Antimicrobial Resistance

Antimicrobial resistance is the ability of microorganisms to survive exposure to agents that once killed or inhibited them. It arises through mutation and the horizontal transfer of resistance genes, and it is selected and amplified by the use of antimicrobial agents, making it both a biological phenomenon and a global public-health threat.

Definition

Microbial drug resistance is the capacity of a microorganism—bacterium, virus, or fungus—to withstand the action of an antimicrobial agent to which it was previously susceptible, whether through an inherent trait or an acquired genetic change.

Scope

This topic covers what microbial drug resistance is, the molecular mechanisms by which it operates, how it is selected and spread, and why it is regarded as a global problem. It is a conceptual reference within antimicrobial principles; for the same term as a basic-science subfield see the cross-linked microbiology node, and it does not provide treatment advice.

Core questions

• What molecular mechanisms allow microbes to resist antimicrobial agents?
• How is resistance acquired and transferred between organisms?
• How does the use of antimicrobials select for resistant populations?
• Why is antimicrobial resistance considered a global health threat?

Key concepts

• Intrinsic versus acquired resistance
• Enzymatic drug inactivation (for example β-lactamases)
• Target modification
• Reduced uptake and active efflux
• Horizontal gene transfer and mobile genetic elements
• Selection pressure from antimicrobial use
• Multidrug resistance

Mechanisms

Resistance operates through a recurring set of molecular strategies: enzymes that inactivate the drug (such as the β-lactamases that hydrolyse β-lactams), modification of the drug's target so it no longer binds, reduced entry of the drug into the cell, and active efflux pumps that expel it. These traits may be intrinsic to a species or acquired by mutation and—importantly—by horizontal gene transfer on plasmids and other mobile genetic elements, which lets resistance spread between organisms. Each exposure to an antimicrobial selects for cells carrying such traits, so that use of the agents drives the expansion of resistant populations, and accumulation of several mechanisms produces multidrug-resistant organisms.

Clinical relevance

Antimicrobial resistance narrows the options available to treat infections and is a central reason for the discipline of stewardship; understanding it is essential to interpreting susceptibility reports and the public-health literature. This entry describes the phenomenon conceptually and is not a source of treatment decisions, which depend on organism-specific susceptibility data and current guidelines.

Epidemiology

Resistance is a worldwide and growing problem, documented across human medicine, agriculture, and the environment. Hospital-acquired infections due to gram-negative bacteria illustrate how resistant organisms accumulate in healthcare settings, and broad reviews describe the drivers of resistance and the case for coordinated global action.

Evidence & guidelines

Authoritative reviews characterise the molecular mechanisms and the global drivers of resistance; the operational response is addressed through stewardship guidelines and surveillance programmes rather than within this conceptual entry.

History

Resistance was observed soon after the introduction of the first antibacterial agents and has tracked the use of every class since. Its recognition as a systemic threat—rather than a series of isolated drug failures—grew over the late twentieth and early twenty-first centuries, culminating in global frameworks for surveillance and control.

Key figures

• Stuart Levy
• Laura Piddock

Seminal works

• alekshun-levy-2007
• laxminarayan-2013
• blair-2015
• holmes-2016

Frequently asked questions

Is it bacteria or people that become resistant to antibiotics?

It is the microorganisms that become resistant, not the human host; antimicrobial resistance refers to changes in the microbe that let it survive a drug, and these resistant microbes can then spread between people and settings.

How does antibiotic use drive resistance?

Every exposure to an antimicrobial kills susceptible organisms preferentially and leaves resistant ones to multiply; over many exposures this selection pressure expands resistant populations, which is why use and resistance are linked.