Antibiotic Resistance Pharmacology
Antibiotic resistance emerges through genetic mechanisms including enzymatic inactivation, target modification, and efflux pumps that can be transmitted horizontally.
Overview
Antibiotic resistance is one of the most urgent public health threats globally. Resistance can be intrinsic (natural property of a species) or acquired (through mutation or horizontal gene transfer). Understanding resistance mechanisms guides empiric antibiotic selection and the development of strategies to combat resistant organisms.
Enzymatic Inactivation
Beta-lactamases: Serine or metallo-enzymes that hydrolyze the beta-lactam ring. Classification by molecular class (Ambler A-D) and functional group (Bush-Jacoby):
- Class A: ESBLs (extended-spectrum beta-lactamases) — TEM, SHV, CTX-M — hydrolyze 3rd-gen cephalosporins and monobactams but not carbapenems; inhibited by clavulanate/tazobactam
- Class A KPC: Carbapenem-hydrolyzing; inhibited by avibactam, vaborbactam
- Class B: Metallo-beta-lactamases (NDM, VIM, IMP) — require zinc; not inhibited by classical BLIs; hydrolyze all beta-lactams including carbapenems (not aztreonam)
- Class D: OXA carbapenemases — variable inhibitor susceptibility
Aminoglycoside-modifying enzymes: Acetyltransferases, phosphotransferases, and nucleotidyltransferases add chemical groups to aminoglycosides, preventing ribosomal binding. Amikacin's bulky structure resists most of these enzymes.
Target Modification
- MRSA (PBP2a): mecA gene encodes a low-affinity PBP2a (PBP2') that has reduced beta-lactam binding; maintains transpeptidase activity when normal PBPs are inhibited
- Vancomycin resistance (VanA/VanB): D-Ala-D-Lac substitution reduces vancomycin binding 1000-fold
- Quinolone resistance: GyrA/GyrB and ParC/ParE mutations reduce fluoroquinolone binding
- Rifampin resistance: RpoB mutations prevent rifampin binding to RNA polymerase beta subunit — emerges rapidly with monotherapy
Efflux Pumps
Active transporters use the proton motive force or ATP to expel antibiotics:
- RND family (AcrAB-TolC in E. coli; MexAB-OprM in Pseudomonas): Broad-spectrum, expelling beta-lactams, fluoroquinolones, macrolides, tetracyclines
- MFS family (tetracycline efflux — Tet pumps)
- ABC transporters: Require ATP; macrolide efflux (mef genes in S. pneumoniae)
Horizontal Gene Transfer
Resistance genes spread between bacteria via:
- Plasmids: Most common vehicle for ESBL, carbapenemase, and aminoglycoside resistance genes; conjugative transfer
- Transposons: "Jumping genes" that can move between plasmids and chromosomes
- Integrons: Gene cassette capture systems; common in gram-negative MDR pathogens
- Bacteriophage transduction: Less common but contributes to resistance spread
Key Takeaways
- Beta-lactamases are the dominant resistance mechanism against beta-lactams; ESBLs and KPC are major threats
- MRSA uses PBP2a (mecA) to bypass beta-lactam inhibition; ceftaroline and vancomycin retain activity
- Efflux pumps (RND family) confer multi-class resistance in gram-negative organisms
- Horizontal gene transfer via plasmids and transposons drives rapid spread of resistance across species