However, each species has its own characteristic and particular range of values in which it grows and reproduces best. These upper and lower values are a function of cell metabolism. At lower temperatures molecules move slower, enzymes cannot mediate in chemical reactions, and eventually the viscosity of the cell interior brings all activity to a halt. As the temperature increases, molecules move faster, enzymes speed up metabolism and cells rapidly increase in size. But, above a certain value all of these activities are proceeding at such high rates, enzymes start to denature, and the total effect is detrimental.
Cellular growth ceases.
Each species of microbe has its own, unique upper and lower limit, which is a defining characteristic for that species. Optimum Values Somewhere between its characteristic upper and lower temperature limits, each species of microbe has a particular temperature at which it grows best. At this temperature all aspects of the cell metabolism function at their optimum values, the cell is able to rapidly increase in size and divide. When members of a species find themselves living at their optimum temperature, their growth rate is at its maximum value. Psychrophiles Bacteria that grow at temperatures in the range of -5 o C to 30 o C, with optimum temperatures between 10 o C and 20 o C, are called psychrophiles .