DEVELOPMENT OF ANTIBIOTIC-RESISTANT STRAINS FOR THE ENUMERATION OF FOODBORNE PATHOGENIC BACTERIA IN STORED FOODS

Strains of Aeromonas spp., Salmonella enteritidis phage type 4, Salmon ella typhimurium, verotoxigenic Escherichia coli 0157:H7 (VTEC) and Ye rsinia enterocolitica resistant to streptomycin, nalidixic acid and a combination of both antibiotics were selected. When compared with the parent strains, most of the antibiotic-resistant strains had slightly slower growth rates at their optimum incubation temperature but the di fference was reduced progressively when the temperature was lowered. S ome antibiotic-resistant strains had considerably slower growth rates in the presence of the relevant antibiotic and these were not used fur ther. Several agar and impedance media with added streptomycin and nal idixic acid were assessed for the enumeration of the antibiotic-resist ant strains in artificially contaminated stored foods. Differential/se lective media were required to enumerate low numbers of antibiotic-res istant strains in certain foods. The following agar and impedance medi a were selected: Aeromonas Agar (Ryan) for Aeromonas spp., Xylose Lysi ne Agar and Lysine Iron Cystine Neutral Red Medium for Salmonella, Eos in Methylene Blue Agar and Coliform Medium for VTEC, and Yersinia Sele ctive Agar without selective agents for Yersinia enterocolitica. The a gar and impedance media have been used successfully to enumerate antib iotic-resistant strains inoculated into foods and stored at different temperatures.