Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/1/2013
Publication Date: 4/1/2014
Citation: Folster, J., Tolar, B., Pecic, G., Sheehan, D., Rickert, R., Hise, K., Zhao, S., Cray, P.J., McDermott, P., Wichard, J.M. 2014. Characterization of blaCMY plasmids and their possible role in source attribution of salmonella enterica serotype typhimurium infections. Foodborne Pathogens and Disease. 11(4):301-306. Interpretive Summary: Salmonella is a food borne pathogen that can be acquired from ingestion of contaminated foodstuffs. Infection results in mild to severe cases of gastroenteritis in humans which often resolves within five to seven days. However, when indicated, particularly in the young, the elderly or the immunocompromised, treatment with antimicrobials may be warranted. One class of antimicrobials is especially important in treating salmonellosis, the cephalosporins, since they are well tolerated in children as well as adults. However, all bacteria develop ways to evade killing by antimicrobials through use of internal enzymes, some of which are called beta-lactamases. When bacteria evade killing they become resistant to the effect of the antimicrobial and treatment may fail. Three main groups of beta-lactamases have been identified among Salmonella and include the cephamycinase bla(CMY) genes which are the most common beta-lactamases in North America. One particular type of Salmonella, serotype Typhimurium, is found in diverse agricultural niches and can be recovered from many different animal sources as well as humans. Typhimurium bacteria can carry the genes responsible for antimicrobial resistance to the cephalosporin antimicrobials on a mobile piece of DNA called plasmids. In North America, the antimicrobial susceptibility of Salmonella is monitored by the U.S. National Antimicrobial Resistance Monitoring System (NARMS). In this study, we determined the antimicrobial resistance of Salmonella Typhimurium to cephalosporins. In 2008, 70 isolates (70/581;12.0%) (34 isolates from retail meats, 23 from food animals, and 13 clinically ill humans) were resistant to cephalosporins. All of the isolates had the bla(CMY) gene which was found on a plasmid in 59 (84.3%) of these isolates. We also discovered that these plasmids were primarily of two different types, one identified as IncI1 and the other as IncA/C. Isolates originating from chickens or chicken products were primarily identified as having IncI1 plasmids (37/40; 92.5%) while all isolates from cattle (6/23;26.1%) exclusively had IncA/C plasmids. Antimicrobial susceptibility patterns within the isolates recovered from IncI1 on IncA/C plasmids were also very similar. This suggests that during a food borne outbreak investigation when the source of an infection is unknown, if the laboratory knows that it is Salmonella Typhimurium and characterizes the type of cephalosporin resistant gene, the type of plasmid they carry and determines the antimicrobial resistance profile, they may be able to identify the food source of the outbreak. This is particularly important for physicians, veterinarians, public health laboratories, and epidemiologists as they respond to foodborne outbreaks.
Technical Abstract: Salmonella is an important cause of foodborne illness; however, identifying the source of these infections can be difficult. This is especially true for Salmonella serotype Typhimurium, which is found in diverse agricultural niches. Cephalosporins are one of the primary treatment choices for complicated Salmonella infections, and are the only treatment option in children. In Salmonella, cephalosporin resistance in the U.S. is mainly mediated by blaCMY genes carried on various plasmids. In this study, we examined whether the characterization of blaCMY plasmids can help us identify potential sources of infection by Typhimurium. In the U.S., antimicrobial resistance monitoring of Salmonella is performed by the National Antimicrobial Resistance Monitoring System (NARMS) from retail meats, food animals, and ill persons. In 2008, 70 isolates (70/581; 12.0 %) (34 isolates from retail meat, 23 food animal, and 13 human) were resistant to ceftriaxone and amoxicillin/clavulanic acid. All were PCR-positive for blaCMY and 59 (84.3%) of these genes were plasmid-encoded. PCR-based plasmid typing (PBRT) identified 42 (71.2%) IncI1-blaCMY and 17 (28.8%) IncA/C-blaCMY plasmids. Isolates from chickens or chicken products with blaCMY plasmids primarily had IncI1-blaCMY plasmids (37/40; 92.5%), while all isolates from cattle (6/23;26.1%) exclusively had IncA/C-blaCMY plasmids. Isolates from humans had either IncA/C- blaCMY (8/12; 66.7%) or IncI1- blaCMY (4/12; 33.3%) plasmids. All of the IncI1-blaCMY plasmids were ST12 or highly related. Antimicrobial susceptibility patterns (AST) and pulsed-field gel electrophoresis (PFGE) patterns of the isolates were also compared. When the source of an infection is unknown during a Typhimurium outbreak investigation, characterizing the type of blaCMY plasmid they carry and knowing the AST and PFGE patterns may be aid in identifying the source of the outbreak.