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

## TL;DR

Exposure to low levels of lactic acid can increase bacterial tolerance to the acid and cross-resistance to beta-lactam antibiotics in certain species.

## Contribution

The study reveals that sub-inhibitory lactic acid exposure can induce cross-resistance to beta-lactam antibiotics in Aerococcus viridans and Staphylococcus haemolyticus.

## Key findings

- Aerococcus viridans isolates showed a 2.3–7.5-fold increase in lactic acid tolerance during sub-inhibitory exposure.
- Aerococcus viridans and two Staphylococcus haemolyticus isolates developed cross-resistance to beta-lactam antibiotics.
- Corynebacterium bovis showed no changes in tolerance to lactic acid or antibiotics after repeated exposure.

## Abstract

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 h for Staphylococcus haemolyticus and Aerococcus viridans and every 72 h for Corynebacterium bovis. Changes of susceptibility towards LA during sub-inhibitory treatment were assessed by recording the first non-turbid concentration, named MICA. 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 MICAs during sub-inhibitory exposure, which was 2.3–7.5-fold. Staphylococcus haemolyticus reflected a 1.3–1.8-fold rise of LA MICAs. 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.