Temperature controlling microbial growth

In this article, I briefly describe how temperature controls the growth of microorganisms.

Table of Contents

Microbial control

Microorganisms are controlled to prevent transmission of disease and infection. Microbial growth is controlled either by killing microorganisms or by inhibiting the growth of microorganisms. Microbes must be controlled to avoid contamination by their undesirable growth and prevent material deterioration and spoilage. These are generally controlled by the use of physical agents and chemical agents.

Different species of microorganisms differ in their susceptibility to physical and chemical agents. In spore-forming species, the growing vegetative cells are much more susceptible than the spore forms. Bacterial spores are the most resistant of all living organisms in their capacity to survive under adverse physical and chemical conditions.

Temperature- The physical method of microbial control

Microorganisms can grow over a wide range of temperatures. They can grow from very low temperatures to very high temperatures. Psychrophiles grow at very low temperatures, whereas thermophiles grow at very high temperatures.

High temperature treatment

Treatment of high temperature along with high moisture is one of the most effective methods of killing microorganisms. Dry heat is used to sterilize surfaces like glass Petri dishes, culture vessels, and metal surgical instruments, which should not contain any liquids and are not likely to break down at high temperatures. Moist heat has higher penetrating power than dry heat.

Moist heat has higher penetrating power than dry heat. It destroys microorganisms by coagulating their proteins and oxidizing their chemical constituents, ultimately leading to their destruction. Moist heat quickly penetrates and is used to sterilize culture solutions, agar preparations surgical instruments, etc. Bacterial endospores are not killed by boiling and thus are annihilated by pressurized steam. Typically a pressure of 15 psi (pounds per square inch) is needed to create steam at a high enough temperature (121°C) to kill endospores. Moist heat can kill spores of Clostridium botulinum within 20 minutes at 120°C. However, dry heat needs nearly two hours to destroy the spores at the same temperature.

The shortest period required to kill a suspension of bacterial spores at a specific temperature and under specific conditions is called the thermal death time. The thermal death point is the lowest temperature at which all microorganisms in a particular liquid will be killed in ten minutes. To kill 90% of a given population of microorganisms at a given temperature, the required time is called the decimal reduction time. It is the time in minutes for the thermal death-time curve to pass through one log cycle.

Decimal reduction time and thermal death time

The time in minutes to reduce the microbial population by 90% is known as decimal reduction time. The graph (figure 1), shows that the cells are dying at a constant rate of 90% each minute. At a logarithmic response, the D value is independent of time.

Figure 1: Graph showing decimal reduction time

The second graph (figure-2), shows the thermal-death-time curve for spores of a bacterial species encountered in a type of canned-food spoilage. These values show a time-temperature relationship for killing. In thermal death time, the temperature is constant with a varied time. Decimal reduction time is a modification of thermal death time which measures a 90 percent rather than 100 percent kill rate.

Figure 2: Graph showing the thermal death time curve

Low temperature treatment

At a low temperature, microorganisms retard their growth by slowing their metabolism. However, it does not always kill them and some bacteria and fungi do grow at near-freezing temperatures. Low temperatures are beneficial for preserving cultures since microorganisms have a unique capacity for surviving extreme cold. Refrigeration at 5° C retards the growth of many bacteria and fungi and freezing at – 10° to – 20° C is also an effective method to retard microbial growth.

Moist heat

Steam under pressure

The most practical agent for sterilization is heat in the form of saturated steam under pressure. Steam under pressure has the advantages of rapid heating, penetration, and moisture in abundance, which facilitates the coagulation of proteins. The autoclave (figure-3) is used to routinely sterilize many media, solutions, discarded cultures, and contaminated materials inside the laboratory. It is a double-jacketed steam chamber equipped with devices that fill the chamber with saturated steam and maintain a specific temperature and pressure for some time.

Figure 3: Cross-sectional view of an autoclave

Fractional sterilization

It is a method, which involves heating a material, e.g., chemical solutions, some microbiological media, chemical solutions, and biological materials at 100°C on three successive days with incubation periods in between. Resistant spores germinate during the incubation periods and on subsequent exposure to heat, the vegetative cells will be destroyed. If spores do not germinate during the incubation periods, the material will not be sterilized.

Boiling water

Boiling water can destroy all vegetative cells within minutes but some bacterial spores can tolerate this for hours. Thus, boiling water can’t thoroughly sterilize the contaminated materials. It can be used as a method of disinfection but not as a method of sterilization.

Pasteurization

The process was first discovered by Louis Pasteur, who discovered that controlled heat treatment was effective in preventing the spoilage of beer and wine. It is a method, which uses much heat to reduce the number of microbes without affecting the quality of the food product. Milk, cream, and certain alcoholic beverages are also subjected to pasteurization, which reduces the microbial load and kills many pathogens. However, it does not kill all bacterial pathogens and endospores.

Dry heat

It includes two methods of sterilizing materials, i.e., hot air sterilization and incineration and flaming.

Hot-air sterilization

Some materials, e.g., certain laboratory glassware, oils, powders, and similar substances avoided direct contact with the autoclave. These materials are sterilized in a special type of apparatus, i.e., in an electric or gas oven. Laboratory glassware needs a 2-hour exposure to a temperature of 160°C for sterilization.

Incineration and flaming

In the laboratory, microorganisms are destroyed by burning. The transfer needle carrying microorganisms is introduced into the flame of the Bunsen burner, destroying microorganisms. The transfer needle is sterilized with utmost care to prevent spattering because the droplets that fly off are likely to carry viable organisms. Spattering can be prevented by using a modified Bunsen burner, which has a tubular space, and the transfer needle is exposed to a flame within a tubular space.

Incineration is used for the destruction of animal carcasses, bags and wipes, contaminated dressings, and infected laboratory materials to be disposed of. It should be done with care so that Care the exhaust fumes do not carry particulate matter containing viable microorganisms into the atmosphere.

Conclusion

Microbial growth is controlled either by killing microorganisms or by inhibiting the growth of microorganisms. Microorganisms can grow from very low temperatures to very high temperatures.

Treatment of high temperature along with high moisture is one of the most effective methods of killing microorganisms. Moist heat has higher penetrating power than dry heat. At a low temperature, microorganisms slow their metabolism and retard their growth.

The most practical agent for sterilization is heat in the form of saturated steam under pressure. The autoclave routinely sterilizes many media, solutions, discarded cultures, and contaminated materials inside the laboratory. Fractional sterilization, boiling, and pasteurization are the other methods of controlling microorganisms.

Dry heat controls microbes by two methods of sterilizing materials, i.e., hot air sterilization and incineration and flaming.