# Functional characterization of a novel class A carbapenemase CAE-1 in carbapenem-resistant Pseudomonas aeruginosa clinical isolates

## TL;DR

This study shows that a new enzyme called CAE-1 contributes to antibiotic resistance in Pseudomonas aeruginosa and could spread easily.

## Contribution

The study identifies CAE-1 as a carbapenemase in Pseudomonas aeruginosa and reveals its potential for horizontal transfer.

## Key findings

- CAE-1 confers resistance to multiple beta-lactam antibiotics in Pseudomonas aeruginosa.
- CAE-1 has lower catalytic efficiency against carbapenems compared to KPC-2.
- The blaCAE-1 gene is located on a mobile genetic element, suggesting potential for spread.

## Abstract

Two carbapenem-resistant Pseudomonas aeruginosa (CRPA) isolates were collected, which lacked known carbapenemases but produced a β-lactamase CAE-1. CAE-1 was recently found in Comamonas aquatica and conferred resistance to penicillins and cephalosporins. In this study, we aim to know whether CAE-1 was responsible for the carbapenem resistance in the CRPA isolates and its enzyme hydrolyzing characteristics. Both CRPA clinical isolates exhibited minimal inhibitory concentrations (MICs) of 8 µg/mL for imipenem and meropenem, with a positive result in the modified Hodge test for meropenem. The blaCAE-1 and blaKPC-2 were cloned and expressed in P. aeruginosa PAO1 and Escherichia coli DH5α, respectively. The blaCAE-1-carrying PAO1 transformant also tested positive using the modified carbapenem inactivation method and was resistant to piperacillin-tazobactam, ticarcillin-clavulanate, and cefoperazone-sulbactam, but susceptible to ceftazidime-avibactam. Expression of blaCAE-1 in P. aeruginosa PAO1 elicited a 64- to 128-fold increase in MICs for piperacillin, ceftazidime, cefepime, and aztreonam, and an eightfold increase for meropenem, exhibiting a broader resistance profile than in E. coli DH5α. Steady-state kinetic assays showed that CAE-1 had catalytic efficiency against all β-lactams tested, with comparatively lower efficiency against three carbapenems relative to KPC-2, while demonstrating approximately equivalent efficiency for the other β-lactam antibiotics tested. Whole-genome sequencing revealed that blaCAE-1 was a class A β-lactamase and encoded on an integrative and conjugative element, which might facilitate its horizontal transfer. The class A β-lactamase CAE-1 is a carbapenemase posing a high risk for horizontal dissemination. Enhanced surveillance for blaCAE-1-harboring isolates is needed.