Immunologic investigations of mucoid strains of Pseudomonas aeruginosa: comparison of susceptibility to opsonic antibody in mucoid and nonmucoid strains.

Mucoid strains of Pseudomonas aeruginosa, isolated from patients with cystic fibrosis, were studied for the prevalence of each of the seven Fisher immunotype antigens and were compared with their nonmucoid transformants, obtained by repeated subculturing, for susceptibility to opsonic antibody. Of the 30 strains tested--one from each of 30 patients--16 were typable and were tested in the opsonophagocytic assay with use of immunotype-specific rabbit antiserum; eight had significant opsonization by complement without antiserum. Of the eight strains requiring antiserum, seven strains required a higher minimum concentration of antiserum for a 1.0 log10 reduction of viable P. aeruginosa than the paired nonmucoid derivative. These end-point titers were significantly greater for nonmucoid Pseudomonas (P = 0.0007). A mucoid strain not requiring antibody for opsonization was shown to use primarily the alternative complement pathway. These results are consistent with the hypothesis that the immunodeterminant for opsonic antibody in nonmucoid strains is blocked in the mucoid strain.