Binding of Actinobacillus pleuropneumoniae to phosphatidylethanolamine

The gram-negative bacterium Actinobacillus pleuropneumoniae is the causative agent of porcine fibrinohemorrhagic necrotizing pleuropneumonia, a disease that causes important economic losses to the swine industry worldwide. In general, the initial step of bacterial colonization is attachment to host cells. The purpose of the present study was to evaluate the binding of A. pleuropneumoniae serotype 1 to phospholipids, which are the major constituents of biological membranes. Phospholipids serve as receptors for several bacteria, including respiratory pathogens. To study this effect, we used thin-layer chromatography overlay binding assays to test commercial phospholipids such as phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, and phosphatidylethanolamine (PE). Our results indicate that A. pleuropneumoniae serotype 1 binds to PE but not to the other phospholipids tested. Serotypes 5b and 7, which, along with serotype 1, are the most prevalent serotypes of A. pleuropneumoniae in North America, share the ability to bind PE. Inhibition of binding with a monoclonal antibody against A. pleuropneumoniae serotype 1 O antigen and the use of isogenic lipopolysaccharide (LPS) mutants of A. pleuropneumoniae serotype 1 showed that the O antigen seems to be implicated in the binding to PE, at least for A. pleuropneumoniae serotype 1. A. pleuropneumoniae was also shown to bind to a phospholipid extracted from swine lungs by using the method of Folch. Chemical staining with molybdenum blue and ninhydrin, migration with neutral, acidic, and basic solvent systems, and mass spectrometry analysis all indicated that this lipid is PE. This study is, to the best of our knowledge, the first description of A. pleuropneumoniae binding to phospholipids. Our data also suggest that LPS O antigens could be involved in binding to PE.