Multidrug-Resistant Salmonella enterica Serovar Muenchen from Pigs and Humans and Potential Interserovar Transfer of Antimicrobial Resistance

More than 2,500 serovars of nontyphoidal Salmonella are known to exist and are considered potential food-borne pathogens. However, most human infections are caused by a few serovars. Salmonella enterica serovar Muenchen ranks among the top 10 serovars that cause clinical salmonellosis in humans in the United States. According to recent reports from the Centers for Disease Control and Prevention (CDC) (10), this serovar accounts for 2% of salmonellosis cases (ranked eighth among the serovars that cause clinical salmonellosis) in the United States. Historically, outbreaks due to this serovar in humans were reported in as early as 1961 (32). Three multistate outbreaks caused by this serovar have been reported in the past decade. These were associated with the consumption of food products, including alfalfa sprouts and unpasteurized orange juice, and the use of marijuana (8, 28, 33). Point sources of each of these products were implicated in the spread of the outbreaks to more than four states. Information on the potential role of commercial swine production in the dissemination of multidrug resistant (MDR) salmonellae in the United States is very limited. In addition, the role that food-producing animals play as a primary source of MDR Salmonella has often been questionable (36). The occurrence of MDR Salmonella strains has been best documented in Salmonella serovar Typhimurium, but MDR is extremely rare in many other relatively common serovars, including Salmonella serovar Muenchen. One major concern to public health has been the emergence of MDR strains such as definitive type 104 (DT104), which was first recognized in the United Kingdom in 1984 (35) and which was later identified in other parts of the world (4, 5, 21, 22, 29). This phage type commonly exhibits resistance to five antimicrobial agents: ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (resistance [R] type ACSSuT). An MDR serovar Newport strain that carries AmpC that has recently emerged and that shows resistance to multiple classes of antimicrobials, including expanded-spectrum cephalosporins, has also been increasing in prevalence and is often associated with dairy products (16). Antimicrobial resistance in various other serovars, including Salmonella serovar Muenchen, may play an important role in the ecology of antimicrobial resistance and may have significance in the development of clinical disease. In the few reported outbreaks attributed to Salmonella serovar Muenchen, either antimicrobial susceptibility testing was not done or the strains responsible for the outbreak were rarely resistant (20). According to a recent report of the human arm of the National Antimicrobial Resistance Monitoring System (23), 86% of Salmonella serovar Muenchen isolates tested in the United States in 2001 were pansusceptible, and none of the isolates showed pentaresistance. Our laboratory recently reported on common MDR strains in various Salmonella serovars from pigs, particularly Salmonella serovar Typhimurium (including variety Copenhagen). The two most common MDR R types identified were ampicillin, kanamycin, streptomycin, sulfamethoxazole, and tetracycline (R type AKSSuT) and amoxicillin-clavulanic acid, ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (R type AxACSSuT) (14, 15). In the present study, we investigated MDR strains of Salmonella serovar Muenchen from pigs and humans, identified and compared the sequences of their resistance genes and class 1 integrons, and compared the clonalities of isolates from humans with those of porcine origin. We also performed a conjugation assay and plasmid restriction pattern analysis to identify and compare the sizes of the conjugative plasmids in Salmonella serovar Muenchen. We then compared the genotypic characteristics with those detected in Salmonella serovar Typhimurium from the same groups of pigs to determine the likelihood of horizontal dissemination of resistance factors between Salmonella serovars Muenchen and Typhimurium.