STATIONARY-PHASE ACID RESISTANCE AND INJURY OF RECENT BOVINE ESCHERICHIA COLI O157 AND NON-O157 BIOTYPE I ESCHERICHIA COLI ISOLATES

Authors: Berry, Elaine; Gallagher, Genevieve; Siragusa, Gregory

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2003
Publication Date: 3/1/2004
Citation: BERRY, E.D., GALLAGHER, G.A., SIRAGUSA, G.R. STATIONARY-PHASE ACID RESISTANCE AND INJURY OF RECENT BOVINE ESCHERICHIA COLI O157 AND NON-O157 BIOTYPE I ESCHERICHIA COLI ISOLATES. JOURNAL OF FOOD PROTECTION. 2004. v. 67. p. 583-590.

Interpretive Summary: Exposure to low pH and acids may cause adaptation to acidic conditions by Escherichia coli O157:H7 that contaminate beef carcasses. In addition, spray washes with acids such as acetic or lactic acid often are used to decontaminate fresh beef carcasses. Thus, we sought to determine both the acid resistance and the ability to develop acid resistance of a collection of fifty-nine E. coli that were recently isolated from beef carcasses. This collection included both E. coli O157 and nonpathogenic E. coli. The E. coli were subjected to stringent acid challenges following their cultivation to stationary phase under conditions that either did or did not allow adaptation to acidic conditions, and their survival following the acid challenges was determined. Survival of the acid challenge was greatly enhanced when the E. coli were first grown under conditions that allowed adaptation to acidic conditions. There was very little variation in population reductions, with 57 of the 59 isolates exhibiting extreme acid resistance. When the E. coli were not previously adapted to acidic conditions, there was a greater range of survival responses. However, most of the E. coli isolates still survived in high numbers. Finally, large differences in acid resistance or ability to adapt to acidic conditions between E. coli O157 and non-O157 nonpathogenic E. coli were not observed. The enhancement of acid survival that was seen among these recent natural isolates of E. coli O157 further supports the idea that the previous environment of this pathogen should be a consideration when designing microbial safety strategies for foods preserved by low pH and acid.

Technical Abstract: Stationary-phase acid resistance and the ability to induce acid resistance were assessed for recent bovine carcass isolates of Escherichia coli, including 39 serotype O157 strains and 20 non-O157 strains. When grown to stationary phase in the absence of glucose and without prior acid exposure, there was a range of responses to a pH challenge of 6 h at pH 2.5. However, populations of 53 of the 59 E. coli isolates examined were reduced by less than 2.00 log10 CFU/ml, and populations of 24 of these isolates were reduced by less than 1.00 log10 CFU/ml. In contrast, there was little variation in population reductions when the E. coli were grown with glucose and pre-adapted to acidic conditions. With few exceptions, acid adaptation improved survival to the acid challenge, with 57 of the 59 isolates exhibiting a log reduction of less than 0.50. Differences in acid resistance or ability to adapt to acidic conditions between E. coli O157:H7 and non-O157 commensal E. coli were not observed. The enhancement of low pH survival that was seen among these recent natural isolates of E. coli O157 further supports the idea that the previous environment of this pathogen should be a consideration when designing microbial safety strategies for foods preserved by low pH and acid.