Legionella pneumophila type II secretome reveals a polysaccharide deacetylase that impacts intracellular infection, biofilm formation, and resistance to polymyxin- and serum-mediated killing

Abstract

Prior analyses suggested that the type II secretion system (T2SS) of Legionella pneumophila secretes ≥47 proteins beyond its 26 known substrates. Upon examination of mutants of wild-type strain 130b that lack those exoproteins most conserved across the Legionella genus, we discovered that protein "06635" majorly promotes L. pneumophila replication within amoebae. Immunoblotting, proteomics, and whole-cell enzyme-linked immunosorbent assay (ELISA) confirmed that 06635 exists in culture supernatants and on the bacterial outer surface and does so in a T2SS-dependent manner. Bioinformatic analyses identified 06635 as a novel member of the carbohydrate esterase family 4, whose members deacetylate bacterial surface polysaccharides, peptidoglycan, chitins, and/or xylans. Given 06635's T2SS-dependent secretion, low-level amino acid similarity to known peptidoglycan deacetylases, and the unaltered lysozyme resistance of a 06635 mutant, we pursued the hypothesis that 06635 deacetylates a polysaccharide on L. pneumophila's surface. Supporting this, the 06635 mutant exhibited increased binding to both wheat germ agglutinin (i.e., more surface N-acetylglucosamine) and antibodies that recognize acetylated lipopolysaccharide (LPS). Nuclear magnetic resonance (NMR) analysis of isolated mutant vs wild-type LPS confirmed that 06635 promotes LPS deacetylation. Thus, we designated 06635 as PdaA, for polysaccharide deacetylase A. Compatible with its altered surface, the pdaA mutant showed greater autoaggregation, increased biofilm formation, and heightened sensitivity to both polymyxin and human serum. Thus, we hypothesize that, following its secretion via the T2SS, PdaA deacetylates LPS, and perhaps other moieties, impacting many significant processes. While defining PdaA, we identified many more putative substrates of the L. pneumophila T2SS, bringing the size of the T2SS output to approximately 120.IMPORTANCELegionella pneumophila is the principal cause of Legionnaires' disease, an increasingly common form of pneumonia. Although prior work demonstrated that the bacterium utilizes its type II protein secretion system (T2SS) to survive in aquatic environments and to cause lung infection, the full scope and impact of this Legionella secretion system is still relatively underappreciated. By utilizing an expanded proteomic approach and testing newly made mutants in a wide range of assays, we have determined that the L. pneumophila type II secretome encompasses approximately 120 proteins, and among these proteins is a novel polysaccharide deacetylase (PdaA) that modulates the L. pneumophila surface and lipopolysaccharide, impacting intracellular infection, biofilm formation, and resistance to both antibiotics and human serum. Moreover, since T2SSs and homologs of PdaA were found in many other bacteria, our findings should also have implications for understanding other infectious diseases and environmental processes.

Keywords: L. pneumophila; PdaA; T2SS; autoaggregation; biofilm formation; intracellular infection; lipopolysaccharide; polysaccharide deacetylase; resistance to polymyxin B; serum-resistance.