The rates of chemical reactions in a cell are determined by the activities of its enzymes. Enzymes are proteins that act as catalysts, substances that increase the rate of a chemical reaction without being permanently altered or consumed. The temperature range over which a microorganism grows reflects the temperatures at which its enzymes function. Within this range, three cardinal temperatures can be identified: minimum, optimum and maximum growth temperatures.
Temperatures higher than the maximum growth temperature cause irreversible denaturation of enzymes and therefore cell death. At temperatures below the minimum, molecular motion and enzymatic activities effectively cease. In addition to the effects of temperature on enzymes, other cell structures, such as the cytoplasmic membrane, are temperature sensitive. At temperatures below the minimum or above the maximum, they will cease to function because of changes in the fluidity of lipid bilayers and membrane transport proteins. According to their growth temperature range, bacteria are classified as:
Psycrophiles: cold-loving organisms, optimum growth temperature is 15°C or lower. These extremophiles are found in the Arctic, Antarctic and lakes fed by glaciers.
Psychrotrophs: these species have an optimum growth temperature between 20 and 30ºC but can grow at 0 to 7º C. These play a major role in spoilage of refrigerated foods
mesophiles: include most bacteria, optimum growth temperature is 20 - 45°C. Many pathogens are mesophiles as their preferred temperature is body temperature (37ºC).
Thermophiles: heat-loving organisms, optimum growth temperature is 55-65°C. Thermophiles can be found in hot springs, compost heaps, and hot water heaters.
Hyperthermophiles: love extreme heat, optimum growth temperature is 70-110°C. Members of this group are generally Archaea.
Temperature Procedure: All 4 microorganisms will be tested at all 4 temperatures. Temperatures: 4˚C, 22˚C (room temp), 37˚C, and 55˚C.
Test Organisms:
Pseudomonas fluorescens
Escherichia coli
Bacillus stearothermophilus
Bacillus subtilis
1. Collect 4 TSA plates from the side bench. On the bottom of the plates use a marker to divide them into 4 quadrants as shown below.
2. Follow the procedure for Innoculating organisms into four quadrants.
3. Repeat the procedure for all four organisms. Streak them in their appropriately labeled quadrants.
4. Repeat the inoculation procedure, as described in the video, for the remaining 3 plates.
5. Invert and incubate the plates in the appropriate temperature incubators for 24 - 48 hours. Place the plate that will be incubated at 55 ˚C into a plastic container with a moist towel to prevent dehydration.
Observe and compare the growth of each organism at the four temperatures
