Pathogenic effects of monocytopenia, granulocytopenia and dexamethasone on the course of experimental Pseudomonas aeruginosa endocarditis in rabbits

Preparation and properties of alginate lyase modified with poly(ethylene glycol)

Modification of the enzyme alginate lyase (AL) with poly(ethylene glycol) (PEG) was attempted for the degradation and removal of alginate biofilms in infectious diseases. The modification of AL with PEG was attempted with three kinds of N-succinimidyl succinate PEG (SS-PEG), which differed in molecular weight (i.e., 2000, 5000 and 12,000 Da). The conjugation of PEG to free amino groups on AL was confirmed by gel permeation chromatography. Quantification of residual free amino groups revealed that PEG modification progressed further with a higher pH and a larger molar ratio of SS-PEG to AL. The reproducibility of the reaction was fairly good. The enzyme activity decreased with increasing PEG modification but the immunoreactivity toward anti-AL antibodies, as evaluated by an ELISA method, was much more remarkably reduced. The immunoreactivity was more reduced by the conjugated PEG with the larger molecular weight. In the reaction with PEG of molecular weight 12,000 Da, we obtained PEG-modified AL retaining approximately 40% enzyme activity but only 0.5% of the immunoreactivity of native AL.

Evidence of less severe aortic valve destruction after treatment of experimental staphylococcal endocarditis with vancomycin and dexamethasone

The beneficial effects of therapy combining an antibiotic and dexamethasone have been reported in human studies on meningitis and in experimental studies on septic arthritis, nephritis, and endophthalmitis. Since most patients with staphylococcal endocarditis need a combination of medical and surgical treatment, the purpose of this study was to determine whether the addition of dexamethasone to vancomycin has any beneficial effect regarding the degree of valve tissue damage or the course of experimental aortic valve endocarditis caused by a methicillin-resistant strain of Staphylococcus aureus. Rabbits with catheter-induced aortic valve vegetations were randomly assigned to a control group and to groups receiving dexamethasone (0.5 mg/kg of body weight, intravenously [i.v.], twice a day [b.i.d]), vancomycin (30 mg/kg, i.v., b.i.d), or dexamethasone plus vancomycin, for a total of 10 doses (two doses per day for 5 days). The severity of valve tissue damage was significantly less in groups receiving vancomycin plus dexamethasone compared with that of the group receiving vancomycin alone (P < 0.001). The severity of tissue damage was inversely correlated with the mean polymorphonuclear leukocyte number in valve tissue. No statistically significant differences were observed between the vancomycin-treated group and the vancomycin-plus-dexamethasone-treated group in survival, blood culture sterilization rate, or reduction of the microbial burden (in CFU per gram) in valvular tissue. In conclusion, treatment with a combination of vancomycin and dexamethasone for 5 days reduces the severity of valve tissue damage in experimental staphylococcal aortic valve endocarditis. These findings could have significant implications in the treatment of staphylococcal endocarditis and deserve further confirmation in clinical trials.

Treatment of experimental staphylococcal endocarditis due to a strain with reduced susceptibility in vitro to vancomycin: efficacy of ampicillin-sulbactam

We evaluated several 3-day antimicrobial regimens in the treatment of experimental endocarditis caused by an oxacillin-resistant Staphylococcus aureus strain exhibiting intermediate susceptibility in vitro to vancomycin (VISA). Neither vancomycin alone nor trovafloxacin exhibited in vivo efficacy; addition of amikacin to vancomycin yielded a modest in vivo effect. In contrast, the combination of ampicillin and sulbactam was highly effective in vivo, causing a mean decrease in VISA vegetation densities of >5 log(10) CFU/g versus those of untreated controls.

Efficacy of trovafloxacin, a new quinolone antibiotic, in experimental staphylococcal endocarditis due to oxacillin-resistant strains

Therapeutic options for severe infections caused by strains of oxacillin-resistant Staphylococcus aureus (ORSA) and coagulase-negative staphylococci (ORSE) are very limited. With the increasing resistance of such strains to aminoglycosides, rifampin, and currently available quinolone agents, as well as the recent documentation of increasing resistance of ORSA to vancomycin (VANCO), new treatment alternatives are imperative. The in vivo efficacy of trovafloxacin (TROVA), a new quinolone agent with excellent antistaphylococcal activity in vitro, against experimental endocarditis (IE) due to beta-lactamase-producing ORSA and ORSE strains (ORSA and ORSE IE) was evaluated. TROVA (25 mg/kg of body weight intravenously [i.v.] twice daily [b.i.d]) was compared to VANCO (20 mg/kg i.v. b.i.d.) and two regimens of ampicillin-sulbactam (AMP-SUL; 200 mg/kg intramuscularly [i.m.] three times a day [t.i.d.] and 20 mg/kg i.m. b.i.d.), with all agents given for 3 or 6 days. AMP-SUL was included as a comparative treatment regimen because of its proven efficacy against experimental ORSA and ORSE IE. For both ORSA and ORSE IE, TROVA, AMP-SUL, and VANCO each reduced staphylococcal densities in vegetations compared to untreated controls (P < 0.01). For ORSA IE, TROVA was the most rapidly bactericidal agent--although not to a statistically significant degree--correlating with its superior bactericidal effect in vitro compared to those of VANCO and AMP-SUL.

Efficacy of deoxycholate amphotericin B and unilamellar liposomal amphotericin B in prophylaxis of experimental Aspergillus fumigatus endocarditis

OBJECTIVE

To evaluate and compare in vivo the protective efficacy of unilamellar liposomal amphotericin B (L-AmB) with that of deoxycholate amphotericin B (D-AmB) in experimental endocarditis.

MATERIAL AND METHODS

In the rabbit model of experimental Aspergillus fumigatus endocarditis, two doses of each antifungal agent (1.5 mg/kg each) were administered intravenously at 4 hours and at 30 minutes before challenge with an inoculum of A. fumigatus. Three days later, the animals were sacrificed, and the aortic vegetations were analyzed.

RESULTS

All 19 animals that did not receive chemoprophylaxis acquired endocarditis. In contrast, endocarditis developed in 2 of 10 animals pretreated with D-AmB (P < 0.01) and 3 of 8 animals pretreated with L-AmB (P < 0.01). Both D-AmB and L-AmB prevented the development of endocarditis due to A. fumigatus and decreased the concentration of fungi in the aortic vegetations by more than 1 log10.

CONCLUSION

In the rabbit experimental model of Aspergillus endocarditis, D-AmB and L-AmB were equally effective in reducing the incidence of the infection and the tissue burden of fungi.

Phenotypic resistance to thrombin-induced platelet microbicidal protein in vitro is correlated with enhanced virulence in experimental endocarditis due to Staphylococcus aureus

Thrombin-induced platelet microbicidal protein (tPMP) is secreted by rabbit platelets following thrombin stimulation, and it kills common endovascular pathogens in vitro, including Staphylococcus aureus. Therefore, pathogens which exhibit tPMP resistance in vitro possess a potential survival advantage in vivo at sites of endovascular damage. We generated an isogenic S. aureus strain pair, differing in tPMP susceptibility, by transposon (Tn551) mutagenesis of a tPMP-susceptible (tPMPs) parental strain (ISP479) to derive a stably tPMP-resistant (tPMPr) strain, ISP479R. ISP479 and ISP479R were equivalent in vitro in the following phenotypes: biotyping, antiobiograms, platelet adherence and aggregation, growth kinetics, cell wall-associated protein A expression, and fibrinogen binding. Genotypic comparisons of chromosomal DNA of strains ISP479 and ISP479R following restriction endonuclease digestion revealed indistinguishable pulsed-field gel electrophoretic patterns. The genotype exhibited by strain ISP479R was linked to the tPMP-resistant phenotype, as it was transducible into the initially tPMP-susceptible parental strain, ISP479. Southern hybridization verified the presence of a single copy of Tn551 in the same chromosomal restriction site of both ISP479R and tPMPr transductants of ISP479. The correlation of in vitro tPMP susceptibility phenotypes with the ability to induce experimental endocarditis (a prototypical endovascular infection) was evaluated. Despite equivalent rates of endocarditis induction, animals infected with strain ISP479R achieved significantly higher vegetation bacterial densities over a 7-day post-challenge period than did animals infected with strain ISP479. These data suggest that tPMPr microbial strains have a selective advantage in experimental staphylococcal endocarditis. Furthermore, the major impact of tPMP resistance upon endocarditis pathogenesis appears to involve a postvalvular adherence event(s), most probably by facilitating bacterial proliferation within vegetations.

Combination therapy with amphotericin B and fluconazole against invasive candidiasis in neutropenic-mouse and infective-endocarditis rabbit models

Although there are an increasing number of new antifungal agents available, the morbidity and mortality due to invasive mycoses remain high. The high rates of polyene toxicities and the development of azole resistance have raised the issue of using antifungal agents of these classes in combination, despite theoretical concerns regarding antagonism between such agents. This study was designed to evaluate the in vivo efficacy of combined therapy with amphotericin B and fluconazole against Candida albicans. Two distinct animal models were used in this study: a neutropenic-mouse model of hematogenously disseminated candidiasis and the infective-endocarditis rabbit model. Treatment efficacy was assessed by determining reductions in mortality as well as decreases in tissue fungal densities. In the neutropenic-mouse model, amphotericin B, as well as combination therapy, significantly prolonged survival compared to untreated controls (P < 10(-5) and P = 0.001, respectively). The fungal densities in the kidneys of neutropenic mice were significantly reduced with either amphotericin B monotherapy or amphotericin B-fluconazole combined therapy compared to those of controls (P < 10(-6)). Fluconazole monotherapy also reduced fungal densities in the kidneys; however, this decrease was not statistically significant (P = 0.17). In contrast, treatment with either fluconazole alone or combined with amphotericin B (but not amphotericin B monotherapy) significantly decreased fungal densities in the brain (P = 0.025). In the rabbit endocarditis model, amphotericin B monotherapy or combined therapy significantly decreased fungal densities in cardiac vegetations (P < 0.01 versus the controls). Although no significant antagonism was seen when fluconazole was given in combination with amphotericin B, combination therapy did not augment the antifungal activity of amphotericin B.

Comparison of fluconazole and amphotericin B in prophylaxis of experimental Candida endocarditis caused by non-C. albicans strains

Amphotericin B (1 mg/kg of body weight, intravenous) and fluconazole (100 mg/kg, intraperitoneal) were compared in the prophylaxis of experimental Candida endocarditis caused by drug-susceptible, non-C. albicans strains C. tropicalis and C. parapsilosis. Neither antifungal agent was effective at preventing endocarditis due to either Candida strain when either agent was administered in a single-dose regimen (1 h prior to fungal challenge); the prophylactic efficacy of both agents increased substantially when a second prophylactic dose was given (24 h postchallenge). The excellent prophylactic efficacy of fluconazole, a fungistatic agent, underscores the importance of microbistatic mechanisms in endocarditis prophylaxis.

Critical influence of timing of administration of granulocyte colony-stimulating factor on antibacterial effect in experimental endocarditis due to Pseudomonas aeruginosa

The effect of human recombinant granulocyte colony-stimulating factor (hrG-CSF) in rabbits with aortic endocarditis due to Pseudomonas aeruginosa was investigated. hrG-CSF significantly increased the number of polymorphonuclear neutrophils in blood and in cardiac vegetations and the expression of the adhesin molecule CD11b on the surface of polymorphonuclear neutrophils compared with those of animals that had not received hrG-CSF. When treatment was started 72 h after bacterial challenge, hrG-CSF alone had no antibacterial effect and did not enhance the efficacy of ciprofloxacin when used in combination, even with the higher dosing regimen used (50 micrograms/kg of body weight subcutaneously every 12 h for 4 days), in terms of number of positive blood cultures, bacterial counts in vegetations, and survival. In contrast, when treatment was started 30 min prior to bacterial challenge, hrG-CSF (50 micrograms/kg injected every 12 h) decreased bacterial titers in vegetations 72 h later (6.5 +/- 0.9 versus 7.9 +/- 0.9 log10 CFU/g of vegetation for hrG-CSF and controls, respectively; P = prophylactic administration of hrG-CSF did not increase the antibacterial effect of ciprofloxacin. We concluded that the antibacterial effect of hrG-CSF in experimental endocarditis was related to the timing of its administration since hrG-CSF demonstrated a significant but transient antimicrobial effect only when treatment was initiated before bacterial challenge.

Dextranase enhances antibiotic efficacy in experimental viridans streptococcal endocarditis

In endocarditis, exopolysaccharide production by viridans streptococci has been associated with delayed antimicrobial efficacy in cardiac vegetations. We compared the efficacies of temafloxacin alone and in combination with dextranase, an enzyme capable of hydrolyzing 20 to 90% of the bacterial glycocalyx, in a rabbit model of endocarditis. In in vivo experiments, rabbits were infected intravenously with 10(8) Streptococcus sanguis organisms and were treated 6 days later with temafloxacin (50 mg/kg of body weight intramuscularly twice a day) alone or combined with dextranase (1,000 U per rabbit per day intravenously). After 4 days of treatment (day 11), the animals were sacrificed and vegetations were quantitatively cultured. For ex vivo experiments, rabbits were infected as stated above and, on day 11, vegetations were excised aseptically and incubated in vitro in rabbit serum alone (control) or with temafloxacin or temafloxacin plus dextranase at concentrations similar to peak levels in plasma. In vitro, dextranase alone had no antimicrobial effect. In vivo and ex vivo, temafloxacin combined with dextranase was more effective than temafloxacin alone (P < 0.05). Our results suggest that dextranase is able to increase the effects of temafloxacin by reducing the amount of bacterial glycocalyx in infected vegetations, as confirmed in vitro by electron microscopy showing a markedly reduced amount of glycocalyx and a more clearly visible fibrin matrix.

Effects of alginase on the natural history and antibiotic therapy of experimental endocarditis caused by mucoid Pseudomonas aeruginosa

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)

Functional role of mucoid exopolysaccharide (alginate) in antibiotic-induced and polymorphonuclear leukocyte-mediated killing of Pseudomonas aeruginosa

We evaluated in vitro the functional role of mucoid exopolysaccharide (MEP) of Pseudomonas aeruginosa in blocking antibiotic-induced and polymorphonuclear leukocyte (PMN)-mediated pseudomonal killing. The serum-resistant P. aeruginosa isolates used were mucoid strain 144MR and its nonmucoid revertant, strain 144NM. By timed kill curves, early bacterial effects of amikacin against mucoid strain 144MR were substantially less than those observed with nonmucoid strain 144NM; this effect was reversible with enzymatic hydrolysis of MEP of strain 144MR by alginase. Also, early tobramycin uptake (15 to 30 min) by mucoid 144MR cells was less than that seen with nonmucoid strain 144NM; pretreatment of 144MR cells with alginase substantially enhanced early tobramycin uptake compared with untreated 144MR cells (P = 0.08). In strain 144NM (but not in strain 114MR) there was a notable postantibiotic leukocidal enhancement effect manifested by increased nonopsonic killing following brief exposure of these cells to supra-MIC amikacin; pretreatment of strain 144MR with alginase rendered these cells more susceptible to amikacin-induced postantibiotic leukocidal enhancement. Similarly, direct PMN-mediated nonopsonic killing of mucoid strain 144MR was significantly less than that observed with strain 144NM (P less than 0.05); pretreatment of 144MR cells with alginase rendered this strain equal to strain 144NM in susceptibility to nonopsonic killing. In addition, exogenous sodium alginate or extracted MEP of strain 144MR interfered with effective nonopsonic killing of strain 144NM by PMNs. Studies also indicated that mucoid strain 144MR was phagocytosed significantly less well than its nonmucoid mate (P less than 0.00001), an effect reversed by pretreatment of the mucoid cells with alginase. These data confirm that P. aeruginosa MEPs functionally decrease the uptake and early bactericidal effect of aminoglycosides in vitro and interfere with effective PMN-mediated nonopsonic phagocytosis and killing of mucoid strains.

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.