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Answering your questions: inoculating inhibitory stool-culture media.

Q We are a pediatric hospital and our laboratory is incorporating BAP with ampicillin and CIN agar into our stool-culture regimen. We currently use blood agar, SMAC, HE, XLD, MacConkey X 2, CVA, and GN broth to recover pathogens from stool specimens. My question is: If three swabs are received, how (in what order and grouping) would you recommend that the plates be inoculated, since several of the media contain antibiotics? Before we included CIN and blood with ampicillin in our set-up, the three swabs were used in the following manner:

* Swab #1: Gram stain, and SMAC;

* Swab #2: XLD, HE and GN broth;

* Swab #3: BAP, MacC X 2, and CVA (Campy agar).

A Standard stool bacterial cultures normally include media that will recover Salmonella, Shigella, Campylobacter, and--depending on the institution and prevalence in our geographic area--Escherchia coli 0157:H7 and/or Yersinia species, which may or may not be looked for routinely. (1) This can be done with the following plate media:

* one HE or XLD plate (for Salmonella and Shigella; both are not necessary);

* one MacConkey plate (for Salmonella and Shigella);

* one Sorbitol-MacConkey plate (for E coli 0157:H7); and

* either a second MacConkey plate incubated at room temperature, or CIN media for Yersinia isolation.

The addition of a BAP is not considered necessary for part of a standard stool bacterial culture. As many pediatric stool cultures are collected on swabs, GN or similar enrichment broth cultures are often added in this population. Many laboratories, however, are electing no longer to include enrichment broth cultures, especially if whole feces is submitted; this is a protocol that can be determined after looking at your own recovery data from broth cultures. In addition, your clinicians should be aware of what pathogens you look for routinely in your stool cultures.


Specially requested diagnostic cultures for additional organisms that might be looked for in cases of gastroenteritis include Vibrio species and Aeromonas/Plesiomonas (although these last two organisms have conflicting reports regarding their pathogenicity). Cultures for Staphylococcus aureus, Clostridium difficile, Clostridium perfringens, and Bacillus cereus are not routinely performed. Other testing and/or sources are more appropriate to use when looking for these organisms when they are suspected of causing gastroenteritis or food poisoning. (1) There would seem to be no reason to include a second MacConkey plate if you are using CIN for Yersinia, nor reason to include a CVA or BAP+ampicillin plate in routine stool cultures in pediatrics.

Conventional wisdom tells us to inoculate a specimen on the most enriched, non-selective media first and then proceed towards the most selective media. When considering stool cultures, however--as all media are selective--it would seem to make little difference which you inoculate first or last. If a BAP media is used, it should be inoculated first followed by the rest. But, again, the use of a BAP is not normally considered part of a standard stool bacterial culture. If more than one swab is collected (and more than two would not be necessary), one can process the specimen using both swabs at the same time on all media.

--Susan E. Sharp, PhD, (DABMM)

Director of Microbiology

Pathology Regional Laboratory

Kaiser Permanente;

Associate Professor

Oregon Health and Science University

Portland, OR


(1.) Pickering, LK, Cleary TG. Approach to patients with gastrointestinal tract infections and food poisoning. In: Feigin RD, Cherry JD, Demmler GJ, Kaplin SL, eds. Pediatric Infectious Diseases. 5th ed. Philadelphia, PA: Saunders:2004.

Edited by Daniel M. Baer, MD

Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.
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Title Annotation:Tips from the clinical experts
Author:Baer, Daniel M.
Publication:Medical Laboratory Observer
Geographic Code:1USA
Date:May 1, 2006
Previous Article:Answering your questions: Rule of Three.
Next Article:Answering your questions: confirming rapid strep tests.

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