The exopolysaccharide (alginate) of mucoid strains of Pseudomonas aeruginosa is believed to be an important virulence factor. The ability of an alginate-deploymerizing enzyme (alginase) to modify the polymorphonuclear leukocyte (PMN)-directed and antibiotic-mediated phagocytosis and killing of mucoid P. aeruginosa was studied both in vitro and in vivo. In vitro, pretreatment of a mucoid P. aeruginosa strain (144MR) resulted in a significant enhancement of PMN phagocytosis and killing of the organism (P less than 0.05), to levels similar to that observed with its nonmucoid mate, strain 144NM. Moreover, alginase treatment of the mucoid strain 144MR caused a substantial removal of bacterial cell surface alginate as assessed by immunofluorescence staining with a murine monoclonal antialginate antibody. The experimental endocarditis model was used to evaluate the in vivo effect of alginase in modifying the course of a deep-seated pseudomonal infection caused by mucoid strain 144MR. In right-sided endocarditis, in which PMNs normally mediate spontaneous clearance of the organism from cardiac vegetations (A. S. Bayer, J. Yih, C. Y. Chiu, and C. C. Nast, Chemotherapy 35:278-288, 1989), the presence of the alginate exopolysaccharide on strain 144MR was associated with an inability to reduce intravegetation pseudomonal counts over a 13-day postinfection period; in contrast, right-sided vegetations infected with the nonmucoid strain 144NM underwent significant reductions in bacterial densities over this same time (P less than 0.05). Administration of alginase intravenously (i.v.) (750 enzyme units per day for 7 days) to animals with right-sided endocarditis caused by the mucoid strain 144MR was associated with a significant reduction in intravegetation pseudomonal counts (P less than 0.05), to levels similar to that seen with endocarditis caused by the nonmucoid strain. In left-sided endocarditis caused by mucoid strain 144MR, animals received either no therapy, amikacin (20 or 40 mg/kg twice a day for 7 or 14 days), or amikacin plus alginase (750 U/day [i.v.]). The coadministration of alginase for 14 days with the higher-dose amikacin regimen rendered more left-sided vegetations culture negative than those in animals receiving the antibiotic alone for 7 or 14 days (P = 0.001 and 0.056, respectively). These salutary effects of alginase in vivo were paralleled by the ability of the enzyme to remove the exopolysaccharide from the surface of mucoid pseudomonal cells within cardiac vegetations, as assessed by transmission electron microscopy.(ABSTRACT TRUNCATED AT 400 WORDS)