Inhibition of enterotoxin production by, and growth of enteropathogens in a lactic acid-fermenting cereal gruel

Growth and enterotoxin production of three strains of Campylobacter jejuni or Escherichia coli known to produce heat labile enterotoxins were evaluate d when added into a lactic acid-fermenting cereal gruel, togwa. A single st rain of each of the enteropathogens was simultaneously inoculated with a la ctic acid starter culture (sc) to reach a level of about 10(7) c.f.u./ml an d was left to ferment for 48 h. Gruels without sc (control gruel), pure cho lera toxin in fermenting or control gruel and the test bacteria inoculated into nutrient broth were used as positive toxin controls; gruel without ino culated test bacteria was the toxin-negative control. Viable colonies were counted by spread plating 0.1 ml of gruel subsamples collected at intervals during the fermentation period onto different selective media and the heat labile enterotoxin production was evaluated using an assay on Chinese hams ter ovary (CHO) cells. In the fermenting gruels, no viable cells of the C. jejuni and E. coli strains were detected after 8 and 24 h incubation, respe ctively, but inocula increased in number or remained at the initial level i n the control gruel and in the nutrient broth. After 24 h incubation, all s upernatants of control gruels with inoculated bacteria showed enterotoxicit y to the CHO cells (indicated by elongation of 20-50% of the cells). No tox in activity was observed in the fermenting gruels with or without added bac teria or in control gruel alone. Pure cholera toxin added to control gruel caused the enterotoxigenic effect in 70-100% of the CHO cells, but no activ ity was detected when it was added to the fermenting gruel. The CHO cells w ere affected instead by a low pH level but were not elongated. Adjusting th e pH of fermented gruels to approximately neutral levels restored the toxin effect when cholera toxin was added but not in the presence of added test bacteria. We conclude that lactic acid fermented cereal gruels with pH I 4 have a high potential to inhibit the growth of enteropathogenic bacteria of the genera C. jejuni and E. coli and to inhibit production of heat labile enterotoxins. Regular consumption of fermented cereal weaning foods will th erefore reduce transmission of enterotoxin-producing bacteria, and ingestio n of enterotoxins.