An Evaluation of the Prevalence of Antibiotic Resistance among Salmonella and Staphylococcus Aureus Isolated from Various Food Animals

Abstract

Within the last decade antibiotic resistant bacteria have become a major public health concern. A possible major contribution to this problem is thought to be the overuse of antibiotics in food animals. An estimated 70% of antibiotics dispensed yearly throughout the United States are distributed to the livestock industry as growth promoters, prophylactic, and therapeutic treatments, according to the Center for Disease Control and FDA. When food animals are exposed to low doses of antibiotics frequently over a long period of time the bacteria are able to develop resistance to antibiotics. Livestock harbor foodborne pathogens that are generally commensal bacteria in the animals themselves but can cause illness to the people exposed. The problem occurs when treatment becomes difficult, there is some speculation that livestock animals are a main contributor to the increase in antibiotic resistant foodborne pathogens. Salmonella spp. and Staphylococcus aureus are pathogens that can be isolated from livestock and cause serious illness in humans. Objectives of this study include isolating S. aureus and Salmonella from samples collected from food animals, investigating the prevalence of antibiotic resistance in the confirmed S. aureus and Salmonella isolates from animals raised in various areas of Southern New Mexico and Arizona. In this study, samples were collected from various food animals post-harvest at a USDA inspected, non-commercial animal harvest facility in Arizona, and evaluated for the presence of S. aureus and Salmonella. Samples were collected from 129 animals of the following types: Bovine (cow), Caprine (goat), Ovine (sheep), and Porcine (pig). S. aureus and Salmonella were isolated from three different types of samples per animal including hide samples, sub iliac and mesenteric lymph nodes, and nasal swabs. Each sample was cultured separately in enrichment media followed by selective/differential media. Once the pathogen was confirmed via 16s rRNA PCR for S. aureus, invA3 PCR for Salmonella, gel electrophoresis, DNA Sequencing, and other biochemical tests, an antibiotic susceptibility test was performed to check the resistance characteristics of each isolate. The pathogen was exposed to eight different antibiotics- Ampicillin, Cefoxitin, Chloramphenicol, Ciprofloxacin, Erythromycin, Streptomycin, Sulfamethoxazole/Trimethoprim, and Tetracycline; commonly used among animals and humans via the disc diffusion assay. A total of 59 and 60 of 369 samples were confirmed positive for S. aureus and Salmonella, respectively. The animal type that harbored the most Salmonella overall were Bovine/cattle and the sample type that harbored the most Salmonella overall were lymph nodes. The animal type that harbored the most S. aureus overall were porcine/pigs and the sample type that harbored the most S. aureus overall were lymph nodes. 18 out of 129 livestock animals sampled in this study were found to carry both Salmonella and S. aureus and were isolated from: 6-Porcine, 5-Bovine, 5-Caprine, and 2-Ovine. The overall antibiotic resistance prevalence in S. aureus and Salmonella were 22.88% and 32.71%, respectively. Antibiotic resistance patterns were seen in both S. aureus and Salmonella isolated from all different livestock and sample types. Of these S. aureus isolates 43 showed resistance to at least one type of antibiotic, and the most resistance was seen to Ampicillin. 53 Salmonella isolates showed resistance to at least one type of antibiotic, and the most resistance was seen to Erythromycin. The implications of this study indicate that there are antibiotic resistant Staphylococcus aureus and Salmonella found in various food animals and sample types. Most of these Salmonella and S. aureus isolates were resistant to more than one antibiotic. Appropriate control measures are needed to mitigate the problem of antibiotic resistant bacteria among food animals. These control measures could also reduce the spread of resistance from one bacterium to another and possibly lessen the antibiotic resistance problem and infections.