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 <i>Staphylococcus haemolyticus</i> (<i>n</i> = 5), <i>Aerococcus viridans</i> (<i>n</i> = 9) and <i>Corynebacterium bovis</i> (<i>n</i> = 6) were exposed <i>in-vitro</i> to sub-inhibitory concentrations of LA. In the 30-day-study-period LA supplemented growth medium was changed every 48 h for <i>Staphylococcus haemolyticus</i> and <i>Aerococcus viridans</i> and every 72 h for <i>Corynebacterium bovis</i>. 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 <i>Aerococcus viridans</i> isolates exerted an increase of LA MIC_As during sub-inhibitory exposure, which was 2.3-7.5-fold. <i>Staphylococcus haemolyticus</i> reflected a 1.3-1.8-fold rise of LA MIC_As. <i>Aerococcus viridans</i> showed a rise of oxacillin, cefoxitin, cefazolin and ceftiofur MICs after sub-inhibitory exposure to LA. Two isolates of <i>Staphylococcus haemolyticus</i> showed a rise of β-lactam-MICs. <i>Corynebacterium bovis</i> did not show any changes of tolerance towards LA or any tested antibiotics following 9 repetitions of sub-inhibitory treatment. The findings suggest that <i>Aerococcus viridans</i> and <i>Staphylococcus haemolyticus</i> have potential to develop tolerance towards LA and cross-resistance to β-lactams antibiotics following sub-inhibitory exposure to LA.