To reduce the development of drug-resistant bacteria and maintain the effectiveness of aztreonam for injection, USP and other antibacterial drugs, aztreonam for injection should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy. Aztreonam for Injection is indicated for the treatment of the following infections caused by susceptible Gram-negative microorganisms: Urinary Tract Infections , including pyelonephritis and cystitis caused by Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Enterobacter cloacae, Klebsiella oxytoca *, Citrobacter species*, and Serratia marcescens *. Lower Respiratory Tract Infections, including pneumonia and bronchitis caused by Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Haemophilus influenzae, Proteus mirabilis, Enterobacter species, and Serratia marcescens *. Septicemia caused by Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Proteus mirabilis *, Serratia marcescens *, and Enterobacter species. Skin and Skin-Structure Infections, including those associated with postoperative wounds, ulcers, and burns, caused by Escherichia coli, Proteus mirabilis, Serratia marcescens, Enterobacter species, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Citrobacter species*. * Efficacy for this organism in this organ system was studied in fewer than 10 infections. Intra-abdominal Infections, including peritonitis caused by Escherichia coli, Klebsiella species including K. pneumoniae, Enterobacter species including E. cloacae *, Pseudomonas aeruginosa, Citrobacter species* including C. freundii *, and Serratia species* including S. marcescens *. Gynecologic Infections, including endometritis and pelvic cellulitis caused by Escherichia coli, Klebsiella pneumoniae *, Enterobacter species* including E. cloacae *, and Proteus mirabilis *. Aztreonam for injection is indicated for adjunctive therapy to surgery in the management of infections caused by susceptible organisms, including abscesses, infections complicating hollow viscus perforations, cutaneous infections, and infections of serous surfaces. Aztreonam for injection is effective against most of the commonly encountered Gram-negative aerobic pathogens seen in general surgery. Concurrent Therapy Concurrent initial therapy with other antimicrobial agents and aztreonam for injection is recommended before the causative organism is known in seriously ill patients who are also at risk of having an infection due to Gram-positive aerobic pathogens. If anaerobic organisms are also suspected as etiologic agents, therapy should be initiated using an anti-anaerobic agent concurrently with aztreonam for injection. Certain antibiotics may induce high levels of beta-lactamase in vitro in some Gram-negative aerobes such as Enterobacter and Pseudomonas species, resulting in antagonism to many beta-lactam antibiotics including aztreonam. These in vitro findings suggest that such beta-lactamase-inducing antibiotics not be used concurrently with aztreonam. Following identification and susceptibility testing of the causative organism, appropriate antibiotic therapy should be continued.