5.1.1: Temperature and Microbial Growth

Microorganisms require specific temperature ranges in order to grow. These growth requirements exist because these are the temperatures at which their enzymes and molecules that make up their cellular structures work best. Deviations from these temperatures can be detrimental to the cell. Temperatures above a cells optimum temperature may result in the denaturation of enzymes needed for metabolism and the destruction of lipids needed for membranous structures, such as the cell membrane, causing them to become too fluid. Temperatures that are lower than that of the optimum for a microorganism may impede the activity of enzymes in their ability to function, and alter lipids in such a manner that membranes become rigid, thereby affecting membrane transport.

Microorganisms can be categorized into groups based on their optimal temperatures. The groups include the psychrophiles, psychrotroph, mesophiles, thermophiles, and hyperthermophiles. Each of these groups demonstrate a range that has a minimum temperature, a maximum temperature, and optimal temperature for the organism’s growth. Psychrophiles, are cold-loving bacteria that have a minimum growth temperature of 0°C or lower, and a maximum temperature around 15C. They are not capable of growth above 20°C. Psychrotrophs are able to tolerate growth temperatures around 0°C - 5°C up to around 35°C. Mesophiles are adapted to moderate temperatures and have optimal growth temperatures between approximately 20°C - 45 °C. Pathogenic (disease-causing) microorganisms generally fall within this group, because the human body temperature is 37°C. Organisms that are adapted to high temperatures are referred to as thermophiles if they tolerate up to 80°C and hyperthermophiles if they have optimal temperatures greater then 80°C.

In this exercise you will inoculate broths with bacterial samples and incubate them at five different temperatures. After allowing them to incubate you will assess the test tube for bacterial growth. You will do this for four different species, so you can determine the temperature ranges that each of the four organisms prefer.

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