Volatile Fatty Acids Effective as Antibacterial Agents against Three Enteric Bacteria during Mesophilic Anaerobic Incubation

Abstract

The antibacterial effects of a selection of volatile fatty acids (acetic, propionic, butyric, valeric, and caproic acids) relevant to anaerobic digestion were investigated at 1, 2 and 4 g/L. The antibacterial effects were characterised by the dynamics of <i>Enterococcus faecalis</i> NCTC 00775, <i>Escherichia coli</i> JCM 1649 and <i>Klebsiella pneumoniae</i> A17. Mesophilic anaerobic incubation to determine the minimum bactericidal concentration (MBC) and median lethal concentration of the VFAs was carried out in Luria Bertani broth at 37 °C for 48 h. Samples collected at times 0, 3, 6, 24 and 48 h were used to monitor bacterial kinetics and pH. VFAs at 4 g/L demonstrated the highest bactericidal effect (<i>p</i> < 0.05), while 1 g/L supported bacterial growth. The VFA cocktail was the most effective, while propionic acid was the least effective. <i>Enterococcus faecalis</i> NCTC 00775 was the most resistant strain with the VFAs MBC of 4 g/L, while <i>Klebsiella pneumoniae</i> A17 was the least resistant with the VFAs MBC of 2 g/L. Allowing a 48 h incubation period led to more log decline in the bacterial numbers compared to earlier times. The VFA cocktail, valeric, and caproic acids at 4 g/L achieved elimination of the three bacteria strains, with over 7 log₁₀ decrease within 48 h.