Antimicrobial Drug Resistance Patterns among Cattle and Human Associated Salmonella: Are Cattle a Reservoir for Multidrug-resistant Salmonella Linked to Human Disease?

Introduction: Salmonella, an important human and animal pathogen worldwide, is transmitted to humans through contaminated food, water or through direct contact with infected hosts. The emergence and spread of multidrug-resistant strains of Salmonella has become a public health concern worldwide. Antimicrobial susceptibility is commonly measured by either broth or agar dilution methods or agar diffusion techniques. Phenotypic methods for determining antimicrobial resistance profiles of Salmonella isolates are widely used for surveillance and diagnostic applications. However, numerous drawbacks can potentially limit the usefulness of such approaches. In house genotypic screening for the absence or presence of genes responsible of antimicrobial resistance may potentially offer an alternative to phenotypic screening, and may provide rapid, reliable, and cost-effective antimicrobial sensitivity results.

Purpose: Our purpose in this study is to better understand the transmission dynamics and genetic determinants of antimicrobial resistance of Salmonella isolates among human and bovine hosts using phenotypic and genotypic methods. 

Methods: Here we described the distribution of multidrug resistance among 336 Salmonella isolates from human and cattle, representing 51 different serotypes. We also investigated genetic determinants of antimicrobial resistance among both human and bovine isolates, which have previously been characterized by MLST and PFGE. Here we tested all isolates for susceptibility to 15 antimicrobial drugs that are part of the National Antimicrobial Monitoring System (NARMS) bovine susceptibility panel.  PCR was performed on a representative subset of unique isolates (n=53) to screen for the presence of 21 known antimicrobial resistance genes; selected fluoroquinolone and nalidixic acid-resistant (n=3) and sensitive (n=6) isolates were also tested for known resistance conferring mutations in gyrA and parC.

Results: Overall, our data confirm that bovine Salmonella isolates have a significantly higher incidence of multidrug resistance (MDR) than human Salmonella isolates. Importantly, genes responsible for antimicirobal resistance are shared among isolates of human and bovine origin, and diverse genotypes can result in similar phenotypic resistance profiles. Notably, the presence of  flo, aadA1, and aphA1-iab genes and groups of sul genes (sulI and sulII) and group of tet genes (tetA, tetB and tetG) indicates phenotypic profile of Chloramphenicol, Gentamicin, Kanamycin, Sulfizoxazole and Tetracycline, respectively,  in our Salmonella isolate set, regardless of source of isolates.

Significance: Our study characterizes the molecular determinants of antimicrobial resistance among Salmonella isolates from humans and cattle, and our findings are consistent with a role of cattle as one important source of human MDR Salmonella strains. In addition, we conclude that instead of one gene, a set of genes should be used to determine antimicrobial profiles of Salmonella isolates.