Prevalence and antimicrobial resistance profiles of Staphylococcus aureus (SA) recovered from growing pigs in the USA

Although current evidence suggests a relatively low prevalence of MRSA in pigs in the USA, concerns remain about spread of multidrug resistance bacteria from the farm environment to swine workers and the wider community. The aim of this study was to characterize antibiotic resistance profiles of SA isolated from growing pigs in the USA.

Twenty nasal swabs were collected from 36 growing pig sites in 11states in the USA, including one known MRSA-positive farm. Antimicrobial susceptibility testing (17 antibiotics) was performed on a subset of 128 SA isolates selected purposively to maximize the diversity by spa type and farm. Susceptibility was based on Clinical Laboratory Standards Institute criteria for all antimicrobials except ceftiofur, tulathromycin and tylosin. To evaluate co-resistance, associations between phenotypic resistance patterns was assessed using Chi square and Fisher’s exact test adjusting for multiple comparisons.

MRSA were not detected in any of the pigs except on the positive control farm. Overall SA prevalence was 76% (558/739) and there was considerable diversity found with 33 spa types detected within 4 MLST sequence types: ST9, ST398, ST5 and ST2007. All isolates were resistant to spectinomycin and most were resistant to tetracycline (94.5%), clindamycin (75.0%) and penicillin (both 71.9%). Multiple resistance (> 3 antibiotics) was common, and 61 resistance patterns were found among the 128 isolates. The most common resistance profile among isolates was SPE-CTC-OXY-CLITIL-FFN.* There were statistically associations found between 10 pairs of antibiotics : TET-CLI, CLI-TIA, CLI-TIL, CLI-FFN, TIL-FFN, TIA-NEO, FFNNEO,TIL-GEN, GEN-NEO, NEO-ENR. The reported use of antibiotics on farms was poorly correlated with the presence of resistance to the same classes of antibiotics in the isolates studied.

The most striking finding was the apparently low MRSA prevalence in these herds, but a relatively high prevalence of multiple resistant SA phenotypes. Although sample size limited the power of analysis of associations between resistance to antibiotics, many pairs of co-resistance were observed suggesting the potential importance for co-selection of multiple resistance genes as a result of antibiotic use. The public health importance, if any, of multiple resistant SA in the swine reservoir remains to be established.

*Abbreviations: AMP (ampicillin), CTC(chlortetracycline), CLI(clindamycin), ENR(enrofloxacin), FFN(florfenicol), GEN(gentamicin), NEO(neomycin), OXY (oxytetracyline), PEN (penicillin), SPE(spectinomycin), TIA(tiamulin), TIL(tilmocosin)

Disclosure of Interest: None Declared.