Intrinsically resistant to ampicillin.

More resistant to cephalosporins than K. pneumoniae.

Almost always susceptible: aminoglycosides, TMP-SMX, fluoroquinolones, carbapenems, aztreonam, tigecycline, colistin.

Resistant strains may be R to third generation cephalosporins (ESBLs), and rarely to carbapenems (i.e. carbapenemase enzymes).

Fosfomycin and nitrofurantoin for UTI only, when confirmed susceptible