Stool Culture

Normal bacterial flora in stool include several potentially pathogenic organisms. Bacteriologic examination is valuable for identifying pathogens that cause overt GI disease - such as typhoid and dysentery - and carrier states. A sensitivity test may follow isolation of the pathogen. Stool culture may also be used to detect certain viruses, such as enterovirus, which can cause aseptic meningitis.

Purpose

• To identify pathogenic organisms, especially in a debilitated patient
• To aid treatment of disease, prevent possibly fatal complications, and confine severely infectious diseases

Patient preparation

• Explain to the patient that this test is used to determine the cause of GI distress or to determine if he's a carrier of infectious organisms.
• Advise him that food and fluids needn't be restricted.
• Tell the patient that the test requires the collection of a stool specimen on 3 consecutive days.
• Check the patient's history for dietary patterns, recent antimicrobial therapy, and recent travel that might suggest endemic infections or infestations.

Equipment

Gloves, waterproof container with tight-fitting lid or sterile swab and commercial sterile collection and transport system, tongue blade, bedpan (if needed)

Procedure and posttest care

• Collect a stool specimen directly into the container. If the patient isn't ambulatory, collect the specimen in a clean, dry bedpan and, using a tongue blade, transfer the specimen to the container.
• If you must collect the specimen by rectal swab, insert the swab past the and sphincter, rotate it gently, and withdraw it. Then, place the swab in the appropriate container.
• Check with the laboratory for the proper collection procedure before obtaining a specimen for a virus test.
• Label the specimen with the patient's name, doctor's name, facility number, and date and time of collection.
• Indicate the suspected cause of enteritis and current antimicrobial therapy on the laboratory slip.

Precautions

• Wear gloves when performing the procedure and handling the specimen.
• If the patient uses a bedpan or a diaper, avoid contaminating the stool specimen with urine.
• The specimen must represent the first, middle, and last portion of the stool passed. Be sure to include mucoid and bloody portions.
• Put the specimen container in a leakproof bag, and send it to the laboratory immediately. Trophozoites and cysts may be destroyed if exposed to heat, cold, or a delay in delivery to the laboratory.

Normal Findings

A large percentage of normal fecal flora consists of anaerobes, including non-spore-forming bacilli, clostridia, and anaerobic streptococci. The remaining percentage consists of aerobes, including gram-negative bacilli (predominantly Escherichia coli and other Enterobacteriaceae, plus small amounts of Pseudomonas), gram-positive cocci (mostly enterococci), and a few yeasts.

Abnormal findings

The most common pathogenic organisms of the GI tract are Shigella, Salmonella, and Campylobacter jejuni. Less common pathogenic organisms include Vibrio cholerae, V. parahaemolyticus, Clostridium botulinum, C. difficile, C. peifringens, Staphylococcus aureus, enterotoxigenic E. coli, and Yersinia enterocolitica. Isolation of some pathogens indicates bacterial infection in patients with acute diarrhea and may require antimicrobial sensitivity tests. Normal fecal flora may include C. difficile, E. coli, and other organisms. Therefore, isolation of these organisms may require further tests to demonstrate invasiveness or toxin production.

Isolation of pathogens such as C. botulinum indicates food poisoning; the pathogens must also be isolated from the contaminated food. In a patient undergoing long-term antimicrobial therapy, isolation of large numbers of S. aureus or yeast may indicate infection. (Asymptomatic carrier states are also indicated by these enteric pathogens.) Isolation of enteroviruses may indicate aseptic meningitis.

If a stool culture shows no unusual growth, detection of viruses by immunoassay or electron microscopy may be used to diagnose nonbacterial gastroenteritis. Highly increased polymorphonuclear leukocytes in fecal material may indicate an invasive pathogen.

Interfering factors

• Failure to use proper collection technique
• Contamination of the specimen by urine (possible injury to or destruction of enteric pathogens)
• Antimicrobial therapy (possible decrease in bacterial growth)
• Failure to transport the specimen promptly or, if delivery is delayed, to use a transport medium, such as buffered glycerol, that stabilizes pH (possible loss of enteric pathogens or over­growth of nonpathogenic organisms)
• Bismuth-containing toilet tissue placed in the collection container
• Barium and mineral or castor oil when testing for protozoa