Mucoid Pseudomonas aeruginosa resists nonopsonic phagocytosis by human neutrophils and macrophages.

Mucoid Pseudomonas aeruginosa is an important respiratory pathogen in patients with cystic fibrosis, and once acquired is virtually impossible to eradicate. Although mucoid P. aeruginosa is generally believed to be resistant to phagocytosis, the mechanism is not understood fully. We studied the nonopsonic phagocytosis by human neutrophils or macrophages of eight mucoid/nonmucoid P. aeruginosa pairs (three isogenic and five "wild-type"). Mucoid strains were relatively resistant to nonopsonic phagocytosis but the nonmucoid types were phagocytosis-susceptible as assessed by visual inspection and chemiluminescence assays. The mucoid and nonmucoid variants had equal numbers of pili but different surface characteristics as determined by biphasic partitioning in polyethylene glycol and dextran. The mucoid exopolysaccharide of mucoid strains appears to alter the surface characteristics of P. aeruginosa thereby rendering them resistant to nonopsonic phagocytosis. The resistance of mucoid variants of P. aeruginosa to nonopsonic phagocytosis may provide a survival advantage to these bacteria early in the course of pulmonary infection before opsonic antibody and complement are present in respiratory secretions.

Nonopsonic phagocytosis of nonmucoid Pseudomonas aeruginosa by human neutrophils and monocyte-derived macrophages is correlated with bacterial piliation and hydrophobicity.

We have shown previously that some strains of Pseudomonas aeruginosa from patients with cystic fibrosis are phagocytized by human polymorphonuclear leukocytes in the absence of serum opsonins. The purpose of this study was to identify the bacterial features which render certain strains susceptible to nonopsonic phagocytosis. Three strains were phagocytized by human neutrophils and monocyte-derived macrophages, and two were not, as determined by luminol-enhanced chemiluminescence, visual inspection of stained smears, and bactericidal assay. Strains that were phagocytized formed pellicles when grown in static broth, but the phagocytosis-resistant strains did not. The phagocytosis-susceptible strains were more heavily piliated and more hydrophobic than the resistant strains. Bacteria exposed to heat (60 degrees C) or UV irradiation were depiliated, as assessed by electron microscopy, and rendered resistant to phagocytosis. When P. aeruginosa was grown on agar, it was piliated, hydrophobic, and susceptible to nonopsonic phagocytosis, but when grown to stationary phase in shaken broth, it was nonpiliated, less hydrophobic, and resistant to phagocytosis. It appears that nonopsonic phagocytosis of certain P. aeruginosa strains by human polymorphonuclear leukocytes and macrophages is facilitated by hydrophobic interactions which may be determined in part by pili.

Fimbriae (pili): molecular basis of Pseudomonas aeruginosa adherence.

Pseudomonas aeruginosa produces polar pili which promote the adherence of the organism to host mucosal surfaces and to blood-borne phagocytic cells such as polymorphonuclear leukocytes. Pseudomonas polar pili are flexible filaments of 52 A diameter and 2500 nm average length. They consist of a single type of protein subunit, pilin, of molecular weight 15,000, which is arranged in a helical mode of 5 subunits per turn and a pitch of 41 A. Purified whole pili, and anti-pilus antiserum both inhibited the interaction of Pseudomonas aeruginosa strains with human buccal epithelial cells and PMNs, suggesting that Pseudomonas adherence to these mammalian cells is pilus mediated. No correlation was found between the level of cell surface fibronectin on human buccal endothelial cells and the adherence of Pseudomonas bacteria. Pseudomonas adherence to buccal endothelial cells obtained from patients with cystic fibrosis was somewhat less than that to buccal endothelial cells obtained from healthy individuals. Fibronectin levels on buccal endothelial cells from patients with cystic fibrosis were not significantly different than those found on buccal endothelial cells from healthy individuals. Studies with peptide fragments derived from purified pili showed that only one peptide encompassing 23 amino acid residues at the C-terminus of the pilus protein was able to inhibit in vitro adherence of P. aeruginosa PAK to human buccal cells. This peptide domain was tentatively assigned the receptor-binding function.

Opsonin-independent phagocytosis of Pseudomonas aeruginosa.

Three nonmucoid revertant P. aeruginosa strains from cystic fibrosis patients were phagocytized by human polymorphonuclear leukocytes in the absence of serum. Phagocytosis was inhibited by D-mannose and by mannose-containing sugars. Bacteria killed by heat or ultraviolet irradiation or grown in shaken broth were devoid of pili and resistant to nonopsonic phagocytosis. The mucoid parents of two phagocytosis-susceptible nonmucoid revertant strains were resistant to nonopsonic phagocytosis. The nonmucoid revertants were more hydrophobic in nature than the mucoid parents, but they were comparably piliated. Nonopsonic phagocytosis of P. aeruginosa by human neutrophils appears to be mediated in part by pili. Other factors such as the mucoid coating of certain strains may interfere with this process.