Stool Culture

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Stool Culture

Definition

A stool culture is a laboratory test used to isolate and identify pathogens in the feces of patients suspected of having digestive tract infections. A sample of the patient's feces is placed on several different types of nutrient media and observed for growth. Any suspicious organisms that grow on the media are identified using microscopic and biochemical tests.

Purpose

Physicians normally order stool cultures on patients with symptoms of gastrointestinal infection, most commonly diarrhea. The purpose of this test is to isolate bacteria or other organisms that might be causing the symptoms so they can be identified. Identification of the causative organism is essential in determining how to treat the patient. For example, administering an antibiotic merely on the basis of the patient's symptoms could, in some cases, make the condition worse.

Precautions

A stool culture is performed only if an infection of the digestive tract is suspected. The test has no harmful effects.

Description

A routine stool culture (also called a fecal culture) is for the isolation of Campylobacter, enterotoxigenic E. coli (O157:H7), Shigella, and Salmonella. Less frequently isolated bacterial causes of diarrhea are Vibrio spp., Yersinia enterocolitica, and Aeromonas spp. Requests for stool cultures for the isolation of other intestinal pathogens should include special instructions. The most common example is Clostridium difficile, which causes pseudomembraneous colitis.

Stool cultures may be performed on rectal swabs containing feces or submitted stool samples. Swabs are placed in a tube containing Stuart or other transport medium and then delivered to the laboratory. Cultures for C. difficile are usually collected by swabbing the rectum (whereas watery stool is needed for immunoassay of C. difficile toxin). The swab must be placed immediately into prereduced (oxygen free) transport medium because this organism is a strict anaerobe. To submit a stool specimen for routine culture, the patient or caregiver collects a stool sample in a special container, taking care not to contaminate the specimen with water, urine, or other materials. Some containers include a transport solution to stabilize the specimen. Although some requests are for stool cultures on two or more consecutive days, a single specimen is considered to be sufficient. It is important to return the specimen to the doctor's office or the laboratory in the time specified by the physician or nurse. Laboratories normally do not accept stool specimens that are contaminated or that arrive after the specified time period.

A routine bacterial stool culture involves placing a sample of the stool on several kinds of enriched and selective media containing nutrients that support the growth of certain types of organisms. Routine culture should include a sheep blood agar plate, MacConkey agar plate, MacConkey agar with sorbitol, Hektoen or XLD (xylose lysine desoxycholate) plate, Campy plate, and GN (gram-negative) broth. Blood agar supports the growth of most bacteria including Staphylococcus aureus, Listeria monocytogenes, and yeast, which are infrequently implicated in food poisoning or gastrointestinal infections, but do not grow on the other media. Most intestinal pathogens are gram-negative bacilli. MacConkey agar is selective for these organisms and differentiates those that can ferment lactose from those that cannot. MacConkey sorbitol substitutes sorbitol for lactose. This allows differentiation of nonpathogenic E. coli that ferment sorbitol well from the O157:H7 strain, which does not. Hektoen or XLD enhance the growth of Salmonella and Shigella by suppressing the growth of gram-positive organisms and gram-negative normal flora. They also differentiate lactose and sucrose fermenters such as E. coli from Salmonella and Shigella, which are not. Several drops of the GN broth can be transferred to Hektoen or XLD agar after a four-hour incubation at 96.8 °F (36 °C). This procedure can yield isolated colonies of a pathogen the next day that can be used to perform biochemical identification, serotyping, and antibiotic susceptibility tests. Campy agar contains10% sheep blood, sodium bisulfite, and three antibiotics. The sodium bisulfite reduces some of the oxygen in the medium which enhances recovery of Campylobacter. The antibiotics prevent other gram-negative bacilli and yeast from growing. All inoculated media except the Campy plate are incubated in air or 5-10% carbon dioxide at 96.8 °F (36 °C) and are examined for growth at 24 hours and again the next day. Campy plates must be incubated at 107 °F (42 °C). Plates are examined at 24 hours and each day for the next two days. Cultures for Clostridium difficile require CCFA agar and thioglycolate broth. These are incubated in an oxygen free environment at 96.8 °F (36 °C) for two days. CCFA is cycloserine-cefoxitin fructose agar and it inhibits the growth of other enteric anaerobes found as normal flora in stool. C. difficile produces large yellow colonies on CCFA agar that will fluoresce yellow-green.

Gram stains are not performed routinely, but may be requested for the semiquantitation of white blood cells. If any suspicious bacterial colonies grow, they are presumptively identified on the basis of colonial growth, physical characteristics, microscopic features, and biochemical tests. The colonies are subcultured (transferred) to an appropriate medium to obtain a pure culture. This is used to make a suspension of the organism that is inoculated onto biochemical media. Commercially prepared systems for rapid identification are used. These contain multiple pads or wells of media used to test for key defining biochemical characteristics. After overnight incubation, reactions are read by an automated computerized instrument that aids in species identification. Pure cultures are also used to perform antibiotic sensitivity testing. This is typically done by the microtube broth dilution method. This test determines the minimum inhibitory concentration (MIC) of each antibiotic required to prevent growth of the organism. Results are used to determine those antibiotics to which the organism is susceptible.

The length of time needed to perform a stool culture depends on the laboratory instrumentation and the culture methods used. A routine stool culture usually takes 72 hours or longer to complete.

Preparation

Before ordering a stool culture, the physician, or other health care professional, will ask the patient for a complete medical history and perform a physical examination to determine possible causes of the gastrointestinal problem. Information about the patient's diet, any medications taken, and recent travel may provide clues to the identity of possible infectious organisms.

A stool culture normally does not require any special preparation. Patients do not need to change their diets before collecting a specimen. Intake of some substances can contaminate the stool specimen and should not be taken the day before collection. These substances include castor oil, bismuth, and laxative preparations containing psyllium hydrophilic mucilloid.

Aftercare

No aftercare is necessary following a stool culture.

Complications

No complications are associated with this test.

Results

Some bacteria that are normal inhabitants of the digestive tract are known as the enteric bacteria. Escherichia coli, Klebsiella, Enterobacter, and Pseudomonas are members of this group. The enteric bacteria usually do not cause infection in the digestive tract, and are reported as normal flora in a stool culture. Because the presence of normal flora helps to protect against pathogens, the absence of normal flora in a stool culture is also reported. When only normal flora are found the results are reported as "no enteric pathogens found." When normal flora are absent from the stool, a heavy growth of an organism not usually pathogenic may be recovered. Such organisms should be reported in this case.

The following bacteria are not normal inhabitants of the digestive tract, and are known to cause gastrointestinal infection:

Campylobacter
Shigella
Salmonella
Yersinia
enterotoxigenic E. coli
Vibrio
Aeromonas

Although non-toxigenic strains of E. coli are normal flora of the intestines, E. coli O157:H7 is an intestinal pathogen. It produces a toxin (poison or harmful chemical) that causes severe inflammation and bleeding of the colon. Infection with this enterotoxigenic strain of E. coli is usually associated with eating contaminated meat, juice, or fruits.

Clostridium difficile, like enterotoxigenic E. coli, can produce a toxin that causes severe diarrhea. However, this bacterium does not become harmful unless the normal intestinal bacteria are suppressed. Patients taking certain antibiotics may be susceptible to infection with Clostridium difficile. In some cases, the stool culture is used to detect the toxin produced by this bacterium. Other bacteria that produce toxins are Staphylococcus aureus and Bacillus cereus.

If bacteria are not the cause of an intestinal infection, a fungal or viral culture might be necessary. Patients with AIDS, or other immune system diseases, sometimes have gastrointestinal infections caused by fungal organisms such as Candida, or by viral organisms including Cytomegalovirus (CMV). Candida can also become an opportunistic intestinal pathogen when antibiotics or radiation have destroyed the normal stool flora.

Several intestinal parasites, such as Giardia lamblia, also cause gastrointestinal infection and diarrhea. Parasites are not cultured, but are identified microscopically with a stool ova and parasites test.

Health care team roles

The physician orders the stool culture, evaluates the results, and determines the most appropriate treatment. Most laboratories provide instructions for the collection of a stool specimen. The physician, nurse, or other health care professional should instruct or assist the patient in collecting the specimen correctly and without contamination. Once the specimen is ready, the patient or health care professional should ensure that the specimen arrives at the laboratory within the time specified. The clinical laboratory scientist, CLS(NCA)/medical technologist, MT(ASCP) performs the stool culture and sensitivity tests.

KEY TERMS

Bismuth— A substance used in medicines to treat diarrhea, nausea, and indigestion.

Enteric— Pertaining to the intestine.

Enterotoxigenic— Refers to an organism that produces toxins in the gastrointestinal tract that cause such things as vomiting, diarrhea, and other symptoms of food poisoning.

Feces— Material excreted by the intestines.

Gastrointestinal— Referring to the digestive tract; the stomach and the intestines.

Normal flora— Refers to normal bacteria found in a healthy person.

Pathogen— An organism that causes disease.

Psyllium hydrophilic mucilloid— A plant material contained in some laxatives.

Toxin— A poison; usually refers to protein poisons produced by bacteria, animals, and some plants.

Resources

BOOKS

Beers, Mark H., and Robert Berkow, eds. "Gastrointestinal Disorders." In The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck & Co, Inc. 2005.

Tierney, Lawrence M., Stephen J. McPhee, and Maxine A. Papadakis. Current Medical Diagnosis and Treatment 2001 (Lange Series). New York: McGraw-Hill Professional Book Group, 2000.

Zaret, Barry L., Peter I. Jatlow, and Lee D. Katz, eds. The Yale University School of Medicine Patient's Guide to Medical Tests. Boston: Houghton Mifflin. 1997.