Relationship between S. aureus gene pattern and dairy herd mastitis prevalence

The importance to evaluate Staphylococcus aureus virulence analysing the combination of virulence genes is largely recognised, and the recent availability of simplified microarray tools allows performing these analyses also in the dairy field. The combined availability of herd-specific S. aureus mastitis prevalence data, isolates from these herds, and microarray technology offered the opportunity to investigate the relationship between S. aureus genetic pattern and their prevalence among dairy herds. Eleven commercial dairy herds following a S. aureus control programme were enrolled in the study, and 33 S. aureus isolates were collected from these herds. Diagnostic DNA microarrays based on the array-tube platform were used for genotyping of staphylococcal DNA. The genetic analysis of the 157-genes microarray showed as only 19 genes were present in all the isolates considered, and among them the genes coding for the leukocidin subunits (LukF, LukS and LukY), haemolysins (hla, hld and an unnamed haemolysin) and enterotoxin X. Several genes considered in the arrays were absent in all the isolates, including the ones encoding the resistance to most of the antimicrobials, except for tetracycline. In our isolates, some agr alleles were never identified (B-III, C-III, D-III, C-IV and D-IV). The comparison of epidemiological data with the genetic pattern suggests that agr type II is associated to the most diffusive isolates, being recovered from the largest number of herds and with the highest frequency. Microarray technique showed to be a useful method to assess the characteristics of virulence of S. aureus isolated in dairy herds and to investigate the relationship with the prevalence of the microorganism. These results support previous evidence that specific gene patterns could be associated to S. aureus mastitis.