Flow chart for identification of enteric fecal pathogens

▪Major pathogens: Salmonella, Shigella, Vibrio, Campylobacter, Enteropathogenic E.coli

▪Other pathogens: Aeromonas

@ Ellen Jo Baron 2007

Hektoen enteric agar: Hektoen agar is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. Hektoen contains indicators of lactose fermentation and H2S production; as well as inhibitors to prevent the growth of gram positive bacteria.
Salm on HEK agar

Oxidase: The oxidase test is used to determine if a bacterium produces certain cytochrome c oxidases. The reagent, tetramethyl-p-phenylenediamine is used as a redox indicator. The reagent turns dark blue when oxidized (oxidase positive). The reagent is colorless when reduced (oxidase-negative). Pseudomonas species and Aeromonas species are gram-negative bacilli that are oxidase-positive. Neisseria species are gram-negative cocci that are oxidase positive.

Oxidase +, moist,

beige colonies

 

Campylobacter

Campy agar: Campylobacter CVA Agar is a selective medium used in the primary isolation and cultivation of Campylobacter jejuni from human fecal specimens. This selective medium contains cefoperazone, vancomycin and amphotericin B; this combination of antimicrobial agents inhibits the normal fecal flora for easier detection of C. jejuni.
Campy agar after 48 h

Oxidase: The oxidase test is used to determine if a bacterium produces certain cytochrome c oxidases. The reagent, tetramethyl-p-phenylenediamine is used as a redox indicator. The reagent turns dark blue when oxidized (oxidase positive). The reagent is colorless when reduced (oxidase-negative). Pseudomonas species and Aeromonas species are gram-negative bacilli that are oxidase-positive. Neisseria species are gram-negative cocci that are oxidase positive.
Hektoen enteric agar: Hektoen agar is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. Hektoen contains indicators of lactose fermentation and H2S production; as well as inhibitors to prevent the growth of gram positive bacteria.

None

Yellow colonies on HEK

Oxidase +

Possible Aeromonas

MacConkey: MacConkey agar is a selective medium that inhibits the growth of Gram-positive bacteria due to the presence of crystal violet and bile salts. Most Gram-negative bacteria grow well on MacConkey. MacConkey agar also contains neutral red (a pH indicator) and lactose (a disaccharide). Lactose fermenting bacteria or Lactose + bacteria on MacConkey will appear as bright pink colonies. Non-lactose fermenting bacteria will be colorless (or, if they have any color, will be their natural color rather than pink).
   Lac neg on Mac
  Lac + on Mac

Negative
Negative
     Positive

 Possible

 pathogenic E. coli

 Need additional tests

   TSI = S. typhi, Shigella,& Salm

Possible

Salmonella, Shigella or

Yersinia enterocolitica or

pathogenic E. coli

Need additional tests

BAP or SBA: An abbreviation for blood agar plate or sheep blood agar. Blood agar contains mammalian blood (usually sheep, rabbit or horse), typically at a concentration of 5-10%. Blood agar is an enriched media used to isolate bacteria and to detect hemolytic activity.

Swarming on BAP

Proteus

   Perform serotyping

Green

  Possible Vibrio parahemolyticus

Yellow

  Possible Vibrio cholerae

  Possible V. alginolyticus

TCBS: TCBS (thiosulfate citrate bile salts sucrose) enriched agar enhances growth of Vibrio cholerae.
                 Growth on TCBS
Oxidase: The oxidase test is used to determine if a bacterium produces certain cytochrome c oxidases. The reagent, tetramethyl-p-phenylenediamine is used as a redox indicator. The reagent turns dark blue when oxidized (oxidase positive). The reagent is colorless when reduced (oxidase-negative). Pseudomonas species and Aeromonas species are gram-negative bacilli that are oxidase-positive. Neisseria species are gram-negative cocci that are oxidase positive.
Oxidase
Oxidase