Overview of Mechanisms of Resistance to Antibacterial Agents

Bacteria can become resistant to antimicrobial drugs through a variety of mechanisms. First, the organism can produce enzymes that chemically modify or break down the antibacterial agent before it can have an effect. Second, bacteria can use one or more efflux pumps to extrude the antibacterial agent out of the cell before it can reach its target site and exert its inhibitory effect.

Third, bacteria can eliminate the target site of an antimicrobial agent through mutation so the drug has no place to bind to exert its inhibitory effect. Fourth, bacteria may alter the permeability of their cell walls and membranes such that they limit access of the antimicrobial agents into the cell. Finally, the organism can acquire genes, forming an alternate metabolic pathway that is not inhibited by antimicrobial agents.

Normally susceptible populations of bacteria may become resistant to antimicrobial agents through spontaneous mutation, or by acquiring the genetic information that encodes resistance from other bacteria. The latter may occur through one of three genetic mechanisms: conjugation (which involves cell-to-cell contact and exchange of DNA), transformation (the uptake of naked DNA from the environment), or transduction (DNA transferred from one bacterial cell to another by viruses).

Through genetic exchange mechanisms, many bacteria have become resistant to multiple classes of antibacterial agents, and these multi-drug-resistant bacteria (defined as resistance to three or more bacterial drug classes) have become a cause for serious concern, particularly in hospitals and other health-care institutions where they tend to occur most commonly.

As noted above, susceptible bacteria can acquire resistance to an antimicrobial agent via de novo mutation. Spontaneous mutations may cause resistance directly by altering the target protein to which the antibacterial agent binds thus modifying or eliminating the binding site (e.g., change in penicillin binding protein (PBP) 2b in pneumococci, which results in penicillin resistance), or indirectly by (1) upregulating the production of enzymes that inactivate the antimicrobial agent (e.g., erythromycin ribosomal methylase in staphylococci), (2) downregulating or altering an outer membrane protein channel that the drug requires for cell entry (e.g., outer membrane protein F (OmpF) in Escherichia colt), or (3) upregulating pumps that expel the drug from the cell (efflux of fluoroquinolones in S. aureus).

In all of these cases, strains of bacteria carrying resistance-conferring mutations are selected by antimicrobial use, which kills the susceptible strains but allows the newly resistant strains to survive and grow. Acquired resistance that develops due to chromosomal mutation and selection is often deemed vertical evolution.

Bacteria also develop resistance through the acquisition of new genetic material from other resistant organisms. This is called horizontal evolution and may occur between strains of the same species or between different bacterial species or genera. During conjugation, a Gram-negative bacterium transfers a plasmid containing resistance genes to an adjacent bacterium, often using an elongated proteinaceous structure known as a pilus to join the two organisms together.

Conjugation among Gram-positive bacteria is usually initiated by production of sex pheromones by the mating pair, which facilitate the clumping of donor and recipient organisms, allowing the exchange of DNA. During transduction, resistance genes are transferred from one bacterium to another via bacteriophage (bacterial viruses).

This is now thought to be a relatively rare event. Finally, transformation, that is, the process whereby bacteria acquire and incorporate DNA segments from other bacteria that have released their DNA complement into the environment after cell lysis, can move resistance genes into previously susceptible strains.

Mutation and selection (selection is the use of antimicrobial agents, particularly in suboptimal concentrations, that allows the new antimicrobial resistant organisms to survive), together with the mechanisms of genetic exchange, enable many bacterial species to adapt quickly to the introduction of antibacterial agents into their environment.

While a single mutation in a key bacterial gene may only reduce the susceptibility of the host bacteria to that antibacterial agent slightly, that may be just enough to allow its initial survival until it acquires additional mutations or additional genetic information resulting in full-fledged resistance to the antibacterial agent. However, in rare cases, a single mutation can be sufficient to confer high-level, clinically significant resistance to an organism (such as with high-level rifampin resistance in S. aureus, or high-level fluoroquinolone resistance in Campylobacter jejuni, both of which can be the result of a single genetic mutation).

Selection of Resistant Organisms. Any use of antimicrobial agents can provide the selective pressure that allows a newly resistant bacterial isolate (whether it developed by mutation or by acquisition of new genetic information) to survive and multiply.

The susceptible isolates will, of course, be eliminated from the population by the antimicrobial agent. Antimicrobial agents are used not only for treatment of human and animal diseases but for a variety of other applications including growth promotion of animals (except in the European Union, where this application has been banned), control of fire blight and other plant diseases in pome fruit orchards (where the drug may be applied from airplanes, similar to crop dusting), control of fish diseases in aquaculture farms (such as for raising salmon), and inhibition of barnacle growth on large ships.

Thus, limiting the selective pressure that promotes the development of antimicrobial-resistant organisms is not simply a matter of optimizing the use of these drugs in humans.