Polysaccharide surface antigens expressed by nonmucoid isolates of Pseudomonas aeruginosa from cystic fibrosis patients.

We tested nonmucoid Pseudomonas aeruginosa isolates obtained from cystic fibrosis (CF) patients for the expression of lipopolysaccharide (LPS) serotype antigens, serum sensitivity, and production of mucoid exopolysaccharide (MEP). When all nonmucoid isolates were compared with a set of random mucoid isolates, 20 of 52 (38%) nonmucoid isolates were typable and serum resistant, compared with 13 of 51 (24%) mucoid isolates (P = 0.16 by chi-square analysis). However, nonmucoid strains from CF patients colonized only with nonmucoid strains were more frequently typable and serum resistant (67%) than were nonmucoid isolates from patients cocolonized with mucoid strains (31%) (P = 0.012, Fisher exact test). An inhibition enzyme-linked immunosorbent assay done with bacterial extracts, a direct-whole-cell enzyme-linked immunosorbent assay done with affinity-purified antibody to MEP, and immune electron microscopy all demonstrated production of MEP by all nonmucoid P. aeruginosa isolates tested, including nonmucoid revertants of mucoid strains. No other bacterial species tested positive in these assays. These findings suggest that MEP is produced by all P. aeruginosa isolates obtained from CF patients, that the initial colonizing nonmucoid strains produce a smooth LPS, and that once LPS-rough, mucoid strains appear in the sputum, the predominant LPS phenotype is rough regardless of colony morphology.