Table 1.
| Classification | Mechanism | Common bacterial species | Examples | Substrate |
|---|---|---|---|---|
| B-lactamase Ambler class A |
Extended-spectrum Or ESBLs |
Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter spp., Kluyvera spp. | SHV-like, CTX-like, KLUG-like | Penicillins, cephalosporins (except cefamycins), aztreonam Frequently co-transferred with VIM |
| B-lactamase Ambler class A |
Serine carbapenemases Acquisition of a mobile genetic element |
Klebsiella spp. | KPC-like, IMI-like | Penicillins, cephalosporins, aztreonam, carbapenems |
| B-lactamase Ambler class B |
Metallo-β-lactamases, carbapenemases Acquisition of a mobile genetic element | Stenotrophomonas maltophilia, P. aeruginosa, Bacteroides fragilis, Acinetobacter baumannii | VIM-like, IMP-like, NDM-like, GIM, SPM, SIM | Penicillins, cephalosporins, and carbapenems. Monobactams are stable |
| B-lactamase Ambler class C |
Extended-spectrum, cephalosporinases, Mainly Chromosomal |
Enterobacter spp., Klebsiella spp., Proteus spp., Citrobacter spp., E. coli | AmpC, P99, ACT-like, CMY-like, MIR-like | – |
| B-lactamase Ambler class D |
Carbapenemases | A. baumannii, P. aeruginosa, E. coli, | OXA-like (OXA-51, OXA-23) | Penicillin, aztreonam, and carbapenems |
| Porin mutations (Loss of outer membrane permeability) |
Chromosomal mutation |
P.aeruginosa, A. baumannii |
OprD CarO, | Imipenem |
| Efflux pumps | Chromosomal mutations Different antimicrobial classes may be substrates of a single pump: exposure to a given class (e.g., beta-lactams) may thereby select mutants with resistance to other classes |
P. aeruginosa | MexAB-OprM | Ticarcillin, aztreonam, cefepime, meropenem, quinolones |
| – | – | A. baumannii | AdeABC | Beta-lactams (variable), aminoglycosides, fluoroquinolones, tigecycline |
| Topoisomerase modifications Gyrase modifications |
Chromosomal mutation | P.aeruginosa, A. Baumannii, Enterobacteriaceae | – | Fluoroquinolones |
| Qnr | Plasmid mediated | Enterobacteriaceae | A, B, C, D and S subtypes | Fluoroquinolones |
| Aminoglycoside-modifying enzymes | Acquisition of a mobile genetic element Aminoglycoside phosphotransferase, APH, aminoglycoside nucleotidyltransferase, ANT and aminoglycoside acetyltransferase, AAC |
Enterobacteriaceae, A. baumannii | AAC(3), AAC(6') and APH(3′) | Aminoglycosides |
| Methylases of the 16S ribosomal subunit | Acquisition of a mobile genetic element | NDM-1 producing strains (mainly Enterobacteriaceae) | ArmA Rmt |
Plazomicin is stable against the majority of AMEs but is being hydrolysed by Rmts |
| Lipid A (LPS) modifications | Chromosomal mutation | P.aeruginosa, K. Pneumoniae, A. baumannii | – | Colistin |
| PmrA–PmrB two-component system genetic modifications | Chromosomal mutation | A. baumannii, P.aeruginosa, K. Pneumoniae | – | Colistin |
| Mcr1 gene mutation | Plasmid mediated | Enterobacteriaceae | – | Colistin |