Stress Adaptation and the General Stress Response

Exposure of a microorganism to stress triggers a series of metabolic responses that may adapt the cell to subsequent lethal levels of the same type of stress or to multiple lethal stresses. The cell's adaptive response is generally referred to as stress adaptation. Stress, ensuing adaptive response, and the manifestation of this phenomenon in food preservation are collectively described as stress hardening. Food microbiota are regularly subjected to stress hardening. Therefore, the stress adaptation phenomenon is of paramount importance when evaluating the efficacy of intervention strategies to achieve food safety and to preserve food quality. Although stress adaptation is usually associated with the undesirable acquired resistance of foodborne pathogens to processing, this phenomenon also plays a key role in the survival of beneficial microorganisms used as probiotics or fermentation starters.

The microbial mechanisms to survive adverse environmental conditions can be divided into two classes, consisting of limited and multiple adaptive responses [8]. A limited or specific adaptive response results when microorganisms are exposed to a sublethal dose of a physical, chemical, or biological stress, which protects cells against subsequent lethal treatment with the same stress [8,9]. A multiple adaptive response, also known as cross protection, occurs when microbial cells adapt to an inherent physiological condition or to an environmental factor, which results in protection against subsequent lethal treatments, including stresses to which the microorganism had not been previously exposed [8,10-12]. This cross-protective response requires the induction of the general stress response, and it is triggered by stresses relevant to produce, both preharvest and postharvest, including cell starvation, exposure to high or low temperatures, high osmolarity, and low pH [12,13]. The activation of the general stress response is characterized by reduced growth rate or induced entry into stationary phase. The regulation of the general stress response has been well characterized in several microorganisms. This regulation is under the control of the alternative sigma factors, which bind to core RNA polymerase, mediating cellular responses through redirection of transcription initiation. Sigma S (aS, also known as RpoS) and aB regulate the general stress response in Escherichia coli and other Gram-negative bacteria, and in Bacillus subtilis and other Gram-positive bacteria, respectively [13,14].

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