Table 5.
Main mechanisms of acquired antimicrobial resistance in Acinetobacter baumannii
| Mechanism | Genetic event | Antimicrobials |
|---|---|---|
| High-level expressed AmpC cephalosporinase | Chromosomal mutation | Penicillins (with or without beta-lactamase inhibitors), 3GC |
| High-level expressed OXA-51-like beta-lactamase | Chromosomal mutation (insertion of ISAba1 upstream of bla OXA-51) | Carbapenems |
| Other beta-lactamases | ||
| Extended-spectrum beta-lactamasesa | MGE acquisition | Penicillins, 3GC |
| Metallo-beta-lactamasesb (carbapenemases) | Penicillins, 3GC, carbapenems | |
| Oxacillinase-type carbapenemases3 | Penicillins, carbapenems | |
| Functional loss of porins (impermeability) | Chromosomal mutation | Variable |
| Altered penicillin-binding proteins | Chromosomal mutation | Variable |
| Active efflux pumps | ||
| AdeABC | Chromosomal mutation | Beta-lactams (variable), aminoglycosides, fluoroquinolones, tigecycline |
| AdeM | Aminoglycosides, fluoroquinolones | |
| AdeIJK | Tigecycline | |
| Aminoglycoside-modifying enzymesd | MGE acquisition | Aminoglycosides |
| 16S rRNA methylases | MGE acquisition | Aminoglycosides |
| Topoisomerases modifications | Chromosomal mutation | Fluoroquinolones |
| Lipid A (LPS) modifications | Chromosomal mutation | Polymyxins |
MGE mobile genetic element (plasmid or transposon), 3GC third-generation cephalosporins.
Most common enzyme types: aPER, VEB and GES (TEM, SHV and CTX-M are rare in A. baumannii); bVIM, SIM, IMP and NDM; cOXA-23-, OXA-40-, OXA-58-, OXA-143 and OXA-235-like; dAAC(3), AAC(6′) and APH(3′).