| Popis: |
Most microorganisms must accommodate a variety of changing conditions and stresses in their environment in order to survive and multiply. Because of the impact of temperature on all reactions of the cell, adaptations to fluctuations in temperature are possibly the most common. Widespread in the environment and well-equipped for cold temperature growth, psychrophilic and psychrotrophic microorganisms may yet make numerous adjustments when faced with temperatures lower than optimum. Phospholipid and fatty acid alterations resulting in increased membrane fluidity at lower temperatures have been described for many cold tolerant microorganisms while others may make no similar adjustment. While the enzymes of cold growing bacteria have been less extensively studied than those of thermophilic bacteria, it appears that function at low temperature requires enzymes with flexible conformational structure, in order to compensate for lower reaction rates. In many organisms, including psychrophilic and psychrotrophic bacteria, specific sets of cold shock proteins are induced upon abrupt shifts to colder temperatures. While this cold shock response has not been fully delineated, it appears to be adaptive, and may function to promote the expression of genes involved in translation when cells are displaced to lower temperatures. The cold shock response of Escherichia coli has been extensively studied, and the major cold shock protein CspA appears to be involved in the regulation of the response. Upon cold shock, the induction of CspA and its counterparts in most microorganisms studied is prominent, but transient; studies of this response in some psychrotrophic bacteria have reported constitutive synthesis and continued synthesis during cold temperature growth of CspA homologues, and it will be interesting to learn if these are common mechanisms of among cold tolerant organisms. Psychrotrophic microorganisms continue to be a spoilage and safety problem in refrigerated foods, and a greater understanding of the physiological mechanisms and implications of cold temperature adaptation and growth should enhance our ability to design more effective methods of preservation. |
