Bacterial Gastroenteritis Workup

Updated: Jan 07, 2017
  • Author: Jennifer Lynn Bonheur, MD; Chief Editor: BS Anand, MD  more...
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Approach Considerations

A stool pH of 5.5 or below or the presence of reducing substances indicates carbohydrate intolerance. This is usually transient in nature.

Enteroinvasive infections of the large bowel cause leukocytes, predominantly neutrophils, to accumulate in the lumen which are then shed into stool. The absence of fecal leukocytes does not eliminate the possibility of enteroinvasive organisms; however, the presence of fecal leukocytes eliminates consideration of enterotoxigenic E coli, Vibrio species, and viruses. Shigella characteristically causes marked bandemia with variable total white blood cell (WBC) count.

Examine any exudate found in the stool for leukocytes. Such exudates are highly suggestive of inflammatory bowel disease, which could be infectious or of another origin.

Antilisteriolysin O (ALLO) is positive during the convalescent phase of bacterial gastroenteritis and when invasive disease has occurred.

Commercially available multiplex molecular panels may be more sensitive and provide more rapid results (<3 hours) than stool cultures, which are labor intensive and whose results may take longer than 1 day (66.5 hours). [21]


Identification of pseudomembranes in the colon by direct visualization is diagnostic for C difficile; however, the yield may be low.


Bacterial Cultures

Identifying the causative agent underlying the gastroenteritis is important in the management of patients with severe or prolonged diarrhea, symptoms consistent with invasive disease, or a history that may predict a complicated disease course. [22] Moreover, stool culture findings are a means for public health officials to identify and track outbreaks of bacterial gastroenteritis.

Table 3, below, lists common bacteria and the optimal culture media for their growth.

Table 3. Common Bacteria and Optimum Culture Media (Open Table in a new window)

Organism Detection Method Microbiologic Characteristics
Aeromonas species Blood agar Oxidase-positive, flagellated GNB
Bacillus species Blood agar Facultatively aerobic, spore-forming GPR; beta hemolytic; reduces nitrates; ferments carbohydrates
Campylobacter species Skirrow agar Rapidly motile, curved GNR; Campylobacter jejuni 90% of infections, Campylobacter coli 5% of infections
C difficile CCFE agar, EIA for toxin, LA for protein Anaerobic, spore-forming GPR; toxin-mediated diarrhea; produces pseudomembranous colitis
C perfringens None available Anaerobic, spore-forming GPR; toxin-mediated diarrhea
E coli MacConkey, EMB, or SM agar Lactose-producing GNR
Listeria species Blood agar Flagellated GPB
Plesiomonas species Blood agar Oxidase-positive GNR
Salmonella species Blood, MacConkey, EMB, XLD, or HE agar Nonlactose, non–H2S-producing GNR
Shigella species Blood, MacConkey, EMB, XLD, or HE agar Nonlactose and H2S-producing GNR; verotoxin (neurotoxin)
Staphylococcus species Blood agar Heat-stable, preformed toxin-mediated GPC
Vibrio species Blood or TCBS agar Oxidase-positive, motile, curved GNB
Y enterocolitica CIN agar Nonlactose-producing, oval GNR
CCFE = cycloserine-cefoxitin-fructose-egg; CIN = cefsulodin-irgasan-novobiocin; EIA= enzyme immunoassay; EMB = e-methylene blue; GNB = gram-negative bacillus; GNR = gram-negative rod; GPB = gram-positive bacillus; GPC = gram-positive cocci; GPR = gram-positive rod; H2S = hydrogen sulfide; HE = Hektoen enteric; LA = latex agglutination; SM = Sorbitol-MacConkey; TCBS = thiosulfate-citrate-bile-sucrose; XLD = xylose-lysine-deoxycholate.

The following is a list of the different culture media used to isolate bacteria. A high index of suspicion is needed to choose the appropriate medium.

  • Blood agar - All aerobic bacteria and yeast; detects cytochrome oxidase production
  • MacConkey EMB agar - Inhibits gram-positive organisms; permits lactose fermentation
  • XLD agar and HE agar - Inhibit gram-positive organisms and nonpathogenic gram-negative bacilli; permit lactose fermentation and H2S production
  • Skirrow agar - Selective for Campylobacter species
  • SM agar - Selective for enterohemorrhagic E coli
  • CIN agar - Selective for Y enterocolitica
  • Thiosulfate-citrate-bile-sucrose agar - Selective for Vibrio species
  • CCFE agar - Selective for C difficile

Stool cultures are useful when positive, but the yield is usually low. Refrigerate stool that is not cultured at 4°C within 2 hours of collection, or place it in a transport medium. Always culture stool for Campylobacter, Salmonella, and Shigella species, especially if stool leukocytes or gross blood is found in the stool.

Serotype Salmonella for S typhimurium DT104, particularly if the gastroenteritis is associated with raw milk or cheese ingestion. S typhimurium DT104 is a multidrug-resistant organism, and antibiotic sensitivities are crucial to guide therapy. [23, 24]

Preformed toxin from Bacillus or Staphylococcus species may cause rapid-onset gastroenteritis. In such cases, the bacteria may not exist in the gastrointestinal tract; therefore, culture the food ingested by the person.

Bloody diarrhea with a history of ground beef ingestion should raise the suspicion for enterohemorrhagic E coli. If E coli is found in the stool, type it to determine if it is O157:H7. Report cases of E coli O157:E7 gastroenteritis (and other infectious problems) to the state health department.

History of raw seafood ingestion or foreign travel should prompt additional screening for Vibrio and Plesiomonas species.