Isolation of Shigellae

Author:

Taylor Welton I.1,Schelhart Dorothy1

Affiliation:

1. Lafayette Medical Laboratory, Lafayette, Louisiana 70501

Abstract

Two enrichment broths and four plating media were compared for efficiency of detection of enteric pathogens from 1,597 stool specimens. Of 170 salmonellae isolated from the composite of all methods, direct streaking yielded but 54%, whereas enrichment in gram-negative broth found 87% and Selenite-F broth 97%. By contrast, gram-negative broth produced 100% of the 17 shigellae, Selenite-F broth but 77%, and direct streaking only 59%. Thus, enrichment methods produced almost twice the number of both pathogens as direct streaking. Comparison of the plating media revealed xylose lysine deoxycholate agar (XLD) and Hektoen enteric agar to be equal in their abilities to find both pathogens. Both were moderately better than Salmonella-Shigella agar and markedly superior to eosin methylene blue agar. XLD fround 83% of salmonellae produced by the composite of four media and 90% of the shigellae. Hektoen enteric agar found 80% of both. Salmonella-Shigella agar detected 74 and 68%, respectively, and eosin methylene blue agar only 42 and 63%. The numbers of false positives accruing to each medium, however, showed Hektoen enteric and Salmonella-Shigella agars to produce more than twice as many false-positive plates as XLD. Similarly, Selenite-F broth resulted in many more false-positives for all plating media than did gram-negative broth. Consequently, the index of validity, which equates successful isolation of pathogens with total pickings, favored XLD and gram-negative broth as the media of choice, with direct streaking the poorest method by all counts.

Publisher

American Society for Microbiology

Subject

General Pharmacology, Toxicology and Pharmaceutics,General Immunology and Microbiology,General Biochemistry, Genetics and Molecular Biology,General Medicine

Reference14 articles.

1. Food and Drug Administration. 1969. BAM 11. Isolation of Shigella. Bacteriological analytical manual 2nd ed. U.S. Government Printing Ofice Washington D.C.

2. Inhibition of Shigella flexneri by the normal intestinal flora. I. Mechanisms of inhibition by Klebsiella;Hentges D. J.;J. Bacteriol.,1967

3. Inhibition of Shigella flexneri by the normal intestinal flora. II. Mechanisms of inhibition by coliform organisms;Hentges D. J.;J. Bacteriol.,1969

4. Isolation of salmonellae and shigellae from an artificial mixture of fecal bacteria;Isenberg H. D.;Appl. Microbiol.,1969

5. A new plating medium for the isolation of enteric pathogens. I. Hektoen enteric agar;King S.;Appl. Microbiol.,1968

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