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THE EFFECT OF LOW TEMPERATURES ON MICROBIAL PHYSIOLOGIC MECHANISMS

The effects that low incubation temperatures have on the growth and activity of foodborne microorganisms, five have received the most attention and are outlined below. Psychrotrophs have a slower metabolic rate. The precise reasons as to why metabolic rates are slowed at low temperatures are not fully understood.

Psychrotrophic growth decreases more slowly than that of mesophilic with decreasing temperatures. The temperature coefficients (Q10) for various substrates such as acetate and glucose have been shown by several investigators to be lower for growing psychrotrophs than for mesophiles. The end products of mesophiles and psychrotrophic metabolism of glucose were shown to be the same, with the differences largely disappearing when the cells were broken.29

In other words, the temperature coefficients are about the same for psychrotrophs and mesophiles when cell-free extracts are employed. As the temperature is decreased, the rate of protein synthesis is known to decrease, and this occurs in the absence of changes in the amount of cellular DNA. One reason may be the increase in intramolecular hydrogen bonding that occurs at low temperatures, leading to increased folding of enzymes with losses in catalytic activity.37

On the other hand, the decrease in protein synthesis appears to be related to a decreased synthesis of individual enzymes at low growth temperatures. Although the precise mechanism of reduced protein synthesis is not well understood, it has been suggested that low temperatures affect the synthesis of a repressor protein40 and that the repressor protein itself is thermolabile.64 Several investigators have suggested that low temperatures may influence the fidelity of the translation of messenger RNA (mRNA) during protein synthesis.

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For example, in studies with E. coli, it was shown that a leucine-starved auxotroph of this mesophile incorporated radioactive leucine into protein at 0◦C.22 It was suggested that at this temperature, all essential steps in protein synthesis apparently go on and involve a wide variety of proteins. The rate of synthesis at 0◦C was estimated to be about 350 times slower than at 37◦C for this organism. It has been suggested that the cessation of RNA synthesis in general may be the controlling factor in determining low temperature growth,26 and the lack of polysome formation in E. coli when shifted to a temperature below its growth minimum has been demonstrated. The formation of polysomes is thus sensitive t

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