Survival of Escherichia coli O157:H7 in traditional African yoghurt fermentation

Growth and survival of a nontoxigenic strain of Escherichia coli O157:H7 (ATCC 43888) was determined in traditionally fermented pasteurized milk. Preheated milk was inoculated with 1% (v/v) of a mixed culture of Lactobacillus delbrueckii ssp. bulgaricus (NCIMB 11778) and Streptococcus salivarius ssp. thermophilus (NCIMB 110368) and incubated at 25, 30, 37 or 43 °C for 24 h. E. coli O157:H7 (105 CFU/ml) were introduced into the milk pre- and post-fermentation. Fermented milk samples were subsequently stored at either 4 °C (refrigerator temperature) or 25 °C (to mimic African ambient temperature) for 5 days. 24 h of fermentation, the pH of the samples fermented at the higher temperatures of 37–43 °C decreased from 6.8 to 4.4–4.0 (±0.2) whereas at the lower temperature of 25 °C, the pH decreased to pH 5.0±0.1. During this period, viable counts for E. coli O157:H7 increased from 105 to 108–109 CFU/ml except in milk fermented at 43 °C wherein viability declined to 104 CFU/ml. In fermented (25–30 °C) milk stored at 4 °C for 5 days, E. coli O157:H7 viability decreased from 108–9 to 106–7 CFU/ml whereas milk fermented at 43 °C resulted in loss of detectable cells. In contrast, storage of fermented milk samples at 25 °C for 5 days eventually resulted in complete loss of viability irrespective of fermentation temperature. Stationary phase E. coli O157:H7 inoculated post-fermentation (25 and 43 °C) survived during 4 °C storage, but not 25 °C storage. tation temperature and subsequent storage temperature are critical to the growth and survival of E. coli O157:H7 in traditional fermented products involving yoghurt starter cultures.