What is the role of a stool culture in the diagnosis of Shigella infection?

Updated: Apr 03, 2018
  • Author: Jaya Sureshbabu, MBBS, MRCPCH(UK), MRCPI(Paeds), MRCPS(Glasg), DCH(Glasg); Chief Editor: Russell W Steele, MD  more...
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Answer

Stool culture

  • A sample for stool culture should be obtained in all suspected cases of shigellosis.

  • The yield from stool cultures is greatest early in the course of disease. Guidelines for obtaining specimens to improve the yield are as follows:

    • Process specimens immediately after collection.

    • If processing is delayed, use a transport medium (eg, buffered glycerol saline).

    • Collect more than one stool or rectal (not anal) swab and inoculate them promptly on at least 2 different culture media.

    • Specimens should be plated lightly onto MacConkey, xylose-lysine-deoxycholate, Hektoen enteric, or Salmonella-Shigella, or eosin-methylene blue agars.

  • If processing is delayed, a rectal-swab sample can be placed in Cary-Blair transport medium or buffered glycerol saline.

  • After overnight incubation, colorless, nonlactose-fermenting colonies may be tested by means of latex agglutination to establish a preliminary identification of Shigella infection.

  • Antimicrobial susceptibility tests of all confirmed isolates should be performed by using the agar diffusion technique. The agar and broth-dilution methods are also widely used. The new Epsilometer strip method (E test) is used to accurately determine the minimum inhibitory concentration (MIC).

  • Despite meticulous care in obtaining and processing specimens from patients infected with Shigella species, approximately 20% may fail to yield Shigella organisms.


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