Survival of gram-negative bacterial pathogens during maize fermentation and in fermented maize weaning food

Abstract

The behaviour of some important food borne bacterial pathogens was studied during maize fermentation and in two fermented maize products. Five Salmonella serotypes, two Shigella flexneri, one Shigella dysenteriae and three VTEC serotypes, one each of enteropathogenic (EPEC) and entero-aggregative (EAggEC) Escherichia coli were inoculated into maize steep water, maize dough, koko and kenkey water, respectively, at a concentration 10⁶ - 10⁷ cfu ml‾1 at ambient temperature of about 28ᵒC. Enumeration was carried out using selective and non selective media. Almost all the bacteria survived in steep water 48 h although there was a reduction in pH. No Salmonella serotypes were detected in dough after 48 h of maize fermentation, when pH fell below 4.0. Some resistant Shigella and E. coli strains survived in the dough after 48 h, but reduced in numbers by more than 3.4 log units. None of the salmonellae was detected after 24 h in koko (pH 3.6) and kenkey water (pH 3.6). Shigella and E. coli were inactivated in kenkey water after 24 h. However, Sh. flexneri II and four E. coli survived in koko for 48 h with less than 2 log reductions. Greater numbers of survivors were obtained with non-selective than with a newly developed selective medium designed to recover Gram-negative bacterial pathogens. Significant differences in the ability to survive maize fermentation and in koko and kenkey water were observed for the species of Salmonella, Shigella and E. coli. The most resistant were Shigella and E. coli. The risk posed by survival of some acid resistant Shigella and E. coli strains was demonstrated for fermented maize products particularly koko