Microbiology, 2021

542 13 • Control of Microbial Growth
used to clean wounds. Hydrogen peroxide is used to sterilize items such as contact lenses. (credit photos: modification of work by Kerry
Ceszyk)
Plasma, a hot, ionized gas, described as the fourth state of matter, is useful for sterilizing equipment because it
penetrates surfaces and kills vegetative cells and endospores. Hydrogen peroxide and peracetic acid, another
commonly used peroxygen, each may be introduced as a plasma. Peracetic acid can be used as a liquid or
plasma sterilant insofar as it readily kills endospores, is more effective than hydrogen peroxide even at rather
low concentrations, and is immune to inactivation by catalases and peroxidases. It also breaks down to
environmentally innocuous compounds; in this case, acetic acid and oxygen.
Other examples of peroxygens include benzoyl peroxide and carbamide peroxide. Benzoyl peroxide is a
peroxygen that used in acne medication solutions. It kills the bacterium Propionibacterium acnes, which is
associated with acne. Carbamide peroxide, an ingredient used in toothpaste, is a peroxygen that combats oral
biofilms that cause tooth discoloration and halitosis (bad breath). 22 Last, ozone gas is a peroxygen with
disinfectant qualities and is used to clean air or water supplies. Overall, peroxygens are highly effective and
commonly used, with no associated environmental hazard.
CHECK YOUR UNDERSTANDING
• How do peroxides kill cells?
Supercritical Fluids
Within the last 15 years, the use of supercritical fluids, especially supercritical carbon dioxide (scCO 2 ), has
gained popularity for certain sterilizing applications. When carbon dioxide is brought to approximately 10
times atmospheric pressure, it reaches a supercritical state that has physical properties between those of
liquids and gases. Materials put into a chamber in which carbon dioxide is pressurized in this way can be
sterilized because of the ability of scCO 2 to penetrate surfaces.
Supercritical carbon dioxide works by penetrating cells and forming carbonic acid, thereby lowering the cell
pH considerably. This technique is effective against vegetative cells and is also used in combination with
peracetic acid to kill endospores. Its efficacy can also be augmented with increased temperature or by rapid
cycles of pressurization and depressurization, which more likely produce cell lysis.
Benefits of scCO 2 include the nonreactive, nontoxic, and nonflammable properties of carbon dioxide, and this
protocol is effective at low temperatures. Unlike other methods, such as heat and irradiation, that can degrade
the object being sterilized, the use of scCO 2 preserves the object’s integrity and is commonly used for treating
foods (including spices and juices) and medical devices such as endoscopes. It is also gaining popularity for
disinfecting tissues such as skin, bones, tendons, and ligaments prior to transplantation. scCO 2 can also be
used for pest control because it can kill insect eggs and larvae within products.
CHECK YOUR UNDERSTANDING
• Why is the use of supercritical carbon dioxide gaining popularity for commercial and medical uses?
Chemical Food Preservatives
Chemical preservatives are used to inhibit microbial growth and minimize spoilage in some foods. Commonly
used chemical preservatives include sorbic acid, benzoic acid, and propionic acid, and their more soluble salts
potassium sorbate, sodium benzoate, and calcium propionate, all of which are used to control the growth of
molds in acidic foods. Each of these preservatives is nontoxic and readily metabolized by humans. They are
also flavorless, so they do not compromise the flavor of the foods they preserve.
Sorbic and benzoic acids exhibit increased efficacy as the pH decreases. Sorbic acid is thought to work by
22 Yao, C.S. et al. “In vitro antibacterial effect of carbamide peroxide on oral biofilm.” Journal of Oral Microbiology Jun 12, 2013.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3682087/. doi: 10.3402/jom.v5i0.20392.
Access for free at openstax.org.