Any species of bacterium grows best at a particular temperature. At much above or below that temperature it grows more poorly or even may die. There are many complications in practice, such as what kind of food it is growing on, what kinds of microbes it is competing with, and so on, but the upshot is that at any temperature from several degrees below zero, to nearly boiling, a particular group of bacteria in any particular environment is likely to dominate.
Certain freak examples exceed those limiting temperatures, but never mind freak species for now. Species that are dominant at low temperatures we call cryophiles or psychrophiles.The temperatures at which a bacterium can thrive depend largely on the temperatures at which its enzymes can work at an acceptable rate without decomposing. Any bacterium that depends absolutely on at least one enzyme that cannot work outside a given temperature range cannot survive and compete outside that range.
Even within its acceptable temperature range any bacterium tends to grow faster towards the warm end of the range rather than the cooler end. If it is competing with another species of bacterium with an overlapping range of acceptable temperatures, the species nearer its own preferred temperature is likely to dominate. That temperature might actually be too warm or too cool for it, but worse for its rival. There are many special circumstances; for instance one microbe might poison another, but a difference in preferred temperature commonly will permit the more comfortable microbe to dominate its rival.
Psychrophiles in a fridge grow slowly in comparison to mesophiles, the bacteria that grow best at room temperatures, as you will discover if ever your fridge warms up for a couple of days, say because of a power failure. The mesophilic bacteria and moulds will rapidly ruin foods that would otherwise have had a shelflife of months in the fridge.
Psychrophiles rule at low temperatures, not because they work as fast as mesophiles in general, but because they work faster at low temperatures than the surviving mesophiles can.
