# Enterococcus faecium sagA mutants have cell envelope defects influencing antibiotic resistance and bacteriophage susceptibility

## TL;DR

This study shows that phage-resistant Enterococcus faecium bacteria have weakened cell walls, making them more vulnerable to antibiotics.

## Contribution

The study identifies the SagA peptidoglycan hydrolase as a key factor linking phage resistance and antibiotic sensitivity in E. faecium.

## Key findings

- Phage-resistant E. faecium mutants have mutations in the SagA gene, leading to cell envelope defects.
- SagA mutations increase β-lactam antibiotic sensitivity due to altered peptidoglycan remodeling.
- Phage resistance in E. faecium is linked to reduced bacterial fitness and altered cell division.

## Abstract

Enterococcus faecium is a gram-positive bacterium that is resident to the intestines of animals including humans. E. faecium is also an opportunistic pathogen that causes multidrug-resistant (MDR) infections. Bacteriophages (phages) have been proposed as therapeutics for the treatment of MDR infections; however, an obstacle for phage therapy is the emergence of phage resistance. Despite this, the development of phage resistance can impact bacterial fitness. Thus, understanding the molecular basis of fitness costs associated with phage resistance can likely be leveraged as an antimicrobial strategy. We discovered that phage-resistant E. faecium harbor mutations in the cell wall hydrolase gene sagA. SagA cleaves crosslinked peptidoglycan (PG) involved in PG remodeling. We show that mutations in sagA compromised E. faecium PG hydrolysis. One sagA mutant, with a defect in cell envelope integrity, increased cellular permeability, and aberrant distribution of penicillin-binding proteins, was also more sensitive to β-lactam antibiotics. These changes correspond to a growth defect where cells have abnormal division septa, membrane blebbing, and aberrant cell shape. The dysregulation of the cell envelope caused by the sagA mutation alters the binding of phages to the E. faecium cell surface, where phage infection of E. faecium requires phages to localize to sites of peptidoglycan remodeling. Our findings show that by altering the function of a single PG hydrolase, E. faecium loses intrinsic β-lactam resistance. This indicates that phage therapy could help revive certain antibiotics when used in combination.

Enterococcus faecium causes hospital-acquired infections and is frequently resistant to frontline antibiotics, including those that target the cell wall. Bacteriophages represent a promising alternative to combat such infections. However, bacterial adaptation to phage predation often results in resistance. Such resistance is frequently accompanied by fitness trade-offs, most notably altered antibiotic susceptibility. This study provides mechanistic insights into phage resistance-associated antibiotic sensitivity in E. faecium. We show that phage-resistant E. faecium carrying a mutation in the peptidoglycan hydrolase SagA has compromised cell envelope integrity, mislocalized penicillin-binding proteins, and become sensitized to β-lactam antibiotics. These findings highlight the potential of reviving antibiotics when used in combination with phages in the clinical setting.