Serotyping, RAPD grouping and antibiotic susceptibility testing of Salmonella enterica isolated from retail foods in Thailand

Abstract

A total of 270 food samples were obtained from twelve free markets (149 samples) and seven supermarkets (121 samples) in Thailand from September 2010 to February 2012. The samples were divided as follows: 51 meat, 37 fish/seafood, 38 vegetable, 11 fermented food and 133 tofu samples. The samples were examined for the presence of Salmonella spp.. Salmonella spp. were detected in 28% of the free market samples and in 7% of the supermarket samples. Meat samples had the highest contamination rates: 78% of the free market and 40% of the supermarket samples. Of the fish/seafood, vegetable, fermented food and tofu samples from the free markets, 41%, 6%, 25%, and 7%, respectively, tested positive for Salmonella spp., whereas the corresponding contamination rates were 5%, 5%, 0% and 0% for the supermarket samples. The resistance of the isolates towards sixteen antibiotics was tested. A high percentage of the isolates were resistant to streptomycin, tetracycline or ampicillin: 78%, 59% and 51%, respectively. In total, 152 isolates were resistant to at least one antibiotic, and of these, 98 (64%) were resistant to at least three antibiotics (multidrug-resistant). The most frequently isolated serovars were Corvallis, Rissen and O4: i:-. Random Amplified Polymorphic DNA (RAPD) was used for grouping 149 isolates into five groups. The grouping of the isolates is baseline information to emphasize the usefulness of subtyping Salmonella spp. isolated from several categories of retail foods obtained from free markets and supermarkets by using antibiotic resistance studies and RAPD grouping.