Detection and Molecular characterisation of Virulence Genes in Antibiotic Resistant Staphylococcus aureus from milk in the North West Province, South Africa

Abstract

Staphylococcus aureus (S. aureus) is a common microorganism present on the skin and mucosal surfaces of humans and animals. However, a variety of infections, ranging from mild skin infections to severe infections such as bacteraemia, osteomylitis, and toxin-mediated diseases can be caused by this bacterium. These infections become more severe if strain harbours antibiotic resistance genes together with virulence genes. The aim of this study was to investigate the occurrence, antibiotic susceptibilities, virulence genes profiles and genetic relationships of S. aureus in milk obtained from the North-West Province. To achieve this, 200 samples of raw, tank and pasteurised milk were obtained randomly from supermarkets, shops and some farms in the North-West Province during the period of May 2012 to April 2013. S. aureus was isolated and positively identified using morphological, biochemical tests, protein profile analysis (MALDI-TOF mass spectrometry) and molecular (PCR) methods. The isolates were characterised by determining their antimicrobial resistance profiles, detection of genes encoding enterotoxins, exfoliative toxins and collagen adhesins. Moreover, the relationships of the isolates from different stations and milk types were compared using their antibiotic inhibition zone diameter data, RAPD-PCR and ERIC-PCR fmgerprinting data.Among all the samples examined, 30 of 40 raw milk samples (75%), 25 of 85 of tank milk samples (29%) and 10 of 75 pasteurised milk samples (13%) were positive for S. aureus. One hundred and fifty-six PCR-confirmed S. aureus isolates were obtained from the 75 contaminated milk samples. A large proportion (60-1 00%) of the isolates was resistant to penicillin G, ampicillin, oxacillin, vancomycin, teicoplanin and erythromycin. On the contrary, low level resistance (8.3 to 40%) was observed for gentamycin, kanamycin and sulphamethoxazole. Methicillin resistance was detected in 59% of the multidrug resistant isolates. However, a small proportion (20.6%) of these isolates possessed the PBP2a which codes for methicillin resistance in S. aureus. In addition, 32.7% of isolates possessed the sec gene whereas the sea, seb sed, see, cna, eta,etb genes were not detected. A total of 18 RAPD patterns were generated from 74 randomly selected milk isolates while 9 banding patterns was obtained with ERIC-PCR indicating a high genetic diversity among the isolates. However, the presence of toxin genes was not associated with any particular genotype. The findings of this study indicate that raw, tank and pasteurised milk in the North-West Province were contaminated with toxigenic and multi-drug resistant S. aureus strains. Moreover, even if some of these S .aureus isolates were genetically diverse, in some situations isolates from different types of milk were very similar based on phenotypic and genotypic typing assays. This emphasise the need for the implementation of better control measures to reduce contamination as well as the spread of virulent S. aureus strains. This may greatly reduce human suffering due to foodborne infections that may occur through the consumption of contaminated food products.