Secreted antigen A peptidoglycan hydrolase is essential for <i>Enterococcus faecium</i> cell separation and priming of immune checkpoint inhibitor therapy

<i>Enterococcus faecium</i> is a microbiota species in humans that can modulate host immunity¹, but has also acquired antibiotic resistance and is a major cause of hospital-associated infections². Notably, diverse strains of <i>E. faecium</i> produce SagA, a highly conserved peptidoglycan hydrolase that is sufficient to promote intestinal immunity^3-5 and immune checkpoint inhibitor antitumor activity⁶. However, the functions of SagA in <i>E. faecium</i> were unknown. Here we report that deletion of <i>sagA</i> impaired <i>E. faecium</i> growth and resulted in bulged and clustered enterococci due to defective peptidoglycan cleavage and cell separation. Moreover, Δ<i>sagA</i> showed increased antibiotic sensitivity, yielded lower levels of active muropeptides, displayed reduced activation of the peptidoglycan pattern-recognition receptor NOD2, and failed to promote cancer immunotherapy. Importantly, plasmid-based expression of SagA, but not its catalytically-inactive mutant, restored Δ<i>sagA</i> growth, production of active muropeptides and NOD2 activation. SagA is therefore essential for <i>E. faecium</i> growth, stress resistance and activation of host immunity.