Geographical variation in the presence of genes encoding superantigenic exotoxins and β-hemolysin among Staphylococcus aureus isolated from bovine mastitis in Europe and USA

Abstract

The object was to examine the geographical variation in the presence of superantigenic exotoxins and β-hemolysin among epidemiologically independent Staphylococcus aureus isolates from bovine mastitis. A total of 462 S. aureus isolates from nine European countries and USA were examined for the presence of genes encoding staphylococcal enterotoxins A–E, and H, toxic shock toxin-1 (TSST-1), and β-hemolysin, and 128 of these were examined for exfoliative toxins A and B. The detection was done by PCR. Phenotypic methods were used to confirm the PCR-results. None of the 128 isolates carried the genes for exfoliative toxin A or B. The total proportion of isolates in which superantigenic exotoxins were detected varied from 2% (one isolate) of the Danish isolates to 65% (32 isolates) of the Norwegian isolates. This marked and highly significant geographical variation was also present for the individual exotoxins. The genes encoding enterotoxin C, TSST-1, and enterotoxin D were the most common superantigens. The present and earlier studies demonstrate that the superantigenic exotoxins that were investigated in this study, do not play a role in the pathogenesis of bovine S. aureus mastitis. In contrast to the geographical variation among superantigenic exotoxins, 97% of the isolates were PCR-positive for and/or produced β-hemolysin on 5% calf blood agar. Except for three isolates, the Norwegian isolates were PCR-negative, but positive on 5% calf blood agar. Sequence variation in the primer regions in the β-hemolysin encoding gene of the Norwegian isolates is suggested, and should be investigated further. The consistent presence of β-hemolysin suggests that this factor, or a co-existing gene correlated to β-hemolysin, may be an active virulence factor in the pathogenesis of bovine S. aureus mastitis.