Microbiology, 2021

536 13 • Control of Microbial Growth
modification of work by D Coetzee; credit b photo: modification of work by Craig Spurrier)
CHECK YOUR UNDERSTANDING
• Name at least three advantages of alcohols as disinfectants.
• Describe several specific applications of alcohols used in disinfectant products.
Surfactants
Surface-active agents, or surfactants, are a group of chemical compounds that lower the surface tension of
water. Surfactants are the major ingredients in soaps and detergents. Soaps are salts of long-chain fatty acids
and have both polar and nonpolar regions, allowing them to interact with polar and nonpolar regions in other
molecules (Figure 13.25). They can interact with nonpolar oils and grease to create emulsions in water,
loosening and lifting away dirt and microbes from surfaces and skin. Soaps do not kill or inhibit microbial
growth and so are not considered antiseptics or disinfectants. However, proper use of soaps mechanically
carries away microorganisms, effectively degerming a surface. Some soaps contain added bacteriostatic
agents such as triclocarban or cloflucarban, compounds structurally related to triclosan, that introduce
antiseptic or disinfectant properties to the soaps.
Figure 13.25
Soaps are the salts (sodium salt in the illustration) of fatty acids and have the ability to emulsify lipids, fats, and oils by
interacting with water through their hydrophilic heads and with the lipid at their hydrophobic tails.
Soaps, however, often form films that are difficult to rinse away, especially in hard water, which contains high
concentrations of calcium and magnesium mineral salts. Detergents contain synthetic surfactant molecules
with both polar and nonpolar regions that have strong cleansing activity but are more soluble, even in hard
water, and, therefore, leave behind no soapy deposits. Anionic detergents, such as those used for laundry, have
a negatively charged anion at one end attached to a long hydrophobic chain, whereas cationic detergents have
a positively charged cation instead. Cationic detergents include an important class of disinfectants and
antiseptics called the quaternary ammonium salts (quats), named for the characteristic quaternary nitrogen
atom that confers the positive charge (Figure 13.26). Overall, quats have properties similar to phospholipids,
having hydrophilic and hydrophobic ends. As such, quats have the ability to insert into the bacterial
phospholipid bilayer and disrupt membrane integrity. The cationic charge of quats appears to confer their
antimicrobial properties, which are diminished when neutralized. Quats have several useful properties. They
are stable, nontoxic, inexpensive, colorless, odorless, and tasteless. They tend to be bactericidal by disrupting
membranes. They are also active against fungi, protozoans, and enveloped viruses, but endospores are
unaffected. In clinical settings, they may be used as antiseptics or to disinfect surfaces. Mixtures of quats are
also commonly found in household cleaners and disinfectants, including many current formulations of Lysol
brand products, which contain benzalkonium chlorides as the active ingredients. Benzalkonium chlorides,
along with the quat cetylpyrimidine chloride, are also found in products such as skin antiseptics, oral rinses,
and mouthwashes.
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