Evaluation of culture media for enrichment and isolation of Shigella sonnei and S-flexneri

The performance of Gram-negative (GN) broth with (10 mug/ml) and without no vobiocin, Shigella broth (SB) with 0.5 and 3.0 mug/ml novobiocin, all incub ated at 37 degreesC (SB with 3.0 mug/ml novobiocin also at 42 degreesC) and buffered brilliant 0'reen bile glucose (EE) broth with 1.0 tg/ml novobioci n incubated at 37 and 42 T were evaluated for resuscitation and growth of S higella sonnei and S. flexneri (eight strains; unstressed, chill-stressed a nd acid-stressed) and non-shigellae (11 strains). GN broth with or without novobiocin supported significantly less growth of Shigella sp. No significa nt differences in growth of shigellae were obtained between the other cultu re media. Performance depended more on the Shigella strain used. None of th e tested media were significantly superior for suppressing the competitive flora. Electivity and selectivity of MacConkey agar (MAC), tergitol-7 agar (T7), d esoxycholate,citrate agar (DCA), xylose lysine desoxycholate agar (XLD), Sa lmonella Shigella agar and Hektoen enteric agar (HEA) were determined by ec ometric testing. HEA confirmed to be a high selective medium for both non-s higellae and stressed Shigella sp. Klebsiella sp., Enterobacter sp., Citrob acter sp., Salmonella sp. and the Escherichia strains can mask the presence of shigellae. In vitro competition experiments and experiments with artificially contamin ated foods showed higher resistance of S. sonnei than S. flexneri towards t he stress imposed by the food matrix and its indigenous flora. Reliable det ection, however, of shigellae in foods with the current enrichment and isol ation media was not achieved. (C) 2001 Elsevier Science B.V. All rights res erved.