thaw cycles |
| Lyophilisation | Long-term dry preservation | Organism must tolerate freezing/drying |
| Liquid nitrogen | Long-term cryopreservation | Specialised storage and handling |
Chapter Summary
- Lyophilisation: Freeze-drying removes water from a frozen culture under reduced pressure, producing a stable dried preparation for organisms that tolerate the process.
- Advantages: Properly prepared lyophilised cultures can remain stable for long periods and are convenient for transport and storage.
- Limitations: Not all organisms survive drying equally well, and careful revival procedures are required.
- Liquid nitrogen: Very low-temperature storage greatly reduces biochemical reactions and can preserve suitable microbial cultures for long periods.
- Culture collections: Long-term preservation protects reference strains from repeated subculture and helps maintain reproducibility in research and teaching.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to lyophilisation and liquid-nitrogen storage.
Integrated Identification Workflow
25.1 Step 1: isolate
Obtain separated colonies or another appropriate pure-culture preparation from the original sample.
25.2 Step 2: describe
Record colony morphology and microscopic cellular characters, including staining response.
25.3 Step 3: test
Select biochemical, physiological or serological tests based on the developing identification hypothesis.
25.4 Step 4: compare
Compare the complete pattern of results with validated identification criteria rather than matching one characteristic.
25.5 Step 5: verify
Repeat doubtful reactions, include controls and ensure culture purity before reporting an identification.
25.6 Scientific reporting
Report observations, methods and interpretations separately so that another microbiologist can evaluate how the conclusion was reached.
Chapter Summary
- Step 1: isolate: Obtain separated colonies or another appropriate pure-culture preparation from the original sample.
- Step 2: describe: Record colony morphology and microscopic cellular characters, including staining response.
- Step 3: test: Select biochemical, physiological or serological tests based on the developing identification hypothesis.
- Step 4: compare: Compare the complete pattern of results with validated identification criteria rather than matching one characteristic.
- Step 5: verify: Repeat doubtful reactions, include controls and ensure culture purity before reporting an identification.
- Scientific reporting: Report observations, methods and interpretations separately so that another microbiologist can evaluate how the conclusion was reached.
Practical Review
- State the principle of the technique.
- List the essential materials or conditions required.
- Describe the procedure in logical sequence.
- State the expected observations and how they should be recorded.
- Explain the interpretation of the result.
- Identify major sources of error or contamination.
- Explain how the technique contributes to integrated identification workflow.
Comprehensive Practical Revision
- Explain aseptic technique and why it is essential.
- Describe preparation and quality assessment of culture media.
- Describe isolation of bacteria from soil, water, food and air.
- Explain streak, pour and spread plate methods.
- Describe subculturing and verification of culture purity.
- Explain simple, Gram and special staining procedures.
- Compare direct and viable enumeration methods.
- Solve serial-dilution and CFU calculations.
- Describe colonial and cellular characterisation.
- Explain biochemical identification procedures.
- Discuss serological identification.
- Explain antibiotic disc sensitivity testing and its interpretation.
- Describe agar-well diffusion and its limitations.
- Explain cultivation using an anaerobic jar.
- Compare preservation by agar slants, refrigeration, deep freezing, lyophilisation and liquid nitrogen.
- Design a logical workflow for isolating, purifying and identifying an unknown bacterial isolate.
Selected References
- National Universities Commission. Core Curriculum and Minimum Academic Standards for the Nigerian University System: Sciences. MCB 231: Basic Techniques in Microbiology.
- Cappuccino, J. G. and Welsh, C. T. Microbiology: A Laboratory Manual.
- Leboffe, M. J. and Pierce, B. E. A Photographic Atlas for the Microbiology Laboratory.
- Willey, J. M. et al. Prescott's Microbiology.