Interactions of organic acids with vancomycin-resistant Enterococcus faecium (VRE) isolated from community wastewater in Texas.

AIMS: Investigate the interactions of organic acids (OAs), acetic, butyric, citric, formic, lactic and propionic acid against 50 Gram-positive VRE strains to determine whether pH, undissociated or dissociated acid forms correlate with bacterial inhibition.

METHODS AND RESULTS: Concentrations of undissociated and dissociated OAs at the molar MICs (MICM s) of the VRE were calculated using the Henderson-Hasselbalch equation. The pH at the MICM s of all VRE strains against acetic, butyric, formic and propionic acids was similar, 4.66 ± 0.07, but there was a 1.1 pH unit difference for all six OAs. Inhibition of VRE by all six OAs did not appear to be solely dependent on pH or on the undissociated OA species. The inhibition of VRE by all six dissociated acids was within Δ = 3.1 mmol l-1 .

CONCLUSIONS: VRE inhibition correlated with the dissociated OA species. A small decrease in concentration of the dissociated OAs from optimum may result in allowing VRE strains to escape disinfection.

SIGNIFICANCE AND IMPACT OF THE STUDY: When an OA is used to disinfect VRE strains the concentration of the dissociated OA should be carefully controlled. A concentration of at least 20 mmol l-1 dissociated OA should be maintained when disinfecting VRE. This article is protected by copyright. All rights reserved.