JAC Advance Access originally published online on October 14, 2005
Journal of Antimicrobial Chemotherapy 2005 56(6):1025-1033; doi:10.1093/jac/dki365
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Molecular mechanisms of resistance in multidrug-resistant serovars of Salmonella enterica isolated from foods in Germany
National Salmonella Reference Laboratory, Federal Institute for Risk Assessment (BfR), Diedersdorfer Weg 1, 12277 Berlin, Germany
Received 22 April 2005; returned 5 June 2005; revised 15 July 2005; accepted 13 September 2005
* Corresponding author. Tel: +49-30-8412-2201; Fax: +49-30-8412-2953; E-mail: a.miko{at}bfr.bund.de
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 six antimicrobial families (ß-lactams, aminoglycosides, phenicols, sulphonamides, tetracycline, and trimethoprim) were identified in the food isolates, many of them integrated as gene cassettes in class 1 and class 2 integrons. Class 1 integrons were detected in 65% of the multidrug-resistant Salmonella isolates comprising 16 different serovars, while class 2 integrons were found in 10% of the isolates belonging to two serovars only. The results demonstrate a clear predominance of both SGI1-borne resistance genes and class 1 integrons in Salmonella serovar Typhimurium DT104 and of class 2 integrons in Salmonella serovar 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 multidrug-resistant Salmonella and underlines the need for continuing surveillance of food-borne zoonotic bacterial pathogens along the food chain.
Keywords: multidrug resistance , resistance genes , integrons , class 1 integrons , class 2 integrons , gene cassettes , food safety
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