The Phosphorylcholine Epitope Undergoes Phase Variation on a 43-Kilodalton Protein in Pseudomonas aeruginosa and on Pili of Neisseria meningitidis and Neisseria gonorrhoeae

Phosphorylcholine (ChoP) is a component of the teichoic acids of Streptococcus pneumoniae and has been recently identified on the lipopolysaccharide of Haemophilus influenzae, also a major pathogen of the human respiratory tract. Other gram-negative pathogens that frequently infect the human respiratory tract were surveyed for the presence of the ChoP epitope as indicated by binding to monoclonal antibodies (MAbs) recognizing this structure. The ChoP epitope was found on a 43-kDa protein on all clinical isolates of Pseudomonas aeruginosa examined and on several class I and II pili of Neisseria meningitidis. The specificity of the anti-ChoP MAb was demonstrated by the inhibition of binding in the presence of ChoP but not structural analogs. As in the case of H. influenzae, the expression of this epitope was phase variable on these species. In P. aeruginosa, this epitope was expressed at detectable levels only at lower growth temperatures. Expression of the ChoP epitope on piliated neisseriae displayed phase variation, both linked to pilus expression and independently of fully piliated bacteria.Choline, a major constituent of eukaryotic membrane lipids, was previously thought to be an unusual structural feature of prokaryotes. The best-known example is Streptococcus pneumoniae, which accumulates environmental choline and incorporates it in the form of phosphorylcholine (ChoP) into its glycolipid, lipoteichoic acid, as well as its cell wall-associated teichoic acid (8). It has been suggested that ChoP contributes to adherence of the pneumococcus to host cells by binding to the receptor for platelet-activating factor, whose natural ligand also contains ChoP (1). Recently, ChoP has been identified as a unique feature of the lipopolysaccharide (LPS) of Haemophilus influenzae (21, 22). In the case of H. influenzae, choline is also acquired from the growth medium and linked to a glucose residue on the outer core region of the rough LPS (22). The expression of ChoP on the H. influenzae glycolipid undergoes phase variation mediated by a translational switch within the gene licA, a putative choline kinase (20, 22). The only significant homology to licA in protein databases is in a gene found in several species of the genus Mycoplasma, including the common respiratory tract pathogen Mycoplasma pneumoniae. In addition, the ChoP structure has been identified on a polar lipid found in the opportunistic pathogen M. fermentans (2). It appears, therefore, that ChoP is a structure common to several important and distantly related pathogens, including S. pneumoniae, H. influenzae, and various mycoplasma species which reside on the mucosal surface and infect the human respiratory tract. In addition, screening of secretions from the human respiratory tract with a monoclonal antibody (MAb) specific to the ChoP epitope has revealed several other gram-positive species which bind the antibody and may contain the ChoP structure (3).The focus of this study was the identification of the ChoP epitope on pathogenic gram-negative species other than Haemophilus. Results of this screening show that this epitope is present and displays phase variation on protein structures of pathogenic neisseriae, and Pseudomonas aeruginosa. In the case of the neisseriae, the ChoP epitope was found to be present on pili.