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