Infectious Diseases
Antibiotic Mechanisms and Resistance
~4 min read4 sections
Antibiotic mechanisms and resistance represent the fundamental pharmacology of antimicrobial therapy and the critical challenge of bacterial adaptation. Understanding how antibiotics kill or inhibit bacteria is essential for appropriate drug selection, dosing, and timing, while comprehending resistance mechanisms explains clinical failures and guides stewardship practices. Resistance has become a major public health crisis, with organisms like methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE), and carbapenem-resistant Enterobacteriaceae (CRE) causing significant morbidity and mortality globally. The interplay between antibiotic use and resistance selection pressure demands that clinicians understand both optimal utilization and emerging resistance patterns.
Antibiotic Mechanisms (How antibiotics work)
- Cell Wall Synthesis Inhibition – Beta-lactams (penicillins, cephalosporins, carbapenems) and vancomycin inhibit cross-linking of peptidoglycan by binding penicillin-binding proteins (PBPs), causing cell wall weakening, osmotic lysis, and bactericidal death. Glycopeptides like vancomycin bind D-Ala-D-Ala residues directly, preventing cross-linking.
- Protein Synthesis Inhibition – 30S ribosomal inhibitors (aminoglycosides, tetracyclines, streptomycin) and 50S inhibitors (macrolides, chloramphenicol, linezolid) bind bacterial ribosomes, preventing translation initiation or elongation. Aminoglycosides are bactericidal; most others are bacteriostatic.
- DNA/RNA Synthesis Inhibition – Fluoroquinolones inhibit bacterial DNA gyrase and topoisomerase IV, preventing DNA replication and transcription (bactericidal). Rifampin inhibits bacterial RNA polymerase. Newer agents like fidaxomicin inhibit RNA polymerase directly.
- Metabolic Pathway Disruption – Trimethoprim-sulfamethoxazole (TMP-SMX) inhibits sequential steps in folate synthesis (dihydropteroate synthase and dihydrofolate reductase), disrupting dNTP synthesis required for nucleic acid synthesis (bacteriostatic).
- Cell Membrane Disruption – Colistin and polymyxins are cationic peptides that disrupt bacterial cell membranes through electrostatic interactions, causing leakage and cell death (bactericidal but highly toxic).
Bacterial Resistance Mechanisms (How bacteria resist)
- Enzymatic Inactivation – Beta-lactamase production (most common mechanism) hydrolyzes the beta-lactam ring of penicillins and cephalosporins, rendering them inactive. Extended-spectrum beta-lactamases (ESBLs) confer resistance to 3rd-generation cephalosporins; carbapenemases (KPC, NDM, VIM) hydrolyze carbapenems. Aminoglycoside-modifying enzymes (AMEs: acetyltransferases, phosphotransferases, nucleotidyltransferases) inactivate aminoglycosides by acetylation or phosphorylation.
- Altered Target Site – PBP mutations reduce beta-lactam binding affinity (e.g., methicillin resistance in MRSA via mecA gene encoding altered PBP2a). Ribosomal mutations (16S/23S rRNA changes) reduce aminoglycoside and macrolide binding. Quinolone resistance arises from mutations in DNA gyrase (gyrA/gyrB) and topoisomerase IV (parC/parE).
- Efflux Pump Overexpression – Multidrug efflux pumps actively transport antibiotics out of the bacterial cell before they reach target sites (e.g., AcrAB-TolC in Gram-negatives, NorA in MRSA). Confers resistance to fluoroquinolones, macrolides, tetracyclines, and beta-lactams.
- Reduced Permeability – Outer membrane porin loss in Gram-negatives (especially Pseudomonas aeruginosa and Acinetobacter) decreases beta-lactam and fluoroquinolone uptake. Gram-positive cell wall changes reduce vancomycin penetration in some strains.
- Target Bypass or Modification – Vancomycin resistance in Enterococcus occurs via van genes replacing D-Ala-D-Ala with D-Ala-D-Lac in peptidoglycan precursors, reducing vancomycin binding affinity. Macrolide resistance via methylation of 23S rRNA (erm genes) or efflux.
- Horizontal Gene Transfer – Plasmids, transposons, and integrons transfer resistance genes between bacteria, enabling rapid spread. Conjugation is the most clinically relevant mechanism for hospital-acquired resistance dissemination.
Resistance itself does not cause symptoms—it manifests as clinical failure despite appropriate antibiotic therapy. Recognize these presentations:
- Persistent bacteremia or sepsis despite adequate antibiotics – Fever, hypotension, and positive blood cultures continuing ≥48-72 hours after culture and initiation of antibiotics strongly suggest resistant pathogen. Classic sign: "patient should be improving but isn't."
- Recurrent or relapsing infection with same organism – Infections that initially improve then return with the same organism (documented by repeat cultures) suggest inadequate antibiotic coverage or emergence of resistance during therapy.
- Infection in high-risk settings without clinical improvement – Healthcare-associated infections (HAIs), ICU-acquired infections, or recent antibiotic exposure with failure to respond within 48-72 hours. Particularly concerning in pneumonia (no improvement in oxygenation, fever curve), UTIs (persistent pyuria/bacteriuria), or surgical site infections (worsening erythema/drainage).
- Atypical presentations or polymicrobial infections – Resistance often emerges in complex biofilms or polymicrobial infections (e.g., diabetic foot ulcers, osteomyelitis) where multiple organisms with varying resistance patterns coexist.
- Important clinical pearl: Absence of symptoms does not exclude resistant infection; bacteremia can persist without fever in immunocompromised patients. Always correlate clinical response with culture results and susceptibility testing.
Diagnosis of resistant infection requires microbiologic documentation and susceptibility testing:
- Culture and Identification – Blood cultures (gold standard for bacteremia), wound cultures, urine cultures, sputum, or other body fluid cultures are mandatory to identify the organism. Time-to-positivity in blood cultures can predict pathogen identity (early positivity suggests S. aureus; delayed suggests coagulase-negative staph or contaminant). Direct Gram stain provides rapid morphologic identification.
- Antibiotic Susceptibility Testing (AST) – Disk diffusion (Kirby-Bauer) or broth microdilution provides minimum inhibitory concentration (MIC) values and categorical interpretations (susceptible, intermediate, resistant) per CLSI or EUCAST standards. E-test is a semi-quantitative strip method. Molecular/PCR testing rapidly identifies common resistance genes (mecA for MRSA, vanA/vanB for VRE, carbapenemase genes for CRE) without waiting for culture/susceptibility (24-48 hours faster).
- Special Testing Considerations – Beta-lactamase testing (chromogenic cephalosporin, nitrocefin disk) is important for Staphylococcus aureus to predict beta-lactam resistance. Vancomycin heteroresistance (reduced susceptibility strains with elevated MICs) may not be detected by standard AST; clinical failure with MRSA despite vancomycin suggests heteroresistance. Carbapenem resistance detection is critical in Gram-negatives; use carbapenem + beta-lactamase inhibitor combinations or modified Hodge test to differentiate carbapenemase from non-enzymatic resistance.
- Important diagnostic considerations – Culture should be obtained before antibiotic initiation when possible, but do not delay empiric antibiotics in septic patients. Repeat cultures 24