Inhibitory Effects of Volatile Fatty Acids and Low pH on the Multiplication of Enteric PathogensIn Vitroin Caecal Contents of Mice

Abstract

The inhibitory effect of volatile fatty acids (VFA) and low pH on the multiplication of Shigella sonnei 3SR and enterotoxigenic Escherichia coli 2SR (ETEC 2SR) in caecal contents of mice was determined. Multiplication rates and population sizes of both pathogens were greater in contents obtained from streptomycin treated mice at pH 6.94 containing 78.32 μeq total VFA per gram than in contents from untreated conventional mice at pH 6.66 containing 109.11 μeq total VFA per gram. When contents from streptomycin treated mice were adjusted to pH 6.66 and to contain 109.11 μeq total VFA per gram, values observed in contents from untreated mice, the multiplication rates and population sizes of both pathogens were lower in the adjusted contents than in unadjusted contents. Lowering the pH alone without VFA addition did not influence the multiplication of ETEC 2SR but caused a moderate decrease in the multiplication rate and population sizes of S. sonnei 3SR. These effects, however, were not as pronounced as when VFA were also added. Conversely, neutralisation of VFA present in caecal contents from untreated mice by increasing the pH from 6.66 to 7.50 reversed inhibition. Growth curves of both S. sonnei 3SR and ETEC 2SR in contents at high pH were almost identical to curves obtained when the organisms were multiplying in contents from streptomycin treated mice. The results provide strong evidence that the combined effects of high concentrations of VFA and low pH in caecal contents of conventional mice are important determinants of colonisation resistance against enteric pathogens in mice. Keywords: Volatile fatty acids; pH; Enteric pathogens; Caecal contents; Mice.