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

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 compromise E. faecium PG hydrolysis rendering them sensitive to β-lactam antibiotics. sagA mutants have cell envelope integrity defects, increased cellular permeability, and aberrant distribution of penicillin binding proteins. This corresponds to a growth defect where cells have abnormal division septa, membrane blebbing, and the formation of mini cells. The dysregulation of the cell envelope in sagA mutants alters the binding of phages to the E. faecium cell surface. Our data support a model where phage infection of E. faecium requires phages to localize to sites of peptidoglycan remodeling at the cell poles and division septa. 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.