14.5 Drug Resistance

Drug resistance occurs when microorganisms evolve to survive the very drugs designed to kill or inhibit them. This is one of the most pressing problems in modern medicine because it shrinks the arsenal of effective treatments, drives up healthcare costs, and makes once-manageable infections potentially life-threatening.

Resistance can be inherent to a microorganism, or it can be acquired through mutations and gene sharing. Selective pressure from antimicrobial use speeds the whole process up. This section covers how resistance develops, the specific mechanisms bacteria, viruses, and fungi use to evade drugs, and what the medical community is doing about it.

Mechanisms and Impact of Drug Resistance

Drug resistance and antimicrobial effectiveness

A microorganism is considered drug-resistant when it can survive and multiply at drug concentrations that would normally inhibit or kill it. This applies to bacteria, viruses, and fungi alike.

The consequences are significant:

• Treatments that once worked become ineffective, forcing clinicians to use higher doses or switch to alternative (often more toxic or expensive) drugs
• Resistant organisms spread through populations, increasing the prevalence of hard-to-treat infections. Methicillin-resistant Staphylococcus aureus (MRSA) is a well-known example that has become widespread in both hospitals and community settings.
• Healthcare costs rise and hospital stays lengthen when first-line treatments fail

Resistance can emerge against virtually any drug class, including antibiotics (penicillins, fluoroquinolones), antivirals, and antifungals (azoles). One measurable indicator of resistance is the minimum inhibitory concentration (MIC), which is the lowest concentration of a drug that prevents visible growth. As resistance develops, the MIC climbs, meaning you need more drug to achieve the same effect.

Development of antimicrobial resistance

Resistance arises through three main routes:

Intrinsic resistance refers to natural characteristics of a microorganism that make it inherently resistant to certain drugs. For example, the outer membrane of Gram-negative bacteria acts as a physical barrier that blocks entry of some antibiotics, such as certain large or hydrophobic molecules. This isn't something the bacterium "learned" to do; it's just part of its biology.

Acquired resistance is what most people think of when they hear "drug resistance." Microorganisms gain new resistance capabilities through two main paths:

• Mutations in chromosomal genes can alter drug binding sites, reduce drug uptake, or change metabolic pathways. A classic example: mutations in the rpoB gene of Mycobacterium tuberculosis alter the target of rifampin, preventing the drug from binding effectively.
• Horizontal gene transfer (HGT) allows resistance genes to spread between organisms, even across species. This happens through three mechanisms:
  1. Conjugation — direct cell-to-cell contact transfers plasmids carrying resistance genes. This is the most clinically significant route because plasmids can carry multiple resistance genes at once.
  2. Transformation — a bacterium picks up free-floating DNA from its environment. Streptococcus pneumoniae can acquire antibiotic resistance this way.
  3. Transduction — bacteriophages (viruses that infect bacteria) accidentally package and transfer resistance genes between bacterial cells, as seen in Escherichia coli.

Selective pressure is the driving force behind resistance spreading through a population. When antimicrobials are present, susceptible organisms die while resistant ones survive and reproduce. Over time, the resistant population dominates. Inappropriate or excessive antimicrobial use accelerates this process. A major contributor is antibiotic use in agriculture, where sub-therapeutic doses given to livestock create ideal conditions for resistance to develop.

Multidrug Resistance and Superbugs

Multidrug resistance (MDR) occurs when a microorganism becomes resistant to three or more classes of antimicrobial drugs. This typically results from accumulating multiple resistance mechanisms over time or acquiring mobile genetic elements (like plasmids) that carry several resistance genes simultaneously.

Superbugs are MDR strains that resist most or all available treatments. Examples include MRSA, carbapenem-resistant Enterobacteriaceae (CRE), and extensively drug-resistant tuberculosis (XDR-TB). These pose enormous challenges in healthcare settings because treatment options become extremely limited.

Two key strategies address this growing threat:

• The One Health approach recognizes that antimicrobial resistance is interconnected across human medicine, veterinary medicine, and the environment. Resistance genes don't stay in one setting; they move between animals, humans, and ecosystems.
• Antibiotic stewardship programs work within healthcare facilities to optimize antimicrobial prescribing. The goal is to use the right drug, at the right dose, for the right duration, reducing unnecessary selective pressure.

Resistance Mechanisms Across Microorganisms

Mechanisms of resistance across microorganisms

Each type of microorganism has its own toolkit for evading antimicrobial drugs. The details differ, but the strategies share common themes: destroy the drug, change the target, or keep the drug out.

Bacteria use three primary strategies:

• Enzymatic degradation of the drug — Bacteria can produce enzymes that chemically modify or break down antimicrobials before they reach their target. The most clinically important example is beta-lactamases, which hydrolyze the beta-lactam ring in penicillins and cephalosporins, rendering them inactive. Extended-spectrum beta-lactamases (ESBLs) are particularly dangerous because they can break down a wide range of beta-lactam antibiotics.
• Alteration of drug targets — Mutations in genes encoding the proteins that drugs bind to can reduce binding affinity. For instance, mutations in DNA gyrase or topoisomerase IV confer fluoroquinolone resistance in Pseudomonas aeruginosa.
• Reduced uptake or increased efflux — Some bacteria modify their cell wall or membrane to restrict drug entry. Vancomycin resistance in Enterococcus involves remodeling of the peptidoglycan precursor target, reducing drug binding. Other bacteria ramp up efflux pumps, which are membrane proteins that actively pump drugs back out of the cell before they can act. Tetracycline resistance in E. coli commonly works this way.

Viruses rely heavily on their high mutation rates:

• Mutations in drug-targeted enzymes — Antiviral drugs often target specific viral enzymes. Mutations in HIV reverse transcriptase, for example, can confer resistance to nucleoside reverse transcriptase inhibitors (NRTIs). The M184V mutation is a well-known cause of lamivudine resistance.
• Alterations in viral proteins — Changes in proteins that interact with antiviral drugs reduce drug efficacy. Mutations in HIV's gp120 envelope protein can decrease the effectiveness of entry inhibitors like maraviroc.
• High mutation rates and genome diversity — Viruses replicate rapidly, and many viral polymerases lack proofreading ability. This means resistant variants emerge frequently. Influenza virus resistance to adamantanes (amantadine, rimantadine) became so widespread that these drugs are no longer recommended for influenza treatment.

Fungi use mechanisms that parallel bacterial resistance:

• Overexpression of drug target enzymes — If a fungal cell produces more of the enzyme a drug inhibits, the drug can't block enough of it to be effective. Increased production of lanosterol 14α-demethylase (the target of azole antifungals) is a common mechanism of fluconazole resistance in Candida albicans.
• Mutations in drug target genes — Alterations in enzymes of the ergosterol biosynthesis pathway reduce the binding affinity of antifungal agents. Echinocandin resistance in Candida glabrata results from mutations in the FKS genes encoding the drug's target, glucan synthase.
• Efflux pump overexpression — Just like bacteria, fungi can upregulate efflux pumps to remove drugs from the cell. In Candida species, pumps such as Cdr1p and Mdr1p actively export azole antifungals, reducing intracellular drug concentrations below effective levels.