Impact of injury and stress

Exposure of microorganisms to sublethal stresses has long been known to injure them. This could have the effect of allowing an overestimation of the lethality of the process when viable, injured cells are not recovered and are presumed dead. Microorganisms are also known to adapt to stressful environments and even become more resistant to the stress. Exposure to physical and chemical stresses encountered during food processing, such as heat, pressure, increased osmolality, or weak organic acids can cause an adaptive response. Exposure to one stress (e.g. low pH) may induce cross-protection to another stress (such as heat). This is particularly true for microorganisms stressed due to starvation in the stationary phase of growth. This phenomenon is a concern because parameters for minimal processes that are based on the response of non-resistant microorganisms may be inadequate for these microorganisms once their stress response mechanisms have become activated by the process. Davidson and Harrison (2002) reviewed the potential for antimicrobials and sanitizers used in food processing to impart resistance to microorganisms. There has not been much evidence generated to demonstrate acquired resistance to food antimicrobials or sanitizers. However, since the stress response is known to occur in the laboratory, more research is needed into the frequency and mechanism of resistance, mechanisms of action of antimicrobials and development of strategies to prevent the acquisition of resistance to stresses in the microbial ecology of food.

Archer (1996) stated that traditional food preservation systems work well to inhibit the growth of toxin-producing bacteria such as S. aureus or C. botulinum that require relatively high numbers for the toxin to cause disease. However, he expressed concern that infectious bacteria such as E. coli 0157:H7 and strains of Salmonella may increase in virulence during stressful conditions of food preservation. Stresses such as starvation and extremes of temperature, pH, and osmolarity cause adaptive responses, one of which may be to potentiate expression of virulence genes or, even worse, create unpredictable mutations in the virulence genes. To date there is little evidence that this occurs in food production, but it warrants vigilance.

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