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

Authors: Beier, Ross; Harvey, Roger; Poole, Toni; Hume, Michael; Crippen, Tawni - Tc; HIGHFIELD, LINDA - University Of Texas Health Science Center; ALALI, WALID - United Arab Emirates; Andrews, Kathleen - Kate; Anderson, Robin; Nisbet, David

Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2018
Publication Date: 2/1/2019
Publication URL: http://handle.nal.usda.gov/10113/6279063
Citation: Beier, R.C., Harvey, R.B., Poole, T.L., Hume, M.E., Crippen, T.L., Highfield, L.D., Alali, W.Q., Andrews, K., Anderson, R.C., Nisbet, D.J. 2019. Interactions of organic acids with vancomycin-resistant Enterococcus faecium isolated from community wastewater in Texas. Journal of Applied Microbiology. 126(2):480-488. https://doi.org/10.1111/jam.14145.
DOI: https://doi.org/10.1111/jam.14145

Interpretive Summary: Organic acids are commonly used as a carcass wash to remove bacterial loads during food processing. In this study, the interactions of six organic acids with 50 vancomycin-resistant Enterococcus faecium (VRE) strains obtained from a nonclinical semiclosed agri-food system in Texas were evaluated. The pH was determined for the VRE strains, and using the determined pH, the concentrations of the undissociated and dissociated organic acids were calculated using the Henderson-Hasselbalch equation at the molar minimum inhibitory concentrations for the VRE strains. Since all VRE strains behaved similarly to each different organic acid, the results obtained from different locations or environments were treated as a single group for each individual organic acid: Acetic, butyric, citric, formic, lactic, and propionic acid. It was determined that bacterial inhibition was not solely dependent on pH or on the undissociated organic acid species, but closely correlated with the dissociated organic acid species for all six organic acids. A small drop in the concentration of the dissociated organic acids may result in a large number of bacteria escaping disinfection. Therefore, if one uses an organic acid to disinfect VRE strains, the concentration of the dissociated organic acid should be carefully controlled. A concentration of the dissociated organic acids of at least 20 mM should be maintained when disinfecting VRE bacteria.

Technical Abstract: Vancomycin-resistant Enterococcus faecium (VRE) from human wastewater effluents in a nonclinical semiclosed agri-food system in Texas were evaluated for susceptibility to organic acids (OAs). Five swine farms are located within the agri-food system and raise swine for their own internal use. OAs are often used to remove bacteria from animal carcasses, and there has been a long-standing understanding that the undissociated organic acid is responsible for bacterial inhibition. The interactions of six OAs, acetic, butyric, citric, formic, lactic, and propionic acids, with 50 Gram-positive VRE strains were studied by determining the molar MICs (MICMs) and pH at the MICMs of the VRE. Concentrations of the undissociated and dissociated OAs at the MICMs of the VRE were calculated using the Henderson-Hasselbalch equation. The results for the 50 different VRE strains obtained from different locations within the agri-food system were treated as a single group for each OA evaluated. The pH at the MICMs of all VRE strains for acetic, butyric, formic, and propionic acids was extremely close at 4.66 plus or minus 0.07, but for all six OAs was 1.1 pH unit difference. Inhibition of VRE by all six different OAs did not appear to be solely dependent on pH or on the undissociated OA species, but the VRE inhibition correlated well with the dissociated OA species. The inhibition of VRE by all six dissociated acids was within the range of 16.66 mM to 19.76 mM, delta = 3.1 mM. A small decrease in the concentration of the dissociated OAs from optimum may then result in allowing VRE strains to escape disinfection. If an OA is used to disinfect VRE strains, then the concentration of the dissociated OA should be carefully controlled. A concentration of the dissociated OAs of at least 20 mM should be maintained when disinfecting VRE bacteria.