Bacterial Genetics and Resistance

Antibiotic resistance arises by mutation and spreads through mobile genetic elements and horizontal gene transfer.

Mutation is a permanent, heritable change in the DNA sequence, the fundamental source of new genetic variation. A mutation conferring survival advantage under a specific selective pressure, antibiotic exposure being the clinically dominant example, expands within the population.

Plasmids are small, circular, extrachromosomal DNA molecules, replicating independently of the chromosome, frequently carrying resistance and virulence genes.

Transposons are mobile DNA segments, sometimes called jumping genes, capable of moving between different locations within a genome, or between the chromosome and a plasmid, or between two plasmids, without requiring the extensive sequence homology normally needed for standard genetic recombination.

This capacity to move between the chromosome and plasmids is clinically significant. A resistance gene that arose on the chromosome can become mobilised onto a plasmid, and thereby transferable to other bacteria via conjugation. Conversely, a plasmid-borne gene can integrate into the chromosome for more stable, long-term inheritance.

Integrons are genetic elements capable of capturing and expressing gene cassettes, small, mobile units, frequently carrying antibiotic resistance genes, via a specific site-specific recombination system.

Integrons are of major clinical significance because a single integron can accumulate and simultaneously express multiple different resistance gene cassettes in sequence. A single horizontal gene transfer event, acquisition of one integron, can therefore confer resistance to several unrelated antibiotic classes at once. This is a key mechanism underlying the emergence of multidrug-resistant strains from what may originally have been a single genetic acquisition event.

Transformation, transduction, and conjugation are the three mechanisms of horizontal gene transfer:

• transformation: uptake of free environmental DNA, requiring recipient competence
• transduction: bacteriophage-mediated transfer
• conjugation: direct cell-to-cell transfer via the sex pilus, the single most clinically significant mechanism for resistance gene spread

Spread is particularly efficient when the transferred plasmid also carries an integron bearing multiple resistance cassettes, combining two mechanisms into one especially efficient resistance-spreading event.

Enzymatic drug destruction occurs when the organism produces an enzyme that chemically inactivates the antibiotic before it can reach its target.

Beta-lactamases, hydrolysing the beta-lactam ring shared by penicillins, cephalosporins, and carbapenems, are the dominant example. Specific beta-lactamase subtypes confer progressively broader resistance across the beta-lactam class.

Aminoglycoside-modifying enzymes, acetylation, phosphorylation, adenylation, represent a parallel enzymatic destruction and inactivation strategy for a different drug class.

Target modification occurs when the organism alters the specific molecule the antibiotic normally binds, reducing or abolishing drug binding while usually preserving the target molecule's normal cellular function. Altered penicillin-binding proteins, the mechanism underlying MRSA, and altered ribosomal binding sites, relevant to some macrolide and aminoglycoside resistance, are the classic examples.

Efflux pumps are membrane transport proteins that actively export the antibiotic out of the bacterial cell before it can accumulate to an effective intracellular concentration.

This mechanism is of particular importance in Gram negative organisms, contributing to Pseudomonas aeruginosa's broad intrinsic resistance profile. It is present across many organism types and drug classes.

Reduced permeability is decreased entry of the antibiotic into the bacterial cell, most classically via loss or mutation of outer membrane porins in Gram negative organisms, reducing the drug's access to its intracellular or periplasmic target even without any change to the target itself.

Metabolic bypass occurs when the organism acquires an alternative metabolic pathway or enzyme that achieves the same essential cellular function as the one the antibiotic blocks, circumventing the drug's effect entirely. The classic example is acquisition of an alternative folate synthesis enzyme with low affinity for sulfonamide and trimethoprim drugs, allowing continued folate synthesis despite the presence of these folate-pathway-inhibiting agents.

MRSA, methicillin-resistant Staphylococcus aureus, achieves resistance via the mecA gene, carried on a mobile genetic element, the staphylococcal cassette chromosome mec.

The gene encodes an altered penicillin-binding protein, PBP2a, with markedly reduced affinity for beta-lactams. This confers resistance across essentially the entire beta-lactam class and is a target modification mechanism.

VRE, vancomycin-resistant Enterococcus, achieves resistance via acquired van genes, notably vanA and vanB, altering the cell wall precursor target that vancomycin normally binds, a target modification mechanism, disproportionately affecting Enterococcus faecium.

ESBL, extended-spectrum beta-lactamase, is a beta-lactamase enzyme class conferring resistance to penicillins and most cephalosporins, including third-generation agents such as ceftriaxone and cefotaxime, though characteristically remaining susceptible to carbapenems and inhibited, at least partially, by beta-lactamase inhibitor combinations, an enzymatic drug destruction mechanism. It is a major and increasing concern particularly within E. coli and Klebsiella pneumoniae.

AmpC beta-lactamase is a distinct beta-lactamase class, chromosomally encoded and often inducible in specific organisms, notably Enterobacter, Serratia, and some other Enterobacterales.

This means resistance can emerge during treatment with certain cephalosporins even when the initial isolate appeared susceptible on testing. This is a clinically important and specific pitfall distinguishing AmpC from the typically stably expressed ESBL enzymes.

Unlike ESBLs, AmpC enzymes are generally not inhibited by standard beta-lactamase inhibitors such as clavulanic acid.

Carbapenemases are a further, even broader beta-lactamase class capable of hydrolysing carbapenems themselves, the class of beta-lactams normally reserved as reliable broad-spectrum options against ESBL-producing organisms. Carbapenemase-producing organisms, notably carbapenem-resistant Klebsiella pneumoniae via enzymes such as KPC, represent one of the most significant contemporary global antimicrobial resistance threats, often leaving only a very limited range of last-resort agents, such as colistin, as viable treatment options.