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