It has been known for years that psychrotrophic microorganisms are generally unable to grow much above 30–35◦C. Among the first to suggest reasons for this limitation of growth were Edwards and Rettger,14 who concluded that the maximum growth temperatures of bacteria may bear a definite relationship to the minimum temperatures of destruction of respiratory enzymes. Their conclusion has been borne out by results from a large number of investigators.
It has been shown that many respiratory enzymes are inactivated at the temperatures of maximal growth of various psychrotrophic types (Table 16–8). Thus, the thermal sensitivity of certain enzymes of psychrotrophs is at least one of the factors that limit the growth of these organisms to low temperatures. When some psychrotrophs are subjected to temperatures above their growth maxima, cell death is accompanied by the leakage of various intracellular constituents.
The leakage substances have been shown to consist of proteins, DNA, RNA, free amino acids, and lipid phosphorus. The last was thought to represent phosphorus of the cytoplasmic membrane. Although the specific reasons for the release of cell constituents are not fully understood, it would appear to involve rupture of the cell membrane. These events appear to follow those of enzyme inactivation.
Whatever the true mechanism of psychrotroph death at temperatures a few degrees above their growth maxima is, their destruction at these relatively low temperatures is characteristic of this group of organisms.
This is especially true of those that have optimum growth temperatures at and below 20◦C. Reports on psychrotrophs isolated and studied over the past four or so decades reveal that all are capable of growing at 0◦C with growth optima at either 15◦C or between 20◦C and 25◦C and growth maxima between 20◦C and 35◦C. Included among these organisms are Gram-negative rods, Gram-positive aerobic and anaerobic rods, spore formers and non-spore formers, Gram-positive cocci, vibrios, and yeasts.
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