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Fecal Culture |
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Fecal Culture |
| Indications for performing this test: A fecal culture may be indicated in suspected cases of infectious diarrhea. Feces may be cultured for the identification of bacterial organisms such as Salmonella species, Campylobacter jejuni, Clostridium perfringens, and certain pathogenic strains of E. coli, as well as for fungal organisms such as Histoplasma capsulatum. |
Goat kids with diarrhea are often cultured for M. avium sbsp. paratuberculosis (causes Johne's disease). |
Disadvantages:
Interpretation of results: Results must be interpreted in concert with the signalment, clinical signs observed, numbers of potentially pathogenic bacteria isolated in relation to numbers of normal flora, and (when they can be observed) the presence of appropriate lesions. Simple isolation of the above bacteria is not necessarily diagnostic, as these organisms can be cultured from the feces of healthy animals as well. (C. jejuni is isolated from up to 50% of healthy dogs, Salmonella from 30% of healthy dogs, and certain strains of C. perfringens and E. coli are normal flora of dogs and cats). Assays for exotoxin production or genes encoding exotoxin (e.g. PCR) may be necessary to determine whether an isolated Clostridium perfringens or E. coli strain is pathogenic.
Turnaround time: Bacterial cultures may take a few days for results, and fungal cultures take considerably longer. A fecal fungal culture for Histoplasma usually takes 7-10 days to grow. Because of the time required for results, you'll often have to go ahead and make decisions regarding treatment or additional diagnostic procedures based on your presumptive or "most likely" diagnoses. In other words, you often don't have the luxury to wait.
| The Test: One to 3 grams of fresh feces is necessary. Certain organisms may require an appropriate transport media. Once in the lab, the specimen is inoculated onto enrichment media, which will vary with the organism being isolated/suspected. For example, numbers of Salmonella organisms may be enhanced by growth in selenite broth, Campylobacter is grown on blood agar plates that inhibit the growth of other fecal flora, and Histoplasma is grown on Sabouraud's dextrose agar at room temperature. Plates are examined at certain time intervals for growth. After growth and isolation, bacterial organisms may be identified by Gram's staining and biochemical reactions, such as their ability to ferment certain sugars and reaction with substances such as urea and indole. Isolates might also be characterized for the presence of required virulence genes using PCR (the polymerase chain reaction). |
* Selenite enrichment
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* Image from the University of Wisconsin School of Veterinary Medicine: "Selenite media is an enrichment media that is used for primary isolation of enteric pathogens such as Salmonella and Shigella species. It is a liquid media containing chemical constituents, which inhibit normal enteric flora and allow pathogens which may be present in very small numbers in the specimen to grow unhampered and thus enriching them. Shown here is uninoculated selenite media on the left, inoculated (but unincubated) media in the middle, and inoculated media that has been incubated for 24 hours to enrich for enteric pathogens. Isolated colonies of these organisms may be obtained by sub-culturing onto solid media"
NOTE: Calf diarrhea: As you might imagine, the intestinal tract and feces in particular are filled with bacteria, most of which are normal flora. The general operating procedure in our diagnostic lab is to look only for known enteric pathogens, notably pathogens that make the most sense based on signalment, clinical signs, epidemiology and/or pathologic findings (i.e. lesions found grossly or microscopically).
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For more information . . .
Textbook of Veterinary Internal Medicine, eds. Ettinger and Feldman, pp1217-1218, 1239, 1241, 1243-1244.
Infectious Diseases of the Dog and Cat, ed. Craig E. Greene, pp182,
227,238.
