Stress-Adapted, Cross-Protected, Resistant: A Concern?

Reports of microbial foodborne illnesses and contaminated product recalls seem to be increasing in recent years, as is the variety of foods serving as vehicles of pathogen transmission. Documentation of the transmission through food of several pathogens (e.g., Escherichia coli O157:H7, Listeria monocytogenes, Campylobacter, etc.) has occurred only in the past 20–30 years.

Among the factors that may contribute to recent food safety concerns is the ability of pathogens to develop adaptive responses and resistances when exposed to sublethal stresses. Adaptation and resistance development may allow pathogens to survive common food processing and preservation treatments and cause human foodborne illness. Examples of resistant pathogens are the multi-drug-resistant Salmonella Typhimurium DT104 and the acid tolerant E. coli O157:H7. The issue of microbial adaptation to stresses and resistance development is the subject of the IFT Scientific Status Summary prepared by P. Michael Davidson and Mark Harrison that appears on pages 69–78.

Research indicates that bacteria possess systems that alert them to the presence of stresses and allow them to activate survival mechanisms. Exposure of bacteria to stresses (e.g., cold, heat, acid, low moisture, and antimicrobials) at sublethal levels may occur in the general environment as well as in foods, which leads to the following concern: Could stress-exposed bacteria in food become “stress-hardened” and able to survive common food processing and preservation systems?

Exposure of bacteria to sublethal stresses is possible during food processing. For example, the manufacture of many foods uses multiple sublethal “hurdles” that work together to yield stable and safe products. However, there is a concern that reduction of hurdle intensities to meet consumer demands for less-processed foods may cause stress-adaptation and result in the failure of hurdles to assure pathogen control and food safety.

Additional processes that may be sublethal to microorganisms and lead to stress-adaptation include food tempering, slow cooling, prolonged drying, lengthy refrigerated storage, and reduced chemical levels. The increasing use of interventions to reduce microbial contamination on produce and animal carcasses, as well as the increased use of antimicrobials in sanitizing solutions and soaps, also may lead to selection for stress-adapted pathogens. Spraying carcasses with hot water or produce with disinfectants, for example, may shock microorganisms, while other processes may lead to adaptation through residual exposure (e.g., chemical) during storage.

In this timely and well written Scientific Status Summary, Davidson and Harrison explore the potential for the development of resistance against food antimicrobials and processing equipment sanitizers, and they examine the relationship between resistance to antimicrobials and environmental controls commonly used by food manufacturers. As indicated by the authors, available information on bacterial stress-adaptation and resistance development in foods is rather limited, and conclusions as to whether these results commonly occur therefore remain elusive. Because evidence indicates that bacteria can acquire resistance or develop tolerance to environmental stresses, there is a concern that this might provide pathogens with the means to survive exposure to antimicrobials used in food manufacture.

Application of treatments to reduce pathogen contamination and inhibit their growth is useful. However, sublethal stresses must be evaluated for potential short- and long-term effects, including induction of adaptation, resistance and cross protection, and alterations in food microbial ecology. Use of antimicrobial hurdles must be optimized to avoid creating potential food safety risks. Of interest, the number of laboratory studies showing stress-adaptation, resistance, and cross protection development in pure culture bacteria is increasing. However, work to evaluate such phenomena in foods is just beginning.

Research should be aimed at determining the proper sequence, timing, and application intensity for antimicrobial treatments to avoid adaptation and resistance development. It may be possible to optimize the use of hurdles by developing processing schemes that do not lead to stress-adaptation, but instead induce cell sensitization to subsequent hurdles, and thus lead to inactivation through metabolic exhaustion. This preferred approach—avoiding resistance development and producing pathogen-free ready-to-eat products—would apply processes at intensities that are lethal to anticipated levels of pathogens. This approach, however, will limit the variety of foods available to consumers.

In summary, many food processing methods have the potential to cause microbial shock, adaptation, or both, and to lead to cross protection and resistance development. This issue needs attention because stress-hardened pathogens, which may be of increased virulence, also may be difficult to control. Objectives of pathogen control strategies should include efforts to minimize the potential for resistance development during food processing and handling and to enhance control of any potentially resistant or stress-adapted pathogens that may be present in foods.

by JOHN N. SOFOS
Professor, Dept. of Animal Sciences
Colorado State University, Fort Collins