Foodborne pathogens face a broad spectrum of stresses in all links of the food chain [1]. During traditional food processing (e.g., pasteurization), microbial cells are more likely to be killed than injured or stressed [2]. Recently, some novel minimal processing techniques have been developed with less deleterious effects on food quality; however, they may constitute mild stresses, which are detrimental to most of the cells but may also enhance the generation of cells with increased resistance [3]. Microorganisms can tolerate small changes in environmental parameters through inducing adaptive responses [4]. Among the known foodborne outbreaks, there is an increasing involvement of stress-adapted strains, which are difficult to control with traditional intervention strategies [5]. Adaptive responses of foodborne pathogens to stresses are thus of paramount significance in food safety.

Microorganisms in the natural environment are exposed to a wide range of stresses. Pathogens in foods are also frequently exposed to stresses with varying magnitudes [6]. Stresses to these microorganisms in foods during processing include physical stresses, such as heat, high pressure, desiccation, and irradiation, chemical stresses, such as acids, salts, and oxidants, and biological stresses, such as microbial antagonism [7].

Once foodborne pathogens sense a certain stress, the microbial cells respond in various ways. Different microorganisms appear to have evolved different mechanisms to cope with environmental stresses [8]. Complicated changes in cell composition and physiological state may occur due to the exposure to stressful environments. Such responses enable foodborne pathogens to maintain their normal functions and thus survive in foods during processing. For example, most Salmonella strains possess the ability to form fimbria- and cellulose-mediated colony under some harsh conditions [9]. This multicellular phenomenon is termed the rdar morphotype (colonies are “red, dry, and rough” when grown on media containing Congo red), which can be isolated from poultry and produce [10]. The rdar morphotype can enhance the resistance of Salmonella to desiccation and starvation [11].

Heat treatment is one of the most commonly used food processing methods [12]. When properly used, heat can eliminate foodborne pathogens from foods. However, some microbial cells may become heat-shocked under sublethal heat stress [13]. In this case, microorganisms can synthesize heat shock proteins, which renders the cells more resistant to subsequent high temperatures normally considered to be lethal [14]. The heat shock response and induced thermotolerance have been reported in a wide range of foodborne pathogens [15]. Responses of foodborne pathogens to heat stress have been a concern, as the adapted pathogenic cells in foods surviving during thermal processing may pose potential health risks to consumers.

Microbial cells adapted to a sublethal stress may exhibit enhanced survival on subsequent exposure to a different stress. This tolerance to multiple stresses after adaptation to an individual stress is called crossprotection [16]. Several stresses, such as heat stress, cold stress, acid stress, and osmotic stress, have been reported to induce crossprotection in microorganisms [3]. Cross-protection has been found to be mediated by the rpoS gene. In some bacteria, the rpoS gene encodes an alternative sigma factor (σS/RpoS) that acts as a master regulator needed for the survival under stressful conditions [17]. The rpoS gene has been identified to be involved in the cross-protection of desiccation-adapted Salmonella typhimurium against high temperatures [18].

The current trend toward the application of mild minimal food processing techniques may result in more stress responses of foodborne pathogens. In view of this, cross-protection has significant implications in ensuring food safety and establishing risk assessment programs, as it will markedly enhance the survival of foodborne pathogens during food processing containing multiple barriers. Therefore, cross-protection of foodborne pathogens should be taken into consideration when assessing the effectiveness of different combinations of minimal food processing techniques.

Stress responses of foodborne pathogens have a much greater impact on food safety than has already been recognized. As the consumers demand foods with higher food quality, the food industry is applying cumulative mild processing steps for the control of pathogens in foods. In turn, this trend facilitates more frequent exposure of foodborne pathogens to sublethal stresses, potentially compromising food safety through inducing resistance responses and crossprotection.

Currently, most of the food processes are tested by inoculation with freshly harvested cells. However, many published studies have recommended that stressed cells should be used when performing challenge studies and validating food processing procedures [19-22], since the use of fresh cultures may not represent the actual survival characteristics under real-world environments.

Future research on the stress responses of foodborne pathogens could be conducted with advanced modern genetic tools. For instance, it is possible to create isogenic mutants of foodborne pathogens to identify the genes involved in specific or general stress responses. Moreover, there are available complete genome sequences and amino acid sequences of expressed open reading frames (ORFs) in foodborne pathogens [23]. This will enable the identification of genes and proteins involved in stress responses, as well as the evaluation of their importance in the physiology of these microorganisms.