Acid adaptation of Escherichia coli O157:H7 increases survival in acidic foods

Abstract

Escherichia coli O157:H7 was adapted to acid by culturing for one to two doublings at pH 5.0. Acid-adapted cells had an increased resistance to lactic acid, survived better than nonadapted cells during a sausage fermentation, and showed enhanced survival in shredded dry salami (pH 5.0) and apple cider (pH 3.4). Acid adaptation is important for the survival of E. coli O157:H7 in acidic foods and should be considered a prerequisite for inocula used in food challenge studies. Since 1982, food-related diarrheal outbreaks caused by highly virulent strains of enterohemorrhagic Escherichia coli O157:H7 have been recognized in the United States (11, 16, 20). Enterohemorrhagic strains of E. coli produce Shiga-like toxins, and infections caused by E. coli O157:H7 can result in death, particularly in young children and the elderly (11, 20). Various foods have been implicated in outbreaks, including ground beef, raw milk, apple cider (2, 3, 11, 16, 19, 20), and most recently fermented hard salami (4). Since salami and apple cider rely in part on fermentation and acidity for microbial stability and safety, we were interested in determining if adaptation to acid (6, 12) would affect survival in these low-pH foods. Several investigators have studied acid tolerance and survival of E. coli O157:H7 in broth and food systems (1, 15, 21, 22). In these studies, acid tolerance was determined, but cells were not adapted by growing them at low pH as is required for acid adaptation of Salmonella typhimurium (6, 7, 9). In S. typhimurium, acid adaptation appears to be distinctly different from acid tolerance. However, in E. coli, habituation to normal lethal acidity is induced in stationary-phase cells (12) or by prior growth of E. coli at a sublethal pH value (9). The survival of stationary-phase cells at extremely low pH is termed acid resistance or acid tolerance. Growth pH does not affect the stationary-phase acid resistance of wild-type cells but instead involves the expression of genes mediated by the alternative sigma factor 38 (12, 17, 18). Sigma 38 is not expressed during adaptation during growth at sublethal pHs. Acid adaptation and tolerance have been studied most extensively in S. typhimurium (5, 6, 7, 12), but these characteristics probably also occur in other enteric bacteria, including E. coli and Shigella flexneri (9, 17, 18). We previously showed that acid adaptation increased the resistance of Salmonella spp. to various organic acids and also greatly increased their survival in certain foods (13). Acid adaptation induced cross-protection against environmental stresses that may be encountered during food processing, including heat, salt, an activated lactoperoxidase system, and surface-active agents (14). We investigated the biochemical

Survival of Escherichia coli O157:H7 in the processing and post‐processing stages of acidophilus yogurt

“…Guraya et al (1998) reported that 10 5 CFU mL )1 of E. coli O157:H7 were eliminated at pH 3.8 in milk during 7 days and became undetectable (<10 CFU mL )1 ) at pH 4.1 after 35 days at 4°C. Kotz et al (1990) stated that Ôtheir studies demonstrate the bactericidal effect of yogurt is not solely to the acid pH, as milk acidified to the pH of yogurt (pH 4.4) is bacteriostatic for E. coli but not bactericidal.Õ In addition, acid adaptation of E. coli O157:H7 has been reported previously (Leyer et al, 1995). In the present study pH values at which E. coli O157:H7 were eliminated are in agreement with previous studies, but elimination times of the pathogen were shorter than those in previous studies.…”

Antagonistic effect of acetic acid and salt for inactivatingEscherichia coliO157:H7 in cucumber puree

“…in acidified foods, such as apple cider and orange juice (that have pH values similar to many acidified pickled products) suggest that pickled vegetables may not be free from these pathogens (CDC 1996(CDC , 1999. Escherichia coli O157:H7 could be a pathogen of concern in acidified pickled vegetables because of its low infectious dose (Gorden and Samll 1993;Keene et al 1994), high acid tolerance (Glass et al 1992;Miller and Kaspar 1994;Benjamin and Datta 1995;Leyer et al 1995;Tsai and Ingham 1997) and severe consequences of infection such as hemorrhagic colitis and hemolytic uremic syndrome (Griffin and Tauxe 1991;Padhye and Doyle 1992). The growth or survival of E. coli O157:H7 in acid and acidified foods (pH £ 4AE5) has been reported in a variety of products, including mayonnaise (Weagant et al 1994;Raghubeer et al 1995), apple cider Zhao et al 1993) and fermented dairy products (Arocha et al 1992).…”

Combined effect of mild heat and acetic acid treatment for inactivating Escherichia coli O157:H7, Listeria monocytogenes and Salmonella typhimurium in an asparagus puree

“…Vegetables naturally contain one or more OAs and many vegetables that are commonly pickled are mildly acidic (pH ranging from 5AE1 to 6AE7). Foodborne pathogens that can grow on vegetables may adapt to the mild acidic conditions and develop inducible acid tolerance during growth in an acidic environment (Goodson and Rowbury 1989;Benjamin and Datta 1995;Leyer et al 1995;Gahan et al 1996;Tsai and Ingham 1997;Castanie-Cornet et al 1999;Michino et al 1999;Smittle 2000;Tetteh and Beuchat 2001).…”