Intrinsically resistant to ampicillin.

Usually susceptible to: cephalosporins including first generation, aminoglycosides, TMP-SMX, fluoroquinolones, and Pip-Tazo.

Almost always susceptible: 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