Antibiotic resistance can be categorized in three types:
1. Natural or intrinsic resistance
- Inaccessibility of the target (i.e. impermeability resistance due to the absence of an adequate transporter: aminoglycoside resistance in strict anaerobes)
- Multidrug efflux systems: i.e. AcrE in E. coli, MexB in P. aeruginosa
- Drug inactivation: i.e. AmpC cephalosporinase in Klebsiella
2. Mutational resistance
- Target site modification (i.e. Streptomycin resistance: mutations in rDNA genes (rpsL), ß-lactam resistance: change in PBPs (penicillin binding proteins))
- Reduced permeability or uptake
- Metabolic by-pass (i.e trimethoprim resistance: overproduction of DHF (dihydrofolate) reductase or thi- mutants in S. aureus)
- Derepression of multidrug efflux systems
3. Extrachromosomal or acquired resistance (Disseminated by plasmids or transposons)
- Drug inactivation (i.e. aminoglycoside-modifying enzymes, ß-lactamases, chloramphenicol acetyltransferase)
- Efflux system (i.e. tetracycline efflux)
- Target site modification (i.e. methylation in the 23S component of the 50S ribosomal subunit: Erm methylases)
- Metabolic by-pass (i.e trimethoprim resistance: resistant DHF reductase)
