Molecular mechanisms of resistance in multidrug-resistant serovars of Salmonella enterica isolated from foods in Germany

Objectives: The objectives of this study were to determine antimicrobial susceptibility and to characterize the molecular mechanisms of multidrug resistance among German food-borne Salmonella isolates of different serovars. Methods: A total of 319 epidemiologically independent multidrug-resistant isolates from German foodstuffs comprising 25 different serovars were tested for their antimicrobial susceptibility by broth microdilution. The presence of antimicrobial resistance genes, integrons of classes 1 and 2 and their integrated resistance gene cassettes as well as the Salmonella genomic island 1 (SGI1) was investigated by PCR and DNA sequencing. Localization of integrons and relevant resistance genes was done by Southern hybridization. Sequence analysis revealed mutations in the quinolone resistance-determining region of the gyrA gene. Results: The most prevalent resistances found in the multidrug-resistant serovars of Salmonella enterica from foods were to streptomycin (94%), sulfamethoxazole (92%), tetracycline (81%), ampicillin (73%), spectinomycin (72%), chloramphenicol (48%) and trimethoprim (27%). Twenty-four resistance genes covering sixantimicrobial families (b-lactams,aminoglycosides, phenicols, sulphonamides, tetracycline,and trimethoprim)wereidentifiedinthefoodisolates,manyofthemintegratedasgenecassettesinclass1and class 2 integrons. Class 1 integrons were detected in 65% of the multidrug-resistant Salmonella isolates comprising16differentserovars,whileclass2integronswerefoundin10%oftheisolatesbelongingtotwo serovars only. The results demonstrate a clear predominance of both SGI1-borne resistance genes and class1integronsinSalmonellaserovarTyphimuriumDT104andofclass2integronsinSalmonellaserovar Paratyphi B (D-tartrate positive). Nalidixic acid resistance found in 15% of the isolates was associated with single mutations in the gyrA gene. Conclusions: This study confirms the role of foods of animal and other origin as a reservoir of multidrugresistant Salmonella and underlines the need for continuing surveillance of food-borne zoonotic bacterial pathogens along the food chain.