In the current study, we characterized the prevalence, antibiotic susceptibility profiles, and genotypes of S. aureus among US meat and poultry samples. S. aureus contaminated a substantial proportion of samples from all meat and poultry types (37–77%), with a notable 52% of isolates being multidrug resistant.
The distinct S. aureus populations on each product type suggest that food animals are the predominant source of contamination. While a portion of the S. aureus isolates may have been the result of human contamination, a uniform pattern of human-associated strains was not observed. Additional studies tracing S. aureus genotypes from farm to retail are required to definitively identify the sources of S. aureus contamination.
MRSA was isolated from one sample each of beef, turkey, and pork. Our sample size was insufficient to accurately estimate prevalence rates, but our data are consistent with a previous US-based study . Higher MRSA contamination rates have been estimated among meat and poultry samples in the Netherlands, where ST398 is the dominant food-borne sequence type . In contrast, the MRSA isolates from the current and previous US-based food studies were ST5 and ST8.
All isolates were screened against antibiotics that are commonly used to treat severe MRSA infections. We identified 1 vancomycin-intermediate-resistant isolate and 1 daptomycin-resistant isolate. Vancomycin, daptomycin, and their analogs were never approved for US food animal production; therefore, these findings were unexpected and may suggest origins other than US food animals.
Fluoroquinolone-resistant S. aureus isolates were uniquely prevalent among chicken products. Fluoroquinolones were used in US broiler production from 1995 to 2005, which may have selected for the fluoroquinolone-resistant S. aureus strains that exist today ; however, isolates collected prospectively starting prior to 1995 would be necessary to make definitive conclusions in this regard.
Our data demonstrate that retail meat and poultry are frequently contaminated with multidrug-resistant S. aureus, but the public health relevance of this finding is unclear. European and North American studies indicate that ST398 can successfully colonize and infect humans [4, 10], but few studies have investigated the risk of human colonization and infection with S. aureus from meat and poultry products [11, 12]. The European Food Safety Authority (EFSA) concluded that the risk for MRSA infection from food handling and consumption was low; however, this was based on a small number of studies . Furthermore, EFSA did not evaluate the risk from methicillin-susceptible multidrug-resistant S. aureus, which is more common than MRSA among food samples.
Conventional concentrated animal feeding operations (CAFOs) provide all the necessary components for the emergence and proliferation of multidrug-resistant zoonotic pathogens. In the United States, billions of food animals are raised in densely stocked CAFOs, where antibiotics are routinely administered in feed and water for extended periods to healthy animals . NARMS has shown that multidrug-resistant E. coli and Enterococcus species are prevalent among US meat and poultry products . Our findings indicate that multidrug-resistant S. aureus should be added to the list of antimicrobial-resistant pathogens that routinely contaminate our food supply.