Aerococcus viridans, Staphylococcus haemolyticus and Corynebacterium bovis: sub-inhibitory exposure of lactic acid and cross-resistance to β-lactams antibiotics

Lactic acid (LA) is a commonly applied post-milking teat disinfectant to prevent bovine mastitis. Subsequent to disinfection, microorganisms are often subjected to sub-inhibitory concentrations of biocide (residues), which can promote resistance to the applied biocide and cross-resistance to various antibiotics. In this study, control strains and 20 field isolates of three bacterial species, including Staphylococcus haemolyticus (n=5), Aerococcus viridans (n=9) and Corynebacterium bovis (n=6) were exposed in-vitro to sub-inhibitory concentrations of LA. In the 30-day-study-period LA supplemented growth medium was changed every 48 hours for Staphylococcus haemolyticus and Aerococcus viridans and every 72 hours for Corynebacterium bovis . Changes of susceptibility towards LA during sub-inhibitory treatment were assessed by recording the first non-turbid concentration, named MIC A . Changes of tolerance towards antibiotics were assessed before and after LA exposure by determining antibiotic MIC through microdilution. Results showed that all Aerococcus viridans isolates exerted an increase of LA MIC A s during sub-inhibitory exposure, which was 2.3–7.5-fold. Staphylococcus haemolyticus reflected a 1.3–1.8-fold rise of LA MIC A s. Aerococcus viridans showed a rise of oxacillin, cefoxitin, cefazolin and ceftiofur MICs after sub-inhibitory exposure to LA. Two isolates of Staphylococcus haemolyticus showed a rise of β-lactam-MICs. Corynebacterium bovis did not show any changes of tolerance towards LA or any tested antibiotics following 9 repetitions of sub-inhibitory treatment. The findings suggest that Aerococcus viridans and Staphylococcus haemolyticus have potential to develop tolerance towards LA and cross-resistance to β-lactams antibiotics following sub-inhibitory exposure to LA.