Interactive Effects of Temperature, pH, and Water Activity on the Growth Kinetics of Shiga ToxinÂŒProducing <i>Escherichia coli</i> O104:H4 3

The risk of non-O157 Shiga toxin–producing Escherichia coli strains has become a growing public health concern. Several studies characterized the behavior of E. coli O157:H7; however, no reports on the influence of multiple factors on E. coli O104:H4 are available. This study examined the effects and interactions of temperature (7 to 46uC), pH (4.5 to 8.5), and water activity (aw; 0.95 to 0.99) on the growth kinetics of E. coli O104:H4 and developed predictive models to estimate its growth potential in foods. Growth kinetics studies for each of the 23 variable combinations from a central composite design were performed. Growth data were used to obtain the lag phase duration (LPD), exponential growth rate, generation time, and maximum population density (MPD). These growth parameters as a function of temperature, pH, and aw as controlling factors were analyzed to generate second-order response surface models. The results indicate that the observed MPD was dependent on the pH, aw, and temperature of the growth medium. Increasing temperature resulted in a concomitant decrease in LPD. Regression analysis suggests that temperature, pH, and aw significantly affect the LPD, exponential growth rate, generation time, and MPD of E. coli O104:H4. A comparison between the observed values and those of E. coli O157:H7 predictions obtained by using the U.S. Department of Agriculture Pathogen Modeling Program indicated that E. coli O104:H4 grows faster than E. coli O157:H7. The developed models were validated with alfalfa and broccoli sprouts. These models will provide risk assessors and food safety managers a rapid means of estimating the likelihood that the pathogen, if present, would grow in response to the interaction of the three variables assessed. Illnesses caused by Shiga toxin–producing Escherichia coli (STEC), both O157 and non-O157 strains, continue to be a significant concern to the food industry, regulatory agencies, and the public. This pathogen can cause both sporadic cases and outbreaks of diarrhea with abdominal cramping that may lead to potentially life-threatening hemorrhagic colitis as well as hemolytic-uremic syndrome, especially in persons who have a weak immune system. E. coli O157:H7 is the most common predominant serotype associated with the most severe form of the disease (26). However, illnesses and disease outbreaks linked to nonO157 STEC strains outnumber O157 STEC cases in the United States (6, 7, 12). In a report from the Centers for Disease Control and Prevention, Scallan et al. (25) estimated that 175,000 illnesses are caused by foodborne STEC infections per year, with 64% of these infections linked to non-O157 STEC. A large number of non-O157 STEC– associated illnesses and disease outbreaks have been reported in several countries, including the United States, Australia, Japan, and Europe (22). In Germany, a large outbreak due to STEC O104:H4 implicating sprouts as the vehicle of transmission that resulted in 3,842 cases of illness was reported in 2011 (4). Thus, there has been a growing public health concern about the risk of non-O157 STEC strains. Ruminants can be colonized and are recognized as asymptomatic carriers of both O157 and non-O157 STEC, with cattle being the natural, principal reservoir (5, 23). Counts of STEC as high as 8 log CFU/g have been found in the excreta of colonized cattle (3, 17). The likelihood of infections in humans with non-O157 STEC is significantly higher than that of infections with O157 STEC because of the high prevalence of the former in animals and as contaminants in foods (27, 28). Infections may occur in humans when they come in contact with infected animals or consume inadequately cooked beef, vegetables, and dairy products contaminated by the feces of infected cattle. Many studies have reported the prevalence, detection, and characterization of non-O157 STEC (1, 2, 6, 13–16, 24). * Author for correspondence. Tel: 215-233-6500; Fax: 215-233-6697; E-mail: vijay.juneja@ars.usda. gov. { Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of