Chapter 15: Diagnosing Infections – Microbial Identification and Laboratory Techniques

Diagnosing Infections: Overview

Introduction

Accurate diagnosis of infectious diseases relies on the identification of microbial agents in patient specimens. This process uses a combination of phenotypic, immunologic, and genotypic methods to create a unique profile for each microorganism.

Major Categories of Microbial Identification Techniques

Phenotypic Methods

• Definition: Observation of an organism’s expressed traits, including morphology, physiology, and biochemistry.

• Key Features:

  • Microscopic and macroscopic appearance (e.g., cell shape, colony morphology)

  • Types of enzymatic activities

  • Growth conditions (temperature, oxygen requirements)

  • Antibiotic susceptibility

  • Chemical composition of cell walls/membranes

• Example: Gram stain, acid-fast stain, and KOH preparation for fungi.

Immunologic Methods

• Definition: Use of serological analysis to detect specific antibodies or antigens in patient samples.

• Key Features:

  • Exploits the antibody response for diagnosis

  • Often easier and faster than culturing the microbe

  • Laboratory kits available for rapid identification

• Example: Agglutination tests, ELISA, immunochromatography (rapid strep test).

Genotypic Methods

• Definition: Analysis of the genetic material (DNA or RNA) of the microorganism.

• Key Features:

  • Does not always require culturing the organism

  • Essential for identifying nonculturable pathogens

• Example: PCR, nucleic acid hybridization, whole-genome sequencing.

Specimen Collection and Handling

Importance of Proper Collection

• Success of identification depends on correct collection, handling, storage, and culturing of specimens.

• Aseptic technique is critical to prevent contamination.

• Use of sterile containers and tools is required.

• Only the infected site should be sampled, avoiding surrounding areas to prevent contamination with normal microbiota.

Sampling Sites and Methods

• Saliva: Patient spits or drools into a container.

• Sputum: Collected by coughing or catheter; avoid saliva contamination.

• Urine:

  • Aseptically from bladder (catheter)

  • "Clean catch" (midstream after cleaning urethra)

  • "Dirty catch" (first-voided urine for some tests)

  • Swabs for mucous lining of urethra, vagina, or cervix

• Skin: Swabbed or scraped; wounds sampled by swab or biopsy.

• Fluids: Blood, cerebrospinal, and tissue fluids collected by sterile needle aspiration.

• Other sources: Eye, ear canal, synovial fluid, nasal cavity, and biopsied tissue.

Key Points for Collection

• Proper labeling and patient history are essential for accurate results.

• Transport and storage conditions must preserve specimen integrity.

Laboratory Techniques for Microbial Identification

Overview

• Patient analysis for infection signs (fever, exudate, lesions, etc.)

• Specimen collection and analysis can take minutes (rapid tests) to weeks (tuberculosis culture).

• Results are documented in the patient chart.

Direct Examination Methods

• Microscopic observation of fresh or stained specimens (e.g., Gram stain, acid-fast stain, KOH for fungi).

• Provides rapid presumptive or confirmatory identification.

Growth-Based Methods

• Selective media: Encourages growth of suspected pathogen from mixed samples.

• Differential media: Identifies specific characteristics or fermentation patterns.

• Specialized media: Enriches pathogens present in low numbers.

Dichotomous Key

• Flowchart-based approach using Gram stain, media growth, and enzymatic tests to identify bacteria.

• Example: Differentiating Streptococcus (catalase-negative, chains) from Staphylococcus (catalase-positive, clusters).

Biochemical Testing

• Tests physiological reactions to nutrients/substrates to reveal enzyme systems.

• Color change indicates presence of specific enzymes.

• Example: API test strips for rapid identification.

Principle: Microbe + substrate → (if enzyme present) product + color change; if enzyme absent, no color change.

Phenotypic Test Example: MGIT System

• Mycobacterial Growth Indicator Tube (MGIT) detects Mycobacterium tuberculosis by monitoring oxygen levels and fluorescence.

• Automated detection of positive cultures.

Antimicrobial Susceptibility Testing

• Determines which drugs are effective against the pathogen.

• Automated systems often include susceptibility testing during identification.

• Important for managing antimicrobial resistance.

Miscellaneous Tests

• Phage typing: Uses bacteriophage specificity to identify bacterial strains.

• Animal testing: Required for some pathogens (e.g., Mycobacterium leprae).

• Embryo/cell culture: Used for rickettsias, chlamydias, and viruses.

Determining Clinical Significance

• Distinguish between true pathogens, contaminants, and normal biota.

• Repeated isolation or high colony counts suggest clinical significance.

Drawbacks of Phenotypic Methods

• Culturing can take 18–24 hours or longer.

• Some pathogens are nonculturable, risking misidentification.

Immunologic (Serological) Methods

Serology

• Definition: In vitro testing of body fluids for specific antibodies or antigens.

• Based on the specificity of antibodies for antigens.

• Used to determine exposure history, diagnose disease, and assess immune status.

Principles of Serological Testing

• Antigen-antibody binding is highly specific.

• Binding can be visualized as clumping (agglutination) or precipitation.

Agglutination and Precipitation

• Agglutination: Antibodies cross-link whole cells (e.g., bacteria, RBCs) to form visible clumps.

• Precipitation: Antibodies cross-link soluble antigens to form insoluble complexes.

Immunochromatography

• Rapid lateral flow tests (e.g., pregnancy, strep tests).

• Sample flows through a cartridge, encountering antibodies and producing a visible color change if positive.

Antibody Titers

• Measure concentration of antibodies by serial dilution.

• Highest dilution that still causes agglutination is the titer.

Serotyping

• Classifies bacteria into serotypes using antisera against cell antigens (capsule, flagellum, cell wall).

• Widely used for Salmonella and Streptococcus differentiation.

Western Blot

• Proteins separated by electrophoresis, transferred to a membrane, and probed with patient antibodies.

• Secondary, labeled antibody detects binding, visualized as bands.

• Used to confirm presence of specific microbial antigens or antibodies.

Immunofluorescence Testing

• Direct: Fluorescent antibody binds directly to antigen in specimen.

• Indirect: Fluorescent antibody binds to patient antibody that has bound to a known antigen.

• Visualized by fluorescence microscopy.

Enzyme-Linked Immunosorbent Assay (ELISA)

• Indirect ELISA: Detects patient antibodies using a known antigen and an enzyme-linked secondary antibody. Color change indicates a positive result.

• Direct ELISA: Detects antigen using a known antibody and an enzyme-linked indicator antibody.

• Commonly used for HIV, hepatitis, and other infections.

Complement Fixation Test

• Detects presence of specific antibodies by their ability to fix complement and prevent lysis of indicator red blood cells.

In Vivo Testing

• Antigen or antibody introduced into patient to elicit a visible reaction (e.g., tuberculin skin test for tuberculosis exposure).

Test Accuracy

• Specificity: Ability to detect only the target antigen/antibody.

• Sensitivity: Ability to detect even small amounts of antigen/antibody.

Genotypic Methods

Polymerase Chain Reaction (PCR)

• Amplifies specific DNA or RNA sequences from pathogens.

• Real-time PCR (qPCR) uses fluorescence to quantify DNA as it is amplified.

• Multiplex PCR detects multiple organisms in one reaction.

• Rapid and sensitive; does not require culturing.

Equation: $N = N_0 \times 2^n$ (where $N$ is the number of DNA copies after $n$ cycles, starting from $N_0$ copies)

Hybridization Techniques

• Use labeled DNA or RNA probes to detect complementary sequences in microbial nucleic acids.

• Fluorescent in situ hybridization (FISH) applies probes to intact cells in specimens, visualized by microscopy.

Whole-Genome Sequencing

• Determines the complete DNA sequence of a pathogen.

• Useful for outbreak analysis, drug resistance detection, and comprehensive diagnosis.

Emerging and Advanced Diagnostic Technologies

Microarrays (Lab-on-a-Chip)

• Contain thousands of gene sequences for simultaneous detection of multiple pathogens.

• Miniaturized, easy-to-use, and suitable for resource-limited settings.

Mass Spectrometry (MALDI-TOF)

• Analyzes protein "fingerprints" from microbial samples for rapid identification.

• Can also assess antibiotic susceptibility.

Imaging Techniques

• X-rays, MRI, CT, and PET scans help localize infections in deep tissues.

• Non-invasive and useful for diagnosis when biopsy is not feasible.

Summary Table: Major Microbial Identification Methods

Category	Description	Example
Phenotypic	Observation of microbe’s morphology, physiology, and biochemical properties	Gram stain, colony morphology, biochemical tests
Immunologic	Analysis using antibodies or antigens	ELISA, agglutination, immunochromatography
Genotypic	Analysis of DNA or RNA	PCR, FISH, whole-genome sequencing

Key Concepts for Exam Preparation

• Understand the strengths and limitations of each identification method.

• Be able to match laboratory techniques to appropriate clinical scenarios.

• Recognize the importance of specimen collection and handling in diagnostic accuracy.

• Be familiar with the principles behind serological and molecular diagnostic tests.