Interactions of Organic Acids with Staphylococcus aureus and MRSA Strains from Swine Mandibular Lymph Node Tissue, Commercial Pork Sausage Meat and Feces

Staphylococcus aureus is a Gram-positive bacterium affecting human health, and a major cause of skin infections, endocarditis, meningitis, and sepsis. Methicillin-resistant S. aureus (MRSA) is a worldwide health concern, occurs in food animals, is consistently found in swine, and improved strategies are needed to ensure the removal of MRSA from food products. A total of 164 S. aureus strains were isolated from swine mandibular lymph node tissue, commercial pork sausage meat, and feces. These strains were tested for methicillin-resistance, and 7 of the strains isolated from the mandibular lymph node tissue and pork sausage meat were resistant to cefoxitin and oxacillin, and tested positive for staph specific rRNA and for the mecA gene and are therefore, MRSA strains. An intracellular MRSA contamination of 8.2% within swine lymph node tissue and 5.8% MRSA contamination in pork sausage meat was demonstrated. Lymph node tissue may be utilized in producing pork sausage; therefore, the prevalence of MRSA in final pork products may not only be caused by surface contamination, but by internal tissue infection. The strains were tested for susceptibility to six organic acids (OAs) citric, L-lactic, butyric, acetic, propionic, and formic acid. The pH was determined at each of the minimum inhibitory concentrations (MICs) observed for the S. aureus strains. The Henderson-Hasselbalch equation was used to calculate the ratio of the undissociated to dissociated OA concentrations, and the molar concentrations of each were calculated from the total OA present and the ratio. Inhibition of S. aureus did not correlate with pH or the undissociated OA concentrations, but it did correlate with the dissociated OA concentrations. A dissociated OA concentration of 21 mM was successful for inhibiting the S. aureus strains tested. Studies must be conducted in vivo to confirm this concentration value. Acetic, butyric, formic, and propionic acid were the most effective OAs tested against S. aureus.