Research Note Stationary-Phase Acid Resistance and Injury of Recent Bovine Escherichia coli O157 and non-O157 Biotype I Escherichia coli Isolates

Stationary-phase acid resistance and the induction of 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 log CFU/ml, and populations of 24 of these isolates were reduced by less than 1.00 log CFU/ml. In contrast, there was little variation in population reductions when the E. coli were grown with glucose and preadapted 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 the ability to adapt to acidic conditions between E. coli O157:H7 and non-O157 commensal E. coli were not observed. However, we did Ž nd that the E. coli O157 were disposed to greater acid injury after the low pH challenge than the non-O157 E. coli, both for cells that were and were not adapted to acidic conditions before the challenge. The enhancement of low pH survival after acid adaptation 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. Acid resistance of Escherichia coli O157:H7 and other foodborne pathogenic bacteria has been an active research area, because of the effect this resistance may have on the ability of these organisms to survive in acidic foods or to survive acid food processing and preservation treatments and to ultimately be ingested and potentially cause disease. E. coli O157:H7 foodborne infections have been transmitted most often via raw or undercooked beef and milk (13, 15, 19, 29). However, outbreaks of E. coli O157 disease also have been associated with apple cider, fermented sausage, yogurt, and mayonnaise-based dressings, which are products that, all or in part, rely upon low pH and acid for preservation and safety (5, 14, 36; as cited in 48). Survival for extended time has been reported for this organism in acidic foods (11, 22, 24, 33, 35, 42, 48). Furthermore, prior adaptation to acid or low-pH environments may improve the survival of E. coli O157 in acid foods (30, 45). In addition, acid adaptationmay provide cross-protection against the adverse effects of heat, irradiation, various preserva* Author for correspondence. Tel: 402-762-4204; Fax: 402-762-4209; E-mail: [email protected]. † Mention of trade names or commercial products in this article is solely for the purpose of providing speciŽ c information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. ‡ Present address: U.S. Department of Agriculture, Agricultural Research Service, Russell Agricultural Research Center, P.O. Box 5677, Athens, GA 30604, USA. tives, and other processes that are used to inactivate and inhibit microorganisms in foods (7–9, 23, 40, 41). We have previously demonstrated that both the degree of acid resistance of E. coli O157:H7 and the adaptation to acidic conditions by this pathogen can negatively affect the ability of acetic acid spray washes to reduce this organism in beef carcass tissue (4). The group of E. coli O157 strains that were examined in the previous study were a small, generally well-studied collection primarily composed of clinical and ground beef isolates. The objective of the current work was to determine the variation of acid resistance and acid adaptability among those E. coli O157 strains that are likely to contaminate beef carcasses, by assessing the acid resistance characteristics of recent bovine isolates of this pathogen. Recent bovine isolates of non-O157 biotype I E. coli were included in these analyses, for purposes of comparison. MATERIALS AND METHODS E. coli isolates. All E. coli O157 and non-O157 isolates (39 and 20 strains, respectively; Table 1) were recovered from commercially slaughtered beef carcasses during or after the carcass dressing process. The isolation of the E. coli O157 strains has been described elsewhere (21), and selected isolates were used in the current study. Similar sponge and swab procedures were used to sample carcasses for generic E. coli during processing at a local cow and bull processing facility. The samples were transported to J. Food Prot., Vol. 67, No. 3 584 BERRY ET AL. TABLE 1. Populations expressed as log CFU/ml of stationary-phase E. coli O157 and non-O157 E. coli grown with and without 1% glucose after 0 and 6 h of exposure to pH 2.5a E. coli strain, serotype, and/or PFGE subtypeb Length of exposure (h) Cells cultured with 1%