Table 3.

Mechanisms of antibiotic resistance found in Acinetobacter species1,7,54,55

Mechanism of resistance	Genetic mechanisms	Antimicrobials affected
A. Antimicrobial inactivating (hydrolysing) enzymes
Amp C Beta-lactamases [Acinetobacter-derived cephalosporinases (ADCs)]	Chromosomal mediated Insertion sequences ISAba1 and IS1135 increase production of beta-lactamase	Extended spectrum cephalosporins (including 3rd generation and cephamycin group); cefepime and carbapenems are spared
Ambler class D OXA-type enzymes	Chromosomal and Plasmid mediated	Carbapenems
Ambler class B metallo–b-lactamases (MBLs), such as VIM and IMP	Mobile genetic elements	Carbapenems
Ambler class A ESBLs (TEM, SHV)	Plasmid, chromosomal or mobile genetic elements	All cephalosporins (including 3rd generation) except cephamycin group
B. Reduced access to bacterial targets
Altered porin channels and other outer membrane proteins	Point mutations	Carbapenems
C. Mutations that change targets or cellular functions
DNA topoisomerase mutations	Point mutations in the bacterial targets gyrA and parC topoisomerase enzymes	Quinolones
Aminoglycoside-modifying enzyme	Plasmid, transposons	Aminoglycosides
Production of efflux pumps	Point mutations	Tigecycline, aminoglycosides, quinolones, tetracyclines
Modification of cell membrane lipopolysccarides	Point mutations	Colistin