Characterization of Staphylococcus aureus isolated from chronically infected dairy goats

A herd of 88 Alpine goats in Northern Italy was monitored for a complete lactation. Milk samples were taken from each udder half during 8 monthly visits. Goats (n=28) with more than or equal to 2 consecutive positive tests for Staphylococcus aureus in the same udder half were identified as chronically infected, and all of those had more than or equal to 4 positive tests of the 8 samples. Goats with no infections in either udder half during any visit were considered healthy (n=26). Linear mixed models were used to examine the relationship between chronic infection by S. aureus and SCC and production traits. The bacteria isolated from one sample from each infected goat were genotyped on the basis of polymorphism in several genes and evaluated for the presence of genes encoding for enterotoxins. The bacteria isolated from each animal were also subject to a test for beta -lactamase production and to minimum inhibitory concentration tests for 11 antimicrobial agents. As expected, SCC (log2) was significantly higher in infected goats than in healthy goats (7.55 vs. 5.50). Also, mean log SCC from infected udder halves (8.02) was greater than that in uninfected udder halves from the same goats (6.44). No significant differences were observed in milk yield or for fat and protein percentages between infected and healthy goats. No genetic variability was observed among the bacteria isolated, suggesting that all were from the same strain, although isolates did vary in susceptibility to various antimicrobial agents. All S. aureus isolates were negative for the beta -lactamase production test. The most effective drugs when tested in vitro were benzylpenicillin, amoxicillin plus clavulanic acid, cloxacillin, and cephalosporins.