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Microbiology

Serotyping

Serotyping is a crucial component of diagnostic bacteriology, especially in the context of gastrointestinal (GI) infections. It involves identifying specific surface antigens on bacterial cells to differentiate between various strains or serotypes. This process is essential for:

  • Epidemiological Investigations: Tracking outbreaks and identifying the source of infection
  • Public Health Surveillance: Monitoring the prevalence of specific serotypes
  • Clinical Management: Guiding treatment decisions and assessing the severity of infection

General Principles

  • Antigen-Antibody Reactions: Serotyping is based on the specific interactions between bacterial antigens and antibodies. The bacteria have antigens on their surfaces
  • Antigens Targeted: Primarily focuses on:
    • O Antigens: Somatic antigens, located on the lipopolysaccharide (LPS) of Gram-negative bacteria. Used for identification of the organism
    • H Antigens: Flagellar antigens, which are proteins that make up the flagella
    • K Antigens: Capsular antigens, (present in some species)
  • Serological Testing: Use of specific antisera (containing antibodies) to react with the bacterial antigens. The bacteria will agglutinate if the antigens are present
  • Applications
    • Epidemiologic investigations
    • Public health surveillance
    • Informing clinical management

Serotyping of Escherichia coli

  • Significance: E. coli has multiple serotypes. Some are pathogenic
  • Key Pathogenic Groups
    • Enterotoxigenic E. coli (ETEC): Produces heat-labile (LT) and heat-stable (ST) toxins
    • Shiga Toxin-Producing E. coli (STEC): Produces Shiga toxins (e.g., E. coli O157:H7)
  • Serotyping Procedure
    • Culture and Isolation: Isolate the E. coli from the stool sample
    • Antigen Testing
      • O Antigens: Use specific antisera to identify the O serogroup (e.g., O157)
      • H Antigens: Use specific antisera to identify the H serotype (e.g., H7)
      • K Antigens: If present, use specific antisera
    • Agglutination: If the antiserum contains the corresponding antibodies, the bacteria will agglutinate (clump together)
    • Interpretation: Determine the O:H serotype (e.g., O157:H7)
  • Clinical Significance: Identifying specific serotypes is important for:
    • STEC: E. coli O157:H7 is a serious cause of bloody diarrhea and hemolytic uremic syndrome (HUS)

Serotyping of Salmonella spp.

  • Significance: Many serotypes. The serotyping is important
  • Procedure
    • Culture and Isolation: Isolate Salmonella from the stool sample
    • Antigen Testing
      • O Antigens: Use specific antisera to identify the O serogroup (e.g., O1, O4, O9)
      • H Antigens: Use specific antisera to identify the phase 1 and phase 2 H antigens
    • Agglutination: If the antiserum contains the corresponding antibodies, the bacteria will agglutinate (clump together)
    • Interpretation: The serotyping will be based on the O and H antigens
  • Clinical Significance: Serotyping is essential for:
    • Identifying the source of the infection
    • Tracking outbreaks
    • Public health surveillance
    • Example Serotypes: S. enterica serotype Typhi (typhoid fever), S. enterica serotype Typhimurium

Serotyping of Shigella spp.

  • Significance: The serotyping helps identify the species
  • Procedure
    • Culture and Isolation: Isolate Shigella from the stool sample
    • Antigen Testing: Use specific antisera to identify the O antigens
    • Agglutination: If the antiserum contains the corresponding antibodies, the bacteria will agglutinate (clump together)
    • Interpretation: Based on serotype
  • Species and Serotypes
    • Shigella dysenteriae (serogroup A)
    • Shigella flexneri (serogroup B)
    • Shigella boydii (serogroup C)
    • Shigella sonnei (serogroup D)
  • Clinical Significance: Serotyping is essential for:
    • Identifying the species of Shigella
    • Tracking outbreaks
    • Public health surveillance

Additional Considerations

  • Commercial Kits: Serotyping is often performed using commercially available kits, which contain specific antisera and standardized protocols
  • Reference Laboratories: Complex serotyping may be performed by reference laboratories
  • Rapid Testing: Sometimes, rapid tests are used

Key Terms

  • Serotyping: Identifying bacterial strains based on their surface antigens
  • Antigen: A substance (e.g., protein, polysaccharide) that triggers an immune response
  • Antibody: A protein produced by the immune system that recognizes and binds to a specific antigen
  • O Antigen: Somatic antigen, located on the lipopolysaccharide (LPS) of Gram-negative bacteria
  • H Antigen: Flagellar antigen, which is a protein that makes up the flagella
  • K Antigen: Capsular antigen
  • Antisera: Serum that contains antibodies
  • Agglutination: Clumping together of cells or particles
  • Escherichia coli: Gram-negative bacteria
  • Enterotoxigenic E. coli (ETEC): Produces heat-labile (LT) and heat-stable (ST) toxins
  • Shiga Toxin-Producing E. coli (STEC): Produces Shiga toxins (e.g., E. coli O157:H7)
  • Hemolytic Uremic Syndrome (HUS): A serious complication of STEC infections
  • TSI (Triple Sugar Iron Agar): A biochemical test
  • Urease Test: Biochemical test
  • String Test: Test for the presence of Vibrio cholerae
  • SMAC: Sorbitol-MacConkey agar
  • Shiga toxins: Toxins produced by E. coli
  • Heat-labile: Destroyed by heat
  • Microaerophilic: Requires reduced oxygen