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

## TL;DR

This study examines antibiotic resistance in bacteria causing infections in elderly patients, finding that resistance levels are similar to adults but with fewer drug-resistant strains.

## Contribution

The study provides updated antimicrobial susceptibility data for elderly patients, focusing on new β-lactamase inhibitor combinations.

## Key findings

- Aztreonam-avibactam showed high susceptibility (>99.9%) against Enterobacterales in elderly patients.
- CRE susceptibility was highest with aztreonam-avibactam (98.8%), cefiderocol (96.3%), and tigecycline (96.3%).
- Elderly patients had lower frequencies of CRE and MDR isolates compared to adults.

## Abstract

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.

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.

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.

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.