Toxigenic Bacillus cereus isolated from Nunu and Wara, two Nigerian fermented dairy foods

Abstract

Traditional fermented dairy foods are produced by small scale processors in different parts of West Africa without adequate attention to good manufacturing practices (GMPs), microbial contamination and associated safety risks are very common. In this study, bacterial population in two Nigerian traditional fermented dairy foods was examined. Analysis of 50 samples tested showed that aerobic viable count of wara was 1.02 x 107 and nunu, 1.11 x 108 cfu/ml. Further enrichment using chromogenic selective agar medium detected Bacillus cereus in 95% of the tested samples. The mean count of 9.8 x 106 and 1.5 x 106 cfu/ml were observed for wara and nunu respectively. Amongst the B. cereus isolates, 86% obtained from wara and 95% from nunu exhibited haemolytic activity identified as β haemolysis on 5% sheep blood agar plates. Reverse Passive latex agglutination-Bacillus cereus enterotoxin (RPLA-BCET) analysis detected diarrhoea toxin type among 60% of wara and 50% of nunu B. cereus isolated from the two fermented foods. There was also detectable in situ toxin extracted directly from fermented samples. Toxigenic B. cereus W5 strain was inoculated into fermented wara and monitored at 4°C, cell counts reached 2.6 x 108 cfu/ml after three days incubation. In the in vitro bio-control assay, B. cereus growth and enterotoxins production were inhibited by antimicrobial produced by B. amyloliquefaciens UL168 in plate antagonistic spot-on-lawn assay. This  information is relevant to the safety challenges of pathogenic bacilli frequently  associated with traditional fermented foods in Africa.

Keywords: Fermentation, B. cereus, enterotoxin, wara, nunu, toxin.