Increased emergence of antimicrobial-resistant pathogens, including Salmonella, has become a public health concern worldwide especially considering the potential for increased rates in morbidity and mortality (Chen et al., 2004). The routine use of antimicrobials in domestic livestock for disease treatment and growth promotion could potentially lead to widespread dissemination of antimicrobial-resistant bacteria (Tollefson et al., 1997) and transfer to humans through the food chain (Angulo et al., 2004). Ground meats are more likely to be internally contaminated with pathogenic bacteria and indeed antibiotic-resistant Salmonella have been isolated with increasing frequency from retail ground beef (Poppe et al., 2006; Talbot et al., 2006; Varma et al., 2006; White et al., 2001) and implicated in outbreaks due to consumption of the same (CDC, 2002; Mclaughlin et al., 2006).
Federal regulations have addressed Salmonella contamination of fully-cooked meat products, calling for a 6.5-log reduction in beef (USDA-FSIS, 1999), however, the pathogen is still found in cooked meat (USDA-FSIS, 2005). Furthermore, the U.S. Department of Agriculture Food Safety and Inspection Service (USDA-FSIS, 1999) has recommended that ground beef be cooked to an internal temperature of 71.1°C since it is a hazard likely to occur and salmonellae attached to meat surfaces are more resistant to heat than those unattached or dispersed in a food or liquid (Doyle and Mazzotta, 2000). Although, regulators have defined critical limits for pathogen reduction in cooked beef, different strains of bacteria react differently to identical heat stress (Bacon et al., 2003; Ng et al., 1969). Various factors can influence the heat resistance of bacteria including strain, metabolic phase, nutrient availability, growth conditions (temperature), intrinsic and extrinsic factors of the substrate, and previous exposure to the same or unrelated stresses.
The ability of bacteria to exhibit increased resistance when either the same or a seemingly unrelated stress is reapplied, cross protection, has been studied extensively (Rowe and Kirk, 1999; Samelis et al., 2001; Stopforth et al., 2004). Exposure of bacteria to environmental pressures, such as the presence of antibiotics, has caused them to develop survival strategies to survive in the presence of the stress. Microorganisms respond to these stresses by inducing specific sets of proteins to protect against damage (VanBogelen et al., 1987). Induction of stress proteins by exposure to nonlethal levels of a stress (i.e., the application of subtherapeutic levels of antibiotics) may confer protection to subsequent exposure to otherwise lethal levels of the same or unrelated stress (Christman et al., 1985; Yamamori and Yura, 1982). There is sparse and conflicting evidence of the relationship between the antimicrobial resistance profile of Salmonella and heat resistance. Bacon et al. (2003) found no relationship between antimicrobial resistance and heat resistance while Walsh et al. (2005) found that multidrug-resistant S. Typhimurium DT104 had higher heat resistance than susceptible S. Typhimurium and S. Enteritidis.
The stated objectives for this work were:
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