Organisms possessing Extended-Spectrum Beta-Lactamases (ES?L) may be clinically resistant to therapy with penicillins, cephalosporins, and aztreonam, despite apparent in vitro susceptibility to some of these agents. The incidence of these strains is on the rise and is a significant health concern in some regions. Because of this, the screening and confirmation of ES?L-producing strains is recommended by all major national guidelines.