# Synergistic activity of ceftazidime/avibactam combined with aztreonam against MBL-producing exoY+/exoT+/exoU+/exoS- extensively drug-resistant Pseudomonas aeruginosa

## TL;DR

This study explores the resistance mechanisms of drug-resistant Pseudomonas aeruginosa and finds that combining two antibiotics can effectively treat certain strains.

## Contribution

The study identifies a synergistic effect of ceftazidime/avibactam and aztreonam against specific drug-resistant Pseudomonas strains.

## Key findings

- XDR-PA isolates predominantly carried NDM-1 and specific T3SS virulence genes.
- The combination of CZA and ATM showed a synergistic effect in 77.78% of CZA-resistant XDR-PA strains.
- ST1971 was the most common sequence type among the isolates.

## Abstract

Extensively drug-resistant Pseudomonas aeruginosa (XDR-PA) has posed a great threat to public health due to their rising incidence and complicated resistance mechanisms and limited treatment options. XDR-PA has demonstrated high resistance rate to new antibiotic ceftazidime-avibactam (CZA). Therefore, this study was conducted to describe the resistance mechanisms, molecular epidemiology, and type III secretion system (T3SS) of XDR-PA, as well as to evaluate the synergistic antibacterial activity of CZA combined with aztreonam (ATM) against XDR-PA via in vitro experiments, aiming at providing insights for the prevention, control and treatment strategies of XDR-PA infections.

The carbapenemase resistance genes (VIM, IMP, NDM, KPC, GES, OXA-40) and T3SS virulence genes of XDR-PA isolates were identified using polymerase chain reaction (PCR) and sequencing. The expression levels of efflux pump systems (mexA and mexC), oprD2 porin and ampC were detected by the real-time fluorescent quantitative PCR (qPCR). The homology analysis of XDR-PA isolates was performed using multilocus sequence typing (MLST). Combined antimicrobial susceptibility testing of CZA and ATM were performed for XDR-PA isolates through in vitro experiments.

A total of 32 XDR-PA strains were isolated from clinical specimens from a tertiary teaching hospital in Southwest China between October 2022 to October 2023. Among the carbapenemase detected, metallo-β-lactamase (MBL) NDM-1 and VIM-2 were detected in 26 strains (81.25%, 26/32) and 1 strain (3.13%, 1/32) respectively. The efflux pump mexA had a higher expression in the XDR-PA group than that in the sensitive-PA (S-PA) group (P = 0.015). The T3SS virulence genes carried by XDR-PA strains mainly were exoY+/exoT+/exoU+/exoS- (87.50%, 28/32). The 32 XDR-PA isolates belonged to 8 different ST types, mainly including ST1971 and ST308, and the predominant ST type was ST1971 (71.88%, 23/32), with carrying both NDM-1 and exoY+/exoT+/exoU+/exoS-. Combined antimicrobial susceptibility testing revealed that among the 27 CZA-resistant XDR-PA strains, CZA and ATM combination showed a synergistic effect on 21 CZA-resistant XDR-PA strains (77.78%, 21/27), of which 20 strains carrying both MBL (95.24%, 20/21) and exoY+/exoT+/exoU+/exoS-.

The underlying resistance mechanisms of XDR-PA isolates involve the overexpression of efflux pump mexA and the existence of MBL. In addition, ST1971 was the predominant ST type in our study, with carrying both NDM-1 and exoY+/exoT+/exoU+/exoS-. Furthermore, combined antimicrobial susceptibility testing suggested that CZA and ATM combination has potential against MBL-producing exoY+/exoT+/exoU+/exoS- XDR-PA. These findings may provide clues for the prevention, control and treatment strategies of XDR-PA infections.