Table 1.

Main resistance mechanisms in Gram-negative pathogens

Microorganism	Mechanism
Pseudomonas aeruginosa	Efflux pumps
	Mutant topoisomerases
	Modifying enzymes
	AmpC/porin reduction combinations
	Metallo-b-lactamases
	Outer membrane permeability
	Extended spectrum β-lactamases
Acinetobacter baumannii	β-lactamases
	Efflux pumps
	Membrane permeability
	Aminoglycoside-modifying enzymes
	Alteration of target sites
Klebsiella pneumoniae	Modifying enzyme
	Efflux pumps
	Mutant topoisomerases
	Extended spectrum β-lactamases
	Carbapenemases
Stenotrophomonas maltophilia	β-lactamases (L1 and L2)
	Multidrug efflux pumps
	Antibiotic-modifying enzymes
	Mutations of bacterial topoisomerase and gyrase genes
	Reduction in outer membrane permeability