6. List ways by which organisms acquire antibiotic resistance.
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) 
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