Prevalence and characteristics of Staphylococcus aureus strains isolated from bulk and composite milk and cattle handlers

The prevalence and characteristics of Staphylococcus aureus strains isolated from bulk and composite milk and from cattle handlers on dairy farms in Trinidad were determined. S. aureus strains were isolated from all 175 bulk milk samples tested (100%) while 280 of 287 composite milk samples (97.6%) yielded S. aureus. The mean counts of S. aureus in bulk milk ranged from 5.9 x 103 to 1.2 x 105 CFU/ml compared with mean S. aureus counts in composite milk which ranged from 2.4 x 103 to 3.0 x 104 CFU/ml. Of the 105 strains of S. aureus from bulk milk tested, 45 (42.9%) were enterotoxigenic elaborating staphylococcal enterotoxin A (SEA), SEB, SEC, SED, or a combination compared to 69 of 146 strains (47.3%) recovered from composite milk which were enterotoxigenic, but the difference was not statistically significant (P > 0.05; χ2). Twenty-two of 42 bulk milk samples containing enterotoxigenic S. Aureus (52.4%) had counts of the organism which exceeded 104 CFU/ml. For S. aureus strains isolated from cattle handlers, 66 of 146 (45.2%) were enterotoxigenic. Prevalence of resistance to nine antimicrobial agents tested was 18.7% and 12.9% among bulk milk and composite milk isolates, respectively, compared to 49.3% and 69.5% among isolates from human anterior nares and hand swabs, respectively. Resistance to ampicillin and penicillin was highest among both milk (12.2%) and human (53.6%) isolates of S. aureus, and the difference was statistically significant (P ≤ 0.001; χ2). It was concluded that bulk milk containing relatively high counts of enterotoxigenic S. aureus may constitute a health hazard to consumers.