Laboratory Diagnosis of Shigella Infections

Laboratory Diagnosis

◗ Specimens

Stool is the specimen of choice. Diagnosis of shigellosis is made by isolating Shigella spp. from feces. Fresh feces are inoculated without delay or transported in a suitable medium, such as Sachs’ buffered glycerol saline, pH 7.0–7.4. Also, rectal swabs may be taken from the site of ulcer by sigmoidoscopy. However, rectal swabs that do not contain copious volume of stool or mucus are not satisfactory.

◗ Microscopy

Routine microscopy of stool may reveal clumps of polymor-phonuclear leukocytes. Fecal blood or leukocytes are detectable in the stool in approximately 70% of cases of shigellosis.

◗ Culture

A sample for stool is obtained in all suspected cases of shigellosis for culture. Usually, more than one stool or rectal swab is collected and inoculated immediately on at least two different culture media, such as MacConkey, XLD, DCA, or eosin-methylene blue agars. For enrichment, one tube each of selenite F and GN broth are inoculated and incubated at 37°C for 12–18 hours before subculture onto selective media. After overnight incubation, Shigella produces pale nonlactose-fermenting colonies on MacConkey and DCA media and red colonies on XLD medium and colorless colonies on SS agar.

◗ Identification of bacteria

Pale non–lactose-fermenting colonies on MacConkey agar are identified by carrying out motility test, biochemical tests, and slide agglutination test with specific Shigella antisera (polyvalent and monovalent sera).

◗ Serodiagnosis

Serological tests are not useful in the diagnosis of shigellosis.