Table 2.

Efflux pump systems and their activity versus antibiotics.

Efflux System	Antibiotics Affected by Altered Expression
MexAB-OprM	Aztreonam, β-lactams a, macrolides, tetracyclines, lincomycin, chloramphenicol, fluoroquinolones [11,50,54], higher in MIC or resistance to relebactam [69] could raise MIC of cefiderocol b [41].
MexXY-OprM	Aminoglycosides, cefepime, fluoroquinolones, tetracyclines and macrolides [53,59,69].
MexCD-OprJ	Fluoroquinolones, penems, and cefepime [59,61,62], carbanecillin, piperacillin, ceftolozane/tazobactam, and ceftazidime/avibactam c [63,64].
MexEF-OprN	Chloramphenicol, fluoroquinolones, trimethoprim, colistin, and imipenem d [65,66,67,68].

^a MexAB-OprM exports all β-lactams, except imipenem. ^b The overexpression of MexAB-OprM alone cannot provide straightforward resistance to cefiderocol. ^c Resistance to carbanecillin, piperacillin, ceftolozane/tazobactam, and ceftazidime/avibactam occurs with a specific mutation for MexD. ^d Resistance to imipenem due to the contemporary downregulation of OprD.