Extended biofilm susceptibility assay for Staphylococcus aureus bovine mastitis isolates: Evidence for association between Agr-type and biofilm susceptibility

Staphylococcus aureus is one of the most prevalent causes of bovine mastitis. The antimicrobial treatment of this disease is currently based on antimicrobial susceptibility testing according to CLSI standards. However, various studies have shown that there is a discrepancy between the results of this standard susceptibility test and the actual cure rate of the applied antimicrobial treatment. Increasing evidence suggests that biofilm formation by S. aureus is associated to with this problem. Previous data obtained with a limited number of strains revealed that the extended biofilm susceptibility assay (EBS assay) allows for differentiation between strains, which cannot be derived from a standard susceptibility test or from a 24-hour biofilm susceptibility assay. Based on the results form a previous study with the same collection of strains (unpublished data, see poster presentations Mastitis control 2008) the effect of Agr-type and the presence of IS257 in Agr-type II strains on the biofilm susceptibility was studied. The results revealed differences in biofilm susceptibility for both the Agr-type of the strains and for the presence of IS257 in Agr-type II strains. The presence of IS257 in Agr-type II strains also has a marked effect on the in vitro biofilm density of these strains. Furthermore, the results confirmed the decreasing antimicrobial efficacy against older biofilms, and the increased antimicrobial efficacy due to longer duration of antimicrobial challenge on biofilms was shown. The data presented here offer an explanation for the higher efficacy of early antimicrobial treatment and for treatments of longer duration in bovine mastitis S. aureus infections described in the literature from several epidemiological studies.