Antibiotic resistance can be categorized in three types:

• Natural or intrinsic resistance (predictable resistance basis of tables 1 & 2)
• 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. AfnpC cephalosporinase in Klebsiella
• Mutational resistance (Unpredictable resistance)
• Target site modification (i.e. Streptomycin resistance: mutations in rDNA genes (rpsL), -lactam resistance: change in PBPs (penicillin binding proteins))
• Reduced permeability or uptake
• Metabolic by-pass (i.e trimethoprim resistance: overproduction of DHF (dihydrofolate) reductase orthi- mutants in S. aureus)
• Derepression of multidrug efflux systems
• Extrachromosomal or acquired resistance (Unpredictable resistance. Disseminated by plasmids or transposons)
• Drug inactivation (i.e. aminoglycoside-modifying enzymes, -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)