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