Staphylococcus aureus Isolates from Irish Domestic Refrigerators Possess Novel Enterotoxin and Enterotoxin-like Genes and Are Clonal in Nature

Abstract

A previous study carried out by the National Food Centre in Dublin on bacterial contamination of Irish domestic refrigeration systems revealed that 41% were contaminated with Staphylococcus aureus. One hundred fifty-seven S. aureus isolates were screened by multiplex PCR analysis for the presence of 15 staphylococcal enterotoxin and enterotoxin-like genes (seasee, seg-sei, selj-selo, and selq) and the toxic shock toxin superantigen tst gene. Of the refrigerator isolates, 64.3% possessed more than one staphylococcal enterotoxin or staphylococcal enterotoxin–like gene. All bar one of the 101 staphylococcal enterotoxin or staphylococcal enterotoxin–like gene-positive strains possessed the egc locus bearing the seg, sei, selm, seln, and selo genes. Twelve random amplified polymorphic DNA (RAPD) types accounted for 119 (75.8%) of the strains, two of these types accounting for 25 (RAPD type 1, 15.9%) and 52 (RAPD type 5, 33.1%), respectively. All of the RAPD type 5 isolates possessed the egc gene cluster only. The RAPD type 5 amplicon profile was identical to that of S. aureus isolates associated with osteomyelitis in broiler chickens in Northern Ireland that also possessed the egc locus only. However, the RAPD type 5 domestic refrigerator and chicken isolates differed in penicillin G sensitivity, production of Protein A and staphylokinase, and crystal violet agar growth type. These findings highlight that the average Irish household refrigerator harbors potential enterotoxin-producing S. aureus that may or may not be of animal origin and, accordingly, is a potential reservoir for staphylococcal food poisoning.