Improved enrichment for recovery of Shigella sonnei from foods

Shigella species were recovered from foods by the procedure described in the Bacteriological Analytical Manual, 5th Ed. The method is effective if Shigella species are present at about 10(6) cells/g. A 25 g food portion was incubated in Gram-negative (GN) and selenite cystine broths for 16 h at 35 degrees C and streaked onto MacConkey, Levine's eosin methylene blue, desoxycholate citrate, and xylose lysine desoxycholate agars. S. sonnei cells were recovered quantitatively at 44.5 degrees C, and along with other Shigella species, were grown with Escherichia coli in a tryptone broth under anaerobic conditions. Shigella species were also grown in a mixed microflora from foods. S. sonnei cells were inoculated into an enrichment broth containing 20 g tryptone, 2 g K2HPO4, 2 g KH2PO4, 1 g glucose, 5 g NaCl, 1.5 mL Tween 80, and 0.5 mg novobiocin/L (pH 7.0) and incubated for 20 h at 44 degrees C. Enrichments were streaked onto MacConkey agar and the plates were incubated 20 h at 35 degrees C. Suspect Shigella colonies were screened in glucose, tryptone, and lysine broths and in triple sugar iron and motility agars. The sensitivity varied from 0.3 to 1000 bacteria/g. The method has been examined with artificially inoculated lettuce, celery, brussels sprouts, mushrooms, and hamburger. It is also applicable to S. flexneri if incubation is conducted at 42 degrees C.