An antimicrobial agent’s adverse effect on cells is known as its mode (or mechanism) of action. Agents affect one or more cellular targets, inflicting damage progressively until the cell is no longer viable. Antimicrobials have a range of cellular targets, with the agents that are least selective in their targeting tending to be effective against the widest range of microbes (examples include heat and radiation). More selective agents (drugs, for example) tend to target only a single cellular component and are much more restricted in their effect on microbes.
The cellular targets of physical and chemical agents fall into four general categories: (1) cell walls, (2) cell membranes, (3) protein and nucleic acid synthesis, and (4) protein structure and function.
The Effects of Agents on the Cell Wall
The cell wall maintains the structural integrity of bacterial and fungal cells. Several types of chemical agents damage the cell wall by blocking its synthesis, digesting it, or breaking down its sur face. A cell deprived of a functioning cell wall becomes fragile and is lysed very easily. Examples of this mode of action include some antimicrobial drugs (penicillins) that interfere with the synthesis of the cell wall in bacteria. Detergents and alcohol can also disrupt cell walls, especially in gram-negative bacteria.
How Agents Affect the Cell Membrane
All microorganisms have a cell membrane composed of lipids and proteins, and even some viruses have an outer membranous envelope. As we learned in previous chapters, a cell’s membrane provides a two-way system of transport. If this membrane is disrupted, a cell loses its selective permeability and can neither pre vent the loss of vital molecules nor bar the entry of damaging chemicals. Loss of those functions usually leads to cell death. Chemicals called surfactants* work as microbicidal agents because they lower the surface tension of cell membranes. Surfactants are polar molecules such as detergents with hydrophilic and hydro phobic regions that can physically bind to the lipid layer and penetrate the internal hydrophobic regions of membranes. In effect, this process “opens up” the once-tight interface, leaving leaky spots that allow harmful chemicals to seep into the cell and important ions to seep out (figure 1). Alcohols exert a related effect by dissolving membrane lipids and stripping membranes away from cells.
Fig1. Mode of action of surfactants on the cell membrane. Surfactants inserting in the lipid bilayer disrupt it and create abnormal channels that alter permeability and cause leakage both into and out of the cell.
Agents that Affect Protein and Nucleic Acid Synthesis
Microbial life depends upon an orderly and continuous supply of proteins to function as enzymes and structural molecules. As we saw in chapter 9, these proteins are synthesized on the ribosomes during translation. Any chemical that blocks this process can affect function and survival. For example, certain drugs bind to the ribosomes of bacteria in a way that stops peptide bonds from forming. In the presence of such a chemical, many bacterial cells are inhibited from forming proteins required in growth and metabolism and are thus inhibited from multiplying. Many of the drugs used to treat infections are chemicals that block protein synthesis in microbes without adversely affecting human cells. This subject is discussed in greater detail in chapter 12.
The nucleic acids are also needed for the continued functioning of microbes. DNA must be regularly replicated and transcribed in growing cells, and any agent that either impedes these processes or changes the genetic code is potentially antimicrobial. Some agents bind irreversibly to DNA, preventing both transcription and translation, whereas others are mutagenic agents. Gamma, ultraviolet, or X radiation causes mutations that result in permanent inactivation of DNA. Chemicals such as formaldehyde and ethylene oxide also interfere with DNA and RNA function.
Agents that Alter Protein Function
A microbial cell contains large quantities of proteins that function properly only if they remain in a normal three-dimensional configuration called the native state. The antimicrobial properties of some agents arise from their capacity to disrupt the structure of, or denature, proteins. In general, denaturation occurs when the bonds that maintain the secondary and tertiary structure of the protein are broken. Breaking these bonds will cause the protein to unfold or create random, irregular loops and coils (figure 2). One way to denature proteins is through coagulation by moist heat (think of the irreversible solidification of egg white when cooked). Chemicals such as strong organic solvents (alcohols, acids) also coagulate proteins. A few antimicrobial agents, such as metallic ions, attach to the active site of the protein and prevent it from interacting with its correct substrate. Regardless of the exact mechanism, such losses in normal protein function can promptly stop metabolism and exert broad effects on all types of microbes.
Fig2. Modes of action affecting protein function. (a) The native (functional) state is maintained by bonds that create active sites to fit the substrate. Some agents denature the protein by breaking all or some secondary and tertiary bonds. Results are (b) complete unfolding or (c) random bonding and incorrect folding. (d) Some agents react with functional groups on the active site and interfere with bonding.
