Microbiology, 2021

532 13 • Control of Microbial Growth
CHECK YOUR UNDERSTANDING
• Why is triclosan more like an antibiotic than a traditional disinfectant?
Heavy Metals
Some of the first chemical disinfectants and antiseptics to be used were heavy metals. Heavy metals kill
microbes by binding to proteins, thus inhibiting enzymatic activity (Figure 13.21). Heavy metals are
oligodynamic, meaning that very small concentrations show significant antimicrobial activity. Ions of heavy
metals bind to sulfur-containing amino acids strongly and bioaccumulate within cells, allowing these metals to
reach high localized concentrations. This causes proteins to denature.
Heavy metals are not selectively toxic to microbial cells. They may bioaccumulate in human or animal cells, as
well, and excessive concentrations can have toxic effects on humans. If too much silver accumulates in the
body, for example, it can result in a condition called argyria, in which the skin turns irreversibly blue-gray. One
way to reduce the potential toxicity of heavy metals is by carefully controlling the duration of exposure and
concentration of the heavy metal.
Figure 13.21
Heavy metals denature proteins, impairing cell function and, thus, giving them strong antimicrobial properties. (a) Copper in
fixtures like this door handle kills microbes that otherwise might accumulate on frequently touched surfaces. (b) Eating utensils contain
small amounts of silver to inhibit microbial growth. (c) Copper commonly lines incubators to minimize contamination of cell cultures stored
inside. (d) Antiseptic mouthwashes commonly contain zinc chloride. (e) This patient is suffering from argyria, an irreversible condition
caused by bioaccumulation of silver in the body. (credit b: modification of work by “Shoshanah”/Flickr; credit e: modification of work by
Herbert L. Fred and Hendrik A. van Dijk)
Mercury
Mercury is an example of a heavy metal that has been used for many years to control microbial growth. It was
used for many centuries to treat syphilis. Mercury compounds like mercuric chloride are mainly bacteriostatic
and have a very broad spectrum of activity. Various forms of mercury bind to sulfur-containing amino acids
within proteins, inhibiting their functions.
In recent decades, the use of such compounds has diminished because of mercury’s toxicity. It is toxic to the
central nervous, digestive, and renal systems at high concentrations, and has negative environmental effects,
including bioaccumulation in fish. Topical antiseptics such as mercurochrome, which contains mercury in low
concentrations, and merthiolate, a tincture (a solution of mercury dissolved in alcohol) were once commonly
used. However, because of concerns about using mercury compounds, these antiseptics are no longer sold in
the United States.
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