Antimicrobial susceptibility of Enterobacterales causing infection in the elderly: focus on aztreonam-avibactam and recently approved β-lactamase inhibitor combinations.

Background: The US elderly population (≥65 years old) increased markedly in the last decades, and infections are responsible for approximately one-third of all deaths in this population. We evaluated the antimicrobial susceptibility of Enterobacterales causing infection in elderly patients in US hospitals.

Methods: Unique patient clinical isolates were consecutively collected from 72 US hospitals in 2021-2023 and tested for susceptibility by broth microdilution. Results for 10 574 Enterobacterales from elderly patients were analysed and compared with 9793 isolates from adult patients (18-64 years old). Carbapenem-resistant Enterobacterales (CRE) were screened for carbapenemases by whole-genome sequencing.

Results: All isolates from elderly patients were inhibited at aztreonam-avibactam MIC of ≤8 mg/L (>99.9% susceptible at ≤4 mg/L). Ceftazidime-avibactam and meropenem-vaborbactam were very active against Enterobacterales overall (≥99.7% susceptible) but exhibited limited activity against CRE (70.4%-71.6% susceptible). The most active agents against CRE were aztreonam-avibactam (98.8% susceptible), cefiderocol (96.3% susceptible) and tigecycline (96.3% susceptible). Susceptibility rates of isolates from the elderly were comparable (±≤ 2.6%) with those from the adult population; however, the frequencies of CRE and MDR phenotypes were lower among the elderly than adults. The most common carbapenemase among CREs from elderly patients were Klebsiella pneumoniae carbapenemases (55.6% of CRE) and NDM (24.7%); a metallo-β-lactamase was identified in 28.4% of CRE isolates.

Conclusions: Enterobacterales causing infections in the elderly population showed a similar antimicrobial resistance profile but a lower frequency of CRE and MDR isolates to those causing infection in the adults.