A Four-Center Study on the Activity of Aztreonam and Aztreonam-Avibactam Combination Against Clinical Isolates of Gram-Negative Bacilli Producing Representatives of Ambler Class A, B, C or D β-Lactamases.

We aimed to investigate the activity of aztreonam and aztreonam-avibactam combination against gram-negative bacilli producing different types of β-lactamases. Activity of aztreonam and aztreonam-avibactam against 166 isolates (33 AmpC-producing, 33 ESBL-producing, 66 carbapenemase-producing Enterobacterales, 34 metallo-β-lactamase-producing P. aeruginosa) from four centers in Türkiye was determined using gradient diffusion method. Aztreonam susceptibility rates among AmpC-producing, ESBL-producing and carbapenemase-producing Enterobacterales were 75.8%, 3% and 6.1%, respectively. All AmpC-producing or ESBL-producing Enterobacterales were susceptible to aztreonam-avibactam. ESBL-producing K. pneumoniae had significantly higher aztreonam-avibactam MICs compared with ESBL-producing E. coli. K. pneumoniae carrying blaCTX-M + blaSHV + blaTEM had significantly higher aztreonam-avibactam MICs when compared to those carrying only blaCTX-M. Aztreonam-avibactam susceptibility rate among carbapenemase-producing K. pneumoniae (100%) was significantly higher than that observed for carbapenemase-producing E. coli (65%). Metallo-β-lactamase-producing P. aeruginosa had significantly higher aztreonam-avibactam MICs (MIC50/90: 16/96 mg/L) when compared to metallo-β-lactamase-producing Enterobacterales (MIC50/90: 0.19/8 mg/L). Aztreonam-avibactam may be an option for treatment of infections caused by ESBL-producing or AmpC-producing K. pneumoniae, E. coli and carbapenemase-producing K. pneumoniae. However, aztreonam-avibactam resistance may be encountered in a considerable portion of carbapenem-resistant E. coli isolates. High aztreonam-avibactam MIC values suggest limited in vitro activity of aztreonam-avibactam against MBL-producing P. aeruginosa isolates in this collection.