Antimicrobial susceptibilities of isolates of Staphylococcus aureus, Listeria species and Salmonella serotypes associated with poultry processing

The broth microdilution method was used to determine the activities of selected antimicrobial agents used in the South African poultry industry (danofloxacin, neomycin, chlortetracycline, oxytetracycline, tylosin and colistin) and vancomycin against bacterial isolates previously obtained from carcasses and selected equipment surfaces and environmental sources associated with poultry processing. The antimicrobial susceptibilities of 38 isolates of Staphylococcus (S.) aureus, 25 Listeria (L.) innocua, 18 L. monocytogenes, and 62 isolates belonging to six Salmonella (Salm.) serotypes (Salm. agona, Salm. blockley, Salm. enteritidis, Salm. isangi, Salm. reading and Salm. typhimurium) were determined. The most active antimicrobial agent against all the isolates tested was danofloxacin with minimum inhibitory concentrations (MICs) for 90% of the isolates (MIC90) not exceeding 0.25 and 2 μg/ml for Gram-negative and Gram-positive isolates, respectively. Conversely, high MICs were recorded for all the isolates tested against chlortetracycline and oxytetracycline (MIC90 range of 32 to >512 μg/ml), except for the L. monocytogenes and Salm. enteritidis isolates (MIC range of ≤0.5–4 μg/ml). Neomycin was found to be active against S. aureus, L. innocua, L. monocytogenes, Salm. enteritidis and Salm. isangi isolates, with MICs not exceeding 8 μg/ml. MIC ranges for tylosin and vancomycin, which were only tested against the Gram-positive isolates, were from 1 to >512 μg/ml and from 1 to 4 μg/ml, respectively. The MIC range for the remaining antimicrobial agent, colistin, which was only tested against the Salmonella isolates, was 0.5–16 μg/ml. The lack of MIC breakpoints for the antimicrobial agents used in the poultry industry did not allow for definite conclusions as to the level of resistant bacteria associated with poultry carcasses and the processing environment in this study.