Group IIA secreted phospholipase A₂in human serum kills commensal but not clinicalEnterococcus faeciumisolates

Abstract

26Human innate immunity employs cellular and humoral mechanisms to facilitate rapid 27 killing of invading bacteria. The direct killing of bacteria by human serum is mainly 28 attributed to the activity of the complement system that forms pores in Gram-negative 29 bacteria. Although Gram-positive bacteria are considered resistant to serum killing, 30 we here uncover that normal human serum effectively kills Enterococcus faecium. 31 Comparison of a well-characterized collection of commensal and clinical E. faecium 32 isolates revealed that human serum specifically kills commensal E. faecium strains 33 isolated from normal gut microbiota, but not clinical isolates. Inhibitor studies show 34 that the human group IIA secreted phospholipase A2 (hGIIA), but not complement, is 35 responsible for killing of commensal E. faecium strains in human normal serum. This 36 is remarkable since hGIIA concentrations in 'non-inflamed' serum were considered 37 too low to be bactericidal against Gram-positive bacteria. Mechanistic studies showed 38 that serum hGIIA specifically causes permeabilization of commensal E. faecium 39 membranes. Altogether, we find that a normal serum concentration of hGIIA 40 effectively kills commensal E. faecium and that hGIIA resistance of clinical E. 41 faecium could have contributed to the ability of these strains to become opportunistic 42 pathogens in hospitalized patients. 43 44 Importance 45 Human normal serum contains antimicrobial components that effective kill invading 46 Gram-negative bacteria. Although Gram-positive bacteria are generally considered 47 resistant to serum killing, here we show that normal human effectively kills the Gram-48 positive Enterococcus faecium strains that live as commensals in the gut of humans. 49 In contrast, clinical E. faecium strains that are responsible for opportunistic infections 50 3 in debilitated patients are resistant against human serum. The key factor in serum 51 responsible for killing is group IIA secreted phospholipase A2 (hGIIA) that 52 effectively destabilizes commensal E. faecium membranes. We believe that hGIIA 53 resistance by clinical E. faecium could have contributed to the ability of these strains 54 to cause opportunistic infections in hospitalized patients. Altogether, understanding 55 mechanisms of immune defense and bacterial resistance could aid in further 56 development of novel anti-infective strategies against medically important multidrug 57 resistant Gram-positive pathogens. 58 59