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 O157:H7 . FS and FC showed a marked inhibitory activity to growth and production of VT2 by O157:H7 under anaerobic conditions. They may have both bacteriostatic and bactericidal effects on O157. The growth of O157 was markedly suppressed by acetic, propionic and butyric acids compared with hydrochloric acid and lactic acid at concentrations between 25 m and 40 m, being proportional to the pH values. At pH 5.5, 40 m of short-chain fatty acids (SCFAs) almost completely inhibited the growth of O157. SCFAs markedly inhibited the growth of O157 at pH 6.0 rather than pH 7.0. Propionic acid is likely to be more suppressive to than acetic and butyric acids. The production of VT2 was approximately proportional to the growth of O157. However, incubation for 24 h showed that the growth and VT2 production of 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 m, 60:25:25, respectively, in molar ratio). It is probable that the colonic microflora could contribute to a reduction of O157:H7 infections 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 O157 infections.


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