Stool Culturing

Advice for Patients Instructions must be given to the patients that are simple, practical and explicit.

If the collection is to be done at the home, advise him/her to:

  1. Defecate into a clean, dry container (an ice-cream bucket is perfect)
  2. Select the portions of stool that are purulent (pussy) or bloody.
  3. Place approximately 2 – 5 grams / mls into the provided sample bottle with the provided sample spoon.
  4. Transport promptly to lab. If delay is unavoidable the sample should be refrigerated until transported. Generally a transport medium will be used for home collection and thorough mixing should be advised.

Two samples per diarrhea episode are sufficient to rule out bacterial diarrhea.

See Lab – Stool Culture for details of laboratory processing.

Principles Stool, obviously, has a great number of microorganisms in it at all times and therein lies the problem. Looking for gastrointestinal pathogens e.g. Salmonella, Shigella, Campylobacter etc. is like looking for a needle in a haystack.

The art of finding these organisms revolves around selective media and growth conditions. Many types of media have been devised that select for and differentiate these GI pathogens from normal stool flora. This exercise is very labor intensive, often involving many days of work.

The yield of important information is limited in the setting of sporadic diarrhea but plays an extremely important role in the investigation of outbreaks of gastrointestinal disease.

Stool is another type of sample that deteriorates rapidly after collection. Of course, we are not concerned with pathogens multiplying within our sample before processing as we are in the case of urine. The problem is that the non-pathogens grow more quickly, change the pH of the specimen and kill the pathogens. If transport is to be delayed, transport medium is available that has high buffering capacity and significantly improves recovery of stool pathogens.