Genetic Basis of Antibiotic Resistance

The genetic basis of antibiotic resistance is the set of heritable changes in a bacterium's DNA that confer the ability to grow despite a drug. Resistance is encoded either intrinsically in the chromosome, generated by spontaneous mutation under selective pressure, or acquired by importing resistance genes from other organisms — and these genetic origins determine how readily resistance emerges and spreads.

Definition

The genetic basis of antibiotic resistance refers to the chromosomal and acquired DNA determinants — intrinsic resistance genes, resistance-conferring mutations, and imported resistance genes — that encode resistance phenotypes and are inherited by daughter cells.

Scope

This topic covers where resistance information resides in the bacterial genome and how it changes: intrinsic chromosomal traits, point mutations and their selection, and the distinction between mutational and acquired resistance. The mobile elements that carry acquired genes between cells are introduced here and developed in the companion topic on resistance genes and horizontal transfer. The treatment is microbiological and genetic, not clinical.

Core questions

• Is a given resistance trait intrinsic, mutational, or acquired?
• How do spontaneous mutations under drug pressure give rise to resistant lineages?
• What distinguishes mutational resistance from horizontally acquired resistance genes?
• Why does the genetic origin of a determinant shape how fast resistance spreads?

Key concepts

• Intrinsic resistance
• Acquired resistance
• Spontaneous mutation and selection
• Resistance-conferring point mutations
• Hypermutator phenotypes
• Fitness cost and compensatory mutations
• Resistome

Mechanisms

Resistance can be written into the genome in two broad ways. Intrinsic resistance reflects genes or structural features a species naturally possesses, such as a low-permeability outer membrane or a chromosomally encoded enzyme. Acquired resistance arises either by mutation — spontaneous changes in chromosomal genes encoding a drug target, a transporter, or a regulator, which are then selected when the drug is present — or by import of resistance genes from other bacteria. Mutational resistance typically alters an existing cellular component, while acquired genes often add an entirely new function such as a degrading enzyme. Resistance mutations may carry a fitness cost that is later offset by compensatory mutations, helping resistant strains persist (Munita & Arias, 2016; Blair et al., 2015).

Clinical relevance

Knowing whether a resistance trait is intrinsic, mutational, or acquired helps explain why some organisms are predictably resistant to certain drugs and why resistance can emerge during therapy; it is reference knowledge for interpreting susceptibility patterns. This entry describes the underlying genetics and does not provide treatment or prescribing guidance.

Epidemiology

Resistance determinants are evolutionarily old and broadly distributed in environmental and commensal bacteria, forming a reservoir of genetic material that antibiotic use selects upon. Mutational resistance arises independently in many settings, while the same acquired genes recur worldwide once mobilized, reflecting both repeated mutation and shared ancestry of resistance genes (Davies & Davies, 2010).

Evidence & guidelines

The genetic framework summarized here is drawn from widely cited reviews of resistance mechanisms and evolution (Munita & Arias, 2016; Davies & Davies, 2010; Blair et al., 2015). The entry is educational and issues no clinical guidelines.

History

As each antibiotic class entered use, resistant strains appeared, and mid- to late-twentieth-century genetics localized resistance to specific chromosomal mutations and to transferable genetic elements. Later work demonstrated that resistance genes long predate clinical antibiotics, establishing that the genetic capacity for resistance is an ancient and intrinsic property of microbial genomes (Davies & Davies, 2010).

Key figures

• Julian Davies
• Stuart B. Levy
• Cesar A. Arias
• Laura J. V. Piddock

Seminal works

• davies-davies-2010
• munita-arias-2016
• blair-2015

Frequently asked questions

What is the difference between intrinsic and acquired resistance?

Intrinsic resistance is a natural property of a species encoded in its own genome, whereas acquired resistance results from new mutations or from resistance genes imported from other bacteria.

How does mutation lead to resistance?

Spontaneous mutations can alter a drug's target, reduce its uptake, or increase its efflux; when the drug is present, cells carrying such mutations survive preferentially and the resistant lineage expands.