Longitudinal study on the susceptibility to bacteriophages of Staphylococcus aureus strains isolated from dairy farms in Trinidad

A 6-month longitudinal study was conducted on 30 dairy cows in early lactat ion and their human handlers on six farms across Trinidad. Weekly samples o f bulk milk, composite milk and anterior nares and hand swabs from human ha ndlers were collected and cultured for Staphylococcus aureus on Baird-Parke r agar (BPA). The susceptibility of S. aureus strains to bacteriophages and the relatedness of strains isolated over the study; period were determined . Sixty-three (51.2 %) of 123 strains of S. aureus from bulk milk were typa ble compared with 111 (57.3 %) of 194 and 82 (61.7 %) of 133 strains isolat ed from composite milk and human handlers, respectively. The differences we re not statistically significant (P > 0.05; chi(2)). Bovine phage 42D lysed 3.3 % (4 of 123), 16.5 % (32 of 194) and 12.0 % (16 of 133) of S. aureus s trains isolated from bulk milk, composite milk and human handlers, respecti vely. The differences were statistically significant (P < 0.001; chi(2)). A mongst bulk milk isolates of S. aureus, 35 (31.8 %) of 110 exhibited relate dness in 11 groups based on their phage patterns and groups. The mean maxim um interval between the first and last detection of related S. aureus strai ns in a group was 11.5 +/- 7.3 weeks. Amongst composite milk strains of S. aureus, 23 (46.0 %) of 50, 25 (62.5 %) of 40 and 22 (53.7 %) of 41 exhibite d relatedness on farms IB 2, IB 27 and IC 23, respectively, but the differe nces were not statistically significant (P > 0.05; chi(2)). On farm IB 2, f ive groups of related strains of S. aureus were detected with a mean maximu m interval of detection of 18.2 +/- 8.5 weeks compared to farm IB 27 where five groups of related strains were also observed bur with an interval of 1 3.8 +/- 8.2 weeks. On farm IC 23, a total of seven groups of related S. aur eus strains were detected with a mean interval of 8.0 +/- 5.5 weeks. For hu man strains of S. aureus from farm IB 2, nine (56.3 %) of 16 strains isolat ed from anterior nares exhibited relatedness in three groups with a mean ma ximum interval of 13.3 +/- 4.7 weeks compared to four (25.0 %) of 16 hand s wab isolates which exhibited relatedness in two groups with mean interval o f detection of 11.0 +/- 1.4 weeks. The differences were not statistically s ignificant (P > 0.05; chi(2)). On farm IB 27, for anterior nares isolates, eight (72.7%) of 11 exhibited relatedness in two groups with a mean maximum interval of detection of 20.5 +/- 2.1 weeks compared to hand swab isolates , with six (50.0%) of 12 showing relatedness in two,groups and a mean inter val of 10.5 +/- 2.1 weeks. It was concluded that dairy cows and their human handlers carried particula r strains of S. aureus at various sites for extended periods, which served as continuous sources of contamination of milk and may play a significant r ole in the occurrence of subclinical mastitis, with an obvious economic imp act.