Adaptation of Microorganisms to Cold Temperatures, Weak Acid Preservatives, Low pH, and Osmotic Stress: A Review.

The application of physical stress to microorganisms is the most widely used method to induce cell inactivation and promote food stability. To survive, microorganisms have evolved both physiological and genetic mechanisms to tolerate some extreme physical conditions. This is clearly of significance to the food industry in relation to survival of pathogens or spoilage organisms in food. In some microorganisms, the “cold shock response” has been observed in response to abrupt changes to lower temperatures. This results in the production of specific sets of proteins (cold shock proteins), the continued synthesis of proteins involved in transcription and translation, and the repression of heat shock proteins. The addition of weak acid preservatives (for example, sorbates, benzoates) also induces a specific pattern of gene expression (for example, ‘Acid Tolerance Response’), which is likely to be required for optimal adaptation of bacteria to weak acid preservatives and low pH. The primary mode of the antimicrobial action of low pH is to reduce the internal cell pH (pHi) below the normal physiological range tolerated by the cell, leading to growth inhibition. Survival mechanisms involve maintaining pH homeostasis, and this is achieved by a combination of passive and active mechanisms. Microorganisms adapt to osmotic stress by accumulating non-ionic or compatible solutes such as trehalose, glycerol, sucrose, and mannitol. These compatible solutes help balance the osmotic pressure and help preserve protein function inside the cells. By understanding and controlling such mechanisms of adaptation, it may be possible to prevent growth of key microorganisms in food products.