Section 1 — Diagnostic Bacteriology
- Laboratory Safety & Quality Management
- Specimen Selection, Collection, Transport & Processing
- Conventional Identification Methods
- Microscopic Methods
- Rapid & Automated Identification
- Serological Methods
- Biotyping, Serotyping & Phage Typing
- Molecular Typing
- Kirby–Bauer Disc Diffusion Method
- Minimum Inhibitory Concentration (MIC)
- ESBL Detection
1. Laboratory Safety & Quality Management
Laboratory Safety in Microbiology
Clinical microbiology laboratories handle potentially pathogenic microorganisms and infectious clinical specimens. Therefore, appropriate safety measures must be followed during specimen collection, processing, culture, identification, antimicrobial susceptibility testing and disposal.
Major Laboratory Hazards
| Hazard | Examples |
|---|---|
| Biological | Pathogenic bacteria, viruses, fungi, parasites and contaminated clinical specimens |
| Chemical | Disinfectants, stains, acids, alkalis and toxic chemicals |
| Physical | Broken glass, centrifuges, autoclaves, heat and electrical equipment |
| Sharps | Needles, blades, broken glass and contaminated sharp objects |
Basic Laboratory Safety Practices
- Wear appropriate personal protective equipment.
- Do not eat, drink or smoke inside the laboratory.
- Wash hands before and after laboratory work.
- Use mechanical devices for pipetting; never mouth pipette.
- Handle clinical specimens as potentially infectious.
- Minimize aerosol generation during specimen processing.
- Disinfect work surfaces before and after work.
- Dispose of infectious waste appropriately.
- Use biosafety cabinets when required.
- Report accidents, spills and exposure incidents immediately.
Quality Management in the Microbiology Laboratory
Quality management is a systematic approach used to ensure that laboratory results are accurate, reliable, reproducible and clinically useful.
Components of Quality Management
- Quality planning
- Standard operating procedures (SOPs)
- Quality control
- Quality assurance
- Equipment maintenance and calibration
- Proper documentation and record keeping
- Internal quality assessment
- External quality assessment
- Staff training and competency assessment
- Corrective and preventive actions
Exam Focus
- 2 marks: Define laboratory safety and quality control.
- 5 marks: Discuss laboratory safety measures.
- 10 marks: Discuss laboratory safety and quality management in a clinical microbiology laboratory.
2. Specimen Selection, Collection, Transport & Processing
Importance of Proper Specimen Collection
Proper specimen collection is one of the most important steps in microbiological diagnosis. An inappropriate, contaminated, insufficient or improperly transported specimen may produce a false-negative or misleading result.
General Principles
- Collect the specimen from the actual site of infection.
- Collect preferably before antimicrobial therapy.
- Use sterile and appropriate containers.
- Collect an adequate quantity.
- Avoid contamination with normal flora.
- Properly label the specimen.
- Transport to the laboratory as soon as possible.
Specimen Collection and Transport
| Specimen | Important Considerations |
|---|---|
| Blood | Collect aseptically, preferably before antimicrobial therapy; adequate volume is important. |
| Urine | Clean-catch midstream urine is commonly collected. |
| Sputum | A deep cough specimen rather than saliva should be collected. |
| Stool | Collect in a clean, sterile container and transport promptly. |
| CSF | Requires urgent processing because many causative organisms are fragile. |
| Pus/wound specimen | Aspirated material is generally preferable to a superficial swab when possible. |
Specimen Processing
Specimen processing may include direct microscopy, inoculation onto appropriate culture media, incubation under suitable environmental conditions, biochemical testing, antigen detection, antimicrobial susceptibility testing and molecular methods.
Exam Focus
- 5 marks: Discuss the principles of clinical specimen collection.
- 10 marks: Discuss specimen selection, collection, transport and processing in clinical bacteriology.
3. Conventional Identification Methods
Conventional bacterial identification involves the use of morphology, cultural characteristics, biochemical reactions and other phenotypic properties to identify bacteria.
Major Steps
- Direct microscopic examination
- Primary culture
- Observation of colony morphology
- Gram staining
- Biochemical testing
- Identification using characteristic reactions
- Antimicrobial susceptibility testing where required
Colony Characteristics
Bacterial colonies are examined for size, shape, elevation, margin, surface, texture, opacity, pigmentation, hemolysis and other characteristic features.
Biochemical Identification
| Test | Major Purpose |
|---|---|
| Catalase | Differentiation of catalase-positive and catalase-negative organisms |
| Oxidase | Detection of cytochrome c oxidase |
| Indole | Detection of tryptophanase activity |
| Urease | Detection of urease enzyme |
| Citrate utilization | Determination of citrate utilization as a carbon source |
| TSI | Assessment of carbohydrate fermentation and H₂S production |
4. Microscopic Methods
Microscopy provides rapid information about the morphology, arrangement and staining characteristics of microorganisms.
Important Microscopic Methods
| Method | Application |
|---|---|
| Gram staining | Differentiation of bacteria into Gram-positive and Gram-negative groups. |
| Acid-fast staining | Detection of acid-fast bacilli such as Mycobacterium. |
| Negative staining | Demonstration of bacterial morphology without directly staining the organism. |
| Special staining | Demonstration of structures such as spores, capsules and flagella. |
Advantages
- Rapid preliminary information
- Simple and inexpensive
- Useful for determining bacterial morphology
- Helps guide initial laboratory processing
Limitations
- Some organisms cannot be reliably identified to species level.
- Low numbers of organisms may not be detected.
- Interpretation requires experience.
Exam Focus
- 2 marks: Mention two applications of Gram staining.
- 5 marks: Discuss microscopic methods used in bacteriology.
5. Rapid & Automated Identification
Rapid and automated identification systems reduce the time required for bacterial identification and can improve standardization and reproducibility.
Rapid Identification Methods
- Latex agglutination
- Rapid biochemical tests
- Antigen detection
- Immunochromatographic assays
- MALDI-TOF mass spectrometry
- Molecular identification methods
Automated Systems
Automated systems use standardized biochemical, metabolic or other characteristics to identify microorganisms and, in some systems, determine antimicrobial susceptibility.
| Method | Principle / Use |
|---|---|
| MALDI-TOF MS | Identification based on characteristic protein profiles. |
| Automated biochemical systems | Identification using multiple biochemical reactions. |
| Automated AST systems | Determination of antimicrobial susceptibility using standardized systems. |
6. Serological Methods
Major Serological Techniques
- Agglutination
- Precipitation
- ELISA
- Immunofluorescence
- Complement fixation
- Immunochromatographic tests
Agglutination
Agglutination occurs when particulate antigens react with corresponding antibodies and form visible clumps.
Applications
- Identification of bacterial antigens
- Detection of antibodies in patient serum
- Serotyping of bacterial isolates
- Diagnosis of selected bacterial infections
ELISA
Enzyme-linked immunosorbent assay uses enzyme-labelled reagents to detect specific antigen or antibody. The enzyme reaction produces a measurable signal, commonly a color change.
Exam Focus
- 2 marks: Define agglutination.
- 5 marks: Describe ELISA.
- 10 marks: Discuss important serological methods used in microbiological diagnosis.
7. Biotyping, Serotyping & Phage Typing
Biotyping
Biotyping is the classification of bacterial strains within a species according to their biochemical or physiological characteristics.
Serotyping
Serotyping is the differentiation of microorganisms into serotypes based on antigenic differences.
Phage Typing
Phage typing differentiates bacterial strains according to their susceptibility to specific bacteriophages.
| Method | Basis | Application |
|---|---|---|
| Biotyping | Biochemical characteristics | Strain differentiation |
| Serotyping | Antigenic characteristics | Epidemiology and strain identification |
| Phage typing | Phage susceptibility | Epidemiological investigation |
8. Molecular Typing
Molecular typing differentiates microorganisms using differences in their genetic material. It is especially useful for epidemiological investigations, outbreak investigation and determination of genetic relatedness.
Important Molecular Typing Methods
- PCR-based typing
- RFLP
- RAPD
- Pulsed-field gel electrophoresis (PFGE)
- MLST
- Whole-genome sequencing
Applications
- Investigation of outbreaks
- Tracking transmission of bacterial strains
- Identification of related isolates
- Hospital infection surveillance
- Epidemiological studies
Exam Focus
- 5 marks: Describe molecular typing methods.
- 10 marks: Discuss molecular methods used for bacterial typing and epidemiological investigation.
9. Kirby–Bauer Disc Diffusion Method
Principle
Antibiotic-impregnated discs are placed on an agar surface inoculated with a standardized bacterial suspension. During incubation, the antimicrobial agent diffuses through the agar and inhibits bacterial growth around the disc, producing a zone of inhibition.
Procedure
- Prepare a pure culture of the test organism.
- Prepare a bacterial suspension equivalent to the required turbidity standard.
- Inoculate the standardized suspension uniformly over Mueller–Hinton agar.
- Allow the surface to dry briefly.
- Place appropriate antimicrobial discs on the agar surface.
- Incubate under standardized conditions.
- Measure the diameter of inhibition zones.
- Interpret the result according to the applicable standardized interpretive criteria.
Factors Affecting the Test
- Inoculum density
- Agar depth
- Medium composition
- Disc potency
- Incubation conditions
- Timing of testing
- Age and purity of culture
Interpretation
The measured inhibition zone is compared with standardized interpretive criteria. The isolate is categorized according to the current applicable breakpoint system used by the laboratory.
Exam Focus
- 5 marks: Describe the Kirby–Bauer disc diffusion method.
- 10 marks: Discuss the principle, procedure, interpretation and factors affecting Kirby–Bauer antimicrobial susceptibility testing.
10. Minimum Inhibitory Concentration (MIC)
Principle
A series of increasing concentrations of an antimicrobial agent is tested against a standardized bacterial inoculum. After incubation, the lowest concentration showing no visible growth is recorded as the MIC.
Methods for Determining MIC
- Broth dilution method
- Macrobroth dilution
- Microbroth dilution
- Agar dilution
- Gradient diffusion methods
Clinical Importance
- Helps determine antimicrobial susceptibility.
- Provides a quantitative measurement of antimicrobial activity.
- Useful when precise susceptibility information is required.
- Can assist antimicrobial therapeutic decision-making when interpreted with appropriate clinical breakpoints.
MIC vs MBC
| MIC | MBC |
|---|---|
| Lowest concentration that prevents visible bacterial growth. | Lowest concentration that results in bacterial killing under defined test conditions. |
Exam Focus
- 2 marks: Define MIC.
- 5 marks: Explain the principle and methods of MIC determination.
- 10 marks: Discuss MIC determination by dilution methods and its clinical significance.
11. ESBL Detection
Common ESBL-Producing Organisms
- Escherichia coli
- Klebsiella pneumoniae
- Klebsiella oxytoca
- Proteus mirabilis
- Other Enterobacterales may also acquire ESBL genes.
Screening Test
Screening involves testing susceptibility to selected extended-spectrum cephalosporins and identifying isolates that show reduced susceptibility according to the applicable laboratory criteria.
Phenotypic Confirmatory Test
A common phenotypic approach compares the activity of a cephalosporin alone with its activity in combination with a β-lactamase inhibitor such as clavulanate. Increased inhibition in the presence of the inhibitor supports ESBL production.
Combined Disc Test
In the combined disc method, discs containing an appropriate cephalosporin alone and the same cephalosporin combined with clavulanic acid are compared.
Double Disc Synergy Test
In the double-disc synergy approach, an appropriate cephalosporin disc and a clavulanate-containing disc are placed at a standardized distance. Enhancement of the inhibition zone toward the inhibitor-containing disc indicates synergy and supports ESBL production.
Importance of ESBL Detection
- Helps identify important antimicrobial resistance mechanisms.
- Supports appropriate laboratory reporting.
- Helps guide antimicrobial therapy.
- Supports infection prevention and control.
- Helps monitor antimicrobial resistance in healthcare settings.
Screening vs Confirmatory Testing
| Screening Test | Confirmatory Test |
|---|---|
| Identifies isolates with reduced susceptibility that may indicate ESBL production. | Uses an inhibitor-based phenotypic approach to demonstrate the characteristic increase in susceptibility associated with ESBL inhibition. |
Exam Focus
- 2 marks: Define ESBL.
- 5 marks: Discuss screening and confirmatory tests for ESBL detection.
- 10 marks: Discuss the principle, screening methods, confirmatory tests and clinical significance of ESBL detection.
Section 1 — Exam-Oriented Summary
Important 2-Mark Areas
- Define laboratory safety.
- Define quality control.
- Define clinical specimen.
- Define serotyping.
- Define molecular typing.
- Define MIC.
- Define ESBL.
- What is Kirby–Bauer method?
Important 5-Mark Areas
- Laboratory safety measures
- Specimen collection and transport
- Conventional bacterial identification
- Microscopic methods
- Serological methods
- Biotyping, serotyping and phage typing
- Molecular typing
- Kirby–Bauer method
- MIC determination
- Screening and confirmatory tests for ESBL
Important 10-Mark Areas
- Laboratory safety and quality management
- Specimen selection, collection, transport and processing
- Conventional and modern methods of bacterial identification
- Serological methods used in bacteriological diagnosis
- Molecular typing of bacteria
- Kirby–Bauer antimicrobial susceptibility testing
- MIC determination and its significance
- ESBL screening and confirmatory testing