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 2.5, 30,37 or 43 degreesC for 24 h. E. coli O157:1-17 (10(5) CFU/ml) were introduced into the milk pre- and post-fermentation. Fermented milk samples were subsequently stored at either 4 degreesC (refrigerator temperature) or 25 degreesC (to mimic African ambient temperature) for 5 days. After 24 h of fermentation, the pH of the samples fermented at the higher temperatures of 37-43 degreesC decreased from 6.8 to 4.4-4.0 (+/-0.2) whereas at the lower temperature of 25 degreesC, the pH decreased to pH 5.0 +/- 0.1. During this period, viable counts for E. coli O157:H7 increased from 10(5) to 10(8)-10(9) CFU/ml except in milk fermented at 43 degreesC wherein viability declined to 10(4) CFU/ml. In fermented (25-30 degreesC) milk stored at 4 degreesC for 5 days, E. coli O157:H7 viability decreased from 10(8-9) to 10(6-7) CFU/ml whereas milk fermented at 43 degreesC resulted in loss of detectable cells. In contrast, storage of fermented milk samples at 25 degreesC 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 degreesC) survived during 4 degreesC storage, but not 25 degreesC storage. Fermentation 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. (C) 2002 Elsevier Science B.V. All rights reserved.