Microbiology, 2021

520 13 • Control of Microbial Growth
Figure 13.10 Cultures and other medical specimens can be stored for long periods at ultra-low temperatures. (a) An ultra-low freezer
maintains temperatures at or below −70 °C. (b) Two people stand in a room with a large nitrogen storage unit emitting a large amount of
fog. (credit a: modification of work by “Expert Infantry”/Flickr; credit b: Credit: US Navy; Public Domain.)
CHECK YOUR UNDERSTANDING
• Does placing food in a refrigerator kill bacteria on the food?
Pressure
Exposure to high pressure kills many microbes. In the food industry, high-pressure processing (also called
pascalization) is used to kill bacteria, yeast, molds, parasites, and viruses in foods while maintaining food
quality and extending shelf life. The application of high pressure between 100 and 800 MPa (sea level
atmospheric pressure is about 0.1 MPa) is sufficient to kill vegetative cells by protein denaturation, but
endospores may survive these pressures. 4 5
In clinical settings, hyperbaric oxygen therapy is sometimes used to treat infections. In this form of therapy, a
patient breathes pure oxygen at a pressure higher than normal atmospheric pressure, typically between 1 and
3 atmospheres (atm). This is achieved by placing the patient in a hyperbaric chamber or by supplying the
pressurized oxygen through a breathing tube. Hyperbaric oxygen therapy helps increase oxygen saturation in
tissues that become hypoxic due to infection and inflammation. This increased oxygen concentration
enhances the body’s immune response by increasing the activities of neutrophils and macrophages, white
blood cells that fight infections. Increased oxygen levels also contribute to the formation of toxic free radicals
that inhibit the growth of oxygen-sensitive or anaerobic bacteria like as Clostridium perfringens, a common
cause of gas gangrene. In C. perfringens infections, hyperbaric oxygen therapy can also reduce secretion of a
bacterial toxin that causes tissue destruction. Hyperbaric oxygen therapy also seems to enhance the
effectiveness of antibiotic treatments. Unfortunately, some rare risks include oxygen toxicity and effects on
delicate tissues, such as the eyes, middle ear, and lungs, which may be damaged by the increased air pressure.
High pressure processing is not commonly used for disinfection or sterilization of fomites. Although the
application of pressure and steam in an autoclave is effective for killing endospores, it is the high temperature
achieved, and not the pressure directly, that results in endospore death.
4 C. Ferstl. “High Pressure Processing: Insights on Technology and Regulatory Requirements.” Food for Thought/White Paper. Series
Volume 10. Livermore, CA: The National Food Lab; July 2013.
5 US Food and Drug Administration. “Kinetics of Microbial Inactivation for Alternative Food Processing Technologies: High
Pressure Processing.” 2000. http://www.fda.gov/Food/FoodScienceResearch/SafePracticesforFoodProcesses/ucm101456.htm.
Accessed July 19, 2106.
Access for free at openstax.org.