Growth and survival of E. coli O157:H7 during the manufacture and ripening of a smear-ripened cheese produced from raw milk.

Abstract

The behaviour of Escherichia coli O157:H7 was studied during the manufacture and ripening of a smear-ripened cheese produced from raw milk. Cheese was manufactured on a laboratory scale using milk (20 l) inoculated with E. coli O157:H7, and enumeration was carried out using CT-SMAC. From an initial level of 1.52 +/- 0.03 log cfu ml-1 in the milk (34 +/- 2 cfu ml-1), the numbers increased to 3.4 +/- 0.05 log cfu g-1 in the cheese at day 1. During ripening, the numbers decreased to <1 cfu g-1 and <10 cfu g-1 in the rind and core, respectively, after 21 days, although viable cells were detected by enrichment after 90 days. The presence of E. coli O157:H7 in the cheese was confirmed by latex agglutination and by multiplex PCR. The results indicate that the manufacturing procedure encouraged substantial growth of E. coli O157:H7 to levels that permitted survival during ripening and extended storage. The presence of low numbers of E. coli O157:H7 in milk, destined for raw milk cheese manufacture, could constitute a threat to the consumer.