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 (cystitis) only, when confirmed susceptible.