Influence of intestinal anaerobes and organic acids on the growth of enterohaemorrhagic Escherichia coli O157:H7

A suspension of human faeces (FS) and its anaerobic culture (FC), bacterial metabolic products and organic acids were examined for inhibitory effects on growth and verotoxin 2 (VT2) production of Escherichia coli O157:H7 in vitro. FS and FC showed a marked inhibitory activity to growth and production of VT2 by E. coli O157:H7 under anaerobic conditions. They may have both bacteriostatic and bactericidal effects on E. coli O157. The growth of E. coli O157 was markedly suppressed by acetic, propionic and butyric acids compared with hydrochloric acid and lactic acid at concentrations between 25 mM and 40 mM, being proportional to the pH values. At pH 5.5, 40 mM of short-chain fatty acids (SCFAs) almost completely inhibited the growth of E. coli O157. SCFAs markedly inhibited the growth of E. coli O157 at pH 6.0 rather than pH 7.0. Propionic acid is likely to be more suppressive to E. coli than acetic and butyric acids. The production of VT2 was approximately proportional to the growth of E. coli O157. However, incubation for 24 h in vitro showed that the growth and VT2 production of E. coli O157 decreased but were not completely inhibited at pH 6.5 and 7.0 in a mixture of acetic, propionic and butyric acids at a physiological concentration (110 mM, 60:25:25, respectively, in molar ratio). It is probable that the colonic microflora could contribute to a reduction of E. coli O157:H7 infections via the activation of intestinal fermentation by dietary manipulation or something similar to give pH 6.0 or <6.0 and that factors such as age, chemical therapy and body condition, which have effects on the balance of the intestinal microflora, would be associated with the incidence rates of E. coli O157 infections.