Examination of the bactericidal and opsonic activity of normal human serum for a mucoid and nonmucoid strain ofPseudomonas aeruginosa

Alginate lyase enhances antibiotic killing of mucoid Pseudomonas aeruginosa in biofilms

Once mucoid (alginate-producing) strains of Pseudomonas aeruginosa have become established in the respiratory tracts of cystic fibrosis patients they can rarely be eliminated by antibiotic treatment alone; we have investigated, in an in vitro biofilm system, the putative role of co-administration of alginate lyase with antibiotic. Biofilms were maintained in continuous flow culture in a medium resembling sputum from CF patients. Antibiotics and/or alginate lyase were added to some of the cultures. Biofilms of two mucoid CF strains of P. aeruginosa were, in most cases, not eradicated by a one-week course of treatment with 64 microg/ml of gentamicin; the same concentration of gentamicin, under the same conditions, led to the apparent elimination of all biofilms of non-mucoid derivatives of these strains. When alginate lyase and gentamicin were administered together the apparent elimination of mucoid bacteria from biofilms was achieved, whereas the mucoid bacteria in most control biofilms treated only with gentamicin persisted. Ceftazidime treatment of biofilms was more effective against those containing the non-mucoid strains than those with mucoid strains. These studies support the view that co-administration of antibiotics with alginate lyase, which degrades the exopolysaccharide produced by mucoid strains of P aeruginosa, might benefit CF patients by increasing the efficacy of antibiotic in the respiratory tract.

Oxygen-dependent up-regulation of mucoid exopolysaccharide (alginate) production in Pseudomonas aeruginosa

We previously showed substantial differences in Pseudomonas aeruginosa exopolysaccharide production in vitro at oxygen tensions reflective of the right versus left cardiac circuits in vivo (40 versus 80 mm Hg, respectively; A. S. Bayer, T. O'Brien, D. C. Norman, and C. C. Nast, J. Antimicrob. Chemother. 23:21-35, 1989). However, those studies did not specifically confirm this exopolysaccharide to be the characteristic P. aeruginosa mucoid alginate seen in patients with cystic fibrosis. With a murine monoclonal antibody prepared against P. aeruginosa alginate, strongly positive immunofluorescence (IF) staining of a nonmucoid P. aeruginosa strain (PA-96) was seen after its exposure in vitro to oxygen tensions (pO2) of approximately 80 mm Hg; the intensity of the IF staining under these conditions was similar to that observed with a phenotypically mucoid P. aeruginosa strain (C1712M) from a cystic fibrosis patient. In contrast, the same nonmucoid strain (PA-96), after exposure to pO2 of approximately 40 mm Hg, showed little IF staining for alginate. Following enzyme treatment with alginase, PA-96 cells previously exposed to the higher pO2 and exhibiting enhanced alginate production, as determined by IF staining, now showed no IF staining. Moreover, treatment of the oxygen-up-regulated PA-96 cells with alginase released amounts of unsaturated alginate breakdown products (uronic acids) quantitatively similar to those released by typically mucoid strains treated with the same enzyme. These data indicated that the P. aeruginosa exopolysaccharide in our studies was, indeed, mucoid alginate and that variations in oxygen tensions represent one of the trigger mechanisms for the up-regulation of mucoid exopolysaccharide production.

Immunologic aspects of cystic fibrosis

Bacterial infections determine life expectancy in the hereditary disease cystic fibrosis (CF). The dominant pathogens are Staphylococcus aureus and Pseudomonas aeruginosa, which persist in the patient's respiratory tract. Current explanations of the chronicity of the infections in the apparently immunocompetent host are based on defective opsonophagocytosis. This may be caused by (1) bacterial exopolysaccharide production, leading to cryptic infection types; (2) cleavage of immunoglobulin, complement, and surface receptors on immunocompetent cells by host proteases; and (3) a change from opsonic to nonopsonic antibody isotypes. Continuous antigenic stimulation of the immune system leads to local immune complex formation and a high chronic hypersensitivity reaction as well as to temporary immune unresponsiveness. Progressive tissue damage caused by lysosomal enzymes and oxygen radicals from polymorphonuclear leukocytes is thought to be ultimately responsible for respiratory failure and death in CF. Besides antibiotic treatment, anti-inflammatory therapy is therefore currently considered beneficial.

Opsonophagocytic killing activity of rabbit antibody to Pseudomonas aeruginosa mucoid exopolysaccharide

We used an in vitro opsonophagocytic killing assay to measure the functional activity of antibody directed at the mucoid exopolysaccharide (MEP) antigen expressed by Pseudomonas aeruginosa strains isolated from cystic fibrosis patients. Rabbit antibodies raised to purified MEP were able to mediate phagocytic killing in the presence of human peripheral blood leukocytes and a low level (final concentration, 0.3%) of fresh normal human serum as a complement source. No bacterial killing was observed when peripheral blood leukocytes, antiserum, or complement was omitted. Specificity of the antibody for the MEP antigen was shown by adsorption and inhibition assays. Affinity-purified antibody to MEP also mediate phagocytic killing. These data indicate that antiphagocytic properties attributable to MEP can be overcome by specific antibody.

Serum sensitivity of a Pseudomonas aeruginosa mucoid strain

The susceptibility of Pseudomonas aeruginosa 144M (a mucoid strain isolated from the sputum of a cystic fibrosis patient) to the bactericidal activity of pooled fresh normal human serum (FHS) was examined. FHS at concentrations of greater than or equal to 2.5% was capable of killing greater than 95% of strain 144M. Strain 144M was killed by FHS in a dose-dependent manner. Although either immunoglobulin M (IgM) or IgG was bactericidal in the presence of complement, IgM was about 10 times as effective as IgG. However, optimal killing activity required both IgM and IgG and complement, activated by the classical pathway. A role for lysozyme in the killing of 144M was demonstrated only when low concentrations of FHS were used. In contrast to 144M, P. aeruginosa strains 144NM and 144M(SR) were totally resistant to FHS at all of the concentrations tested (up to 50%). Neither the FHS susceptibility of 144M nor the FHS resistance of 144NM or 144M(SR) was altered by choice of growth medium, growth phase, or temperature of growth. Results of absorption studies with whole organisms, isolated outer membrane preparations, or lipopolysaccharide (LPS) from each strain suggest that the antigen(s) which binds the bactericidal immunoglobulins is accessible on the surface of 144M but not on the surface of 144NM or 144M(SR), is insensitive to trypsin treatment, and is believed to be LPS. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the three LPS preparations demonstrated that 144M LPS contained primarily lipid-A-core polysaccharide components, whereas the LPS from 144NM and 144M(SR) were heterogeneous, with various degrees of O-side-chain substitution. These results suggest that at least one target for bactericidal antibody on the surface of 144M is contained in the rough LPS of this strain.

Development of an enzyme-linked immunosorbent assay for studying Pseudomonas aeruginosa cell surface antigens

An enzyme-linked immunosorbent assay for the measurement of antibodies directed against Pseudomonas aeruginosa cell surface antigens was developed. Formalin-killed whole cells of P. aeruginosa, adsorbed to polystyrene acrylic copolymer cuvettes, were used as immobilized antigens. Antisera to P. aeruginosa mucoid strain 144M and to its spontaneous nonmucoid derivative, 144NM, were raised in rabbits by immunization with Formalin-killed bacteria. By using this enzyme-linked immunosorbent assay, anti-144M serum was found to have a ca. 10-fold-higher antibody titer to 144M than did anti-144NM serum, suggesting that 144M may have either immunogenic determinants not present on 144NM or perhaps simply more antigenic determinants. In contrast, anti-144M and anti-144NM immune sera were found to have nearly identical antibody titers to 144NM, suggesting that these strains share many determinants. Anti-P. aeruginosa immune serum was found to contain Pseudomonas-specific antibodies as well as antibodies which cross-reacted with other gram-negative bacteria. Finally, absorption studies demonstrated that this assay can detect both LPS and non-LPS surface-exposed antigenic determinants. Thus, this whole bacterial cell enzyme-linked immunosorbent assay should prove useful in monitoring patient sera and secretions for potentially protective immunoglobulins directed at P. aeruginosa cell surface antigens.