Microorganisms in fresh ground meats: the relative safety of products with low versus high numbers.

The two outbreaks of haemorrhagic colitis (HC) that were traced to ground beef in 1982 were the first foodborne cases known to be caused by Escherichia coli 0157:H7. The 1993 outbreak in the U.S. Pacific Northwest is the largest foodborne disease outbreak ever traced to ground beef. Why these events occurred continues to be a matter of speculation and debate. It is the thesis of this review that HC-causing strains of E. coli, which could have been in the meat supply as early as the mid-1950s, can persist in meats that contain too few of the background bacterial biota. The antagonistic effect of background organisms against pathogenic bacteria (microbial interference) is well established. Fresh ground meats that contain 10(5)-10(6)/g of background organisms are inherently safer than those that contain, say, 10(3)/g. Although the production of fresh ground meats with as few microorganisms as possible would seem to be the ideal, there is little or no evidence to support the superior safety of such products. It is suggested that when pathogen-reduction strategies are applied to animal carcasses, the carcasses should be 'protected' against subsequent colonization by pathogens by actually adding appropriate mixtures of harmless bacteria.