Microbiology, 2021

13.3 • Using Chemicals to Control Microorganisms 533
Silver
Silver has long been used as an antiseptic. In ancient times, drinking water was stored in silver jugs. 15
Silvadene cream is commonly used to treat topical wounds and is particularly helpful in preventing infection
in burn wounds. Silver nitrate drops were once routinely applied to the eyes of newborns to protect against
ophthalmia neonatorum, eye infections that can occur due to exposure to pathogens in the birth canal, but
antibiotic creams are more now commonly used. Silver is often combined with antibiotics, making the
antibiotics thousands of times more effective. 16 Silver is also commonly incorporated into catheters and
bandages, rendering them antimicrobial; however, there is evidence that heavy metals may also enhance
selection for antibiotic resistance. 17
Copper, Nickel, and Zinc
Several other heavy metals also exhibit antimicrobial activity. Copper sulfate is a common algicide used to
control algal growth in swimming pools and fish tanks. The use of metallic copper to minimize microbial
growth is also becoming more widespread. Copper linings in incubators help reduce contamination of cell
cultures. The use of copper pots for water storage in underdeveloped countries is being investigated as a way
to combat diarrheal diseases. Copper coatings are also becoming popular for frequently handled objects such
as doorknobs, cabinet hardware, and other fixtures in health-care facilities in an attempt to reduce the spread
of microbes.
Nickel and zinc coatings are now being used in a similar way. Other forms of zinc, including zinc chloride and
zinc oxide, are also used commercially. Zinc chloride is quite safe for humans and is commonly found in
mouthwashes, substantially increasing their length of effectiveness. Zinc oxide is found in a variety of
products, including topical antiseptic creams such as calamine lotion, diaper ointments, baby powder, and
dandruff shampoos.
CHECK YOUR UNDERSTANDING
• Why are many heavy metals both antimicrobial and toxic to humans?
Halogens
Other chemicals commonly used for disinfection are the halogens iodine, chlorine, and fluorine. Iodine works
by oxidizing cellular components, including sulfur-containing amino acids, nucleotides, and fatty acids, and
destabilizing the macromolecules that contain these molecules. It is often used as a topical tincture, but it may
cause staining or skin irritation. An iodophor is a compound of iodine complexed with an organic molecule,
thereby increasing iodine’s stability and, in turn, its efficacy. One common iodophor is povidone-iodine, which
includes a wetting agent that releases iodine relatively slowly. Betadine is a brand of povidone-iodine
commonly used as a hand scrub by medical personnel before surgery and for topical antisepsis of a patient’s
skin before incision (Figure 13.22).
15 N. Silvestry-Rodriguez et al. “Silver as a Disinfectant.” In Reviews of Environmental Contamination and Toxicology, pp. 23-45.
Edited by GW Ware and DM Whitacre. New York: Springer, 2007.
16 B. Owens. “Silver Makes Antibiotics Thousands of Times More Effective.” Nature June 19 2013. http://www.nature.com/news/
silver-makes-antibiotics-thousands-of-times-more-effective-1.13232
17 C. Seiler, TU Berendonk. “Heavy Metal Driven Co-Selection of Antibiotic Resistance in Soil and Water Bodies Impacted by
Agriculture and Aquaculture.” Frontiers in Microbiology 3 (2012):399.