To study the role of platelet microbicidal activity in host defense against infective endocarditis (IE) due to viridans streptococci (VS), the susceptibility to platelet releasate of blood and oral VS isolates from patients with and without IE was compared. The influence of neutralization of platelet microbicidal activity was studied in 2 experimental IE models. Resistance to platelet releasate was more prevalent among VS from blood of patients with IE than from blood of bacteremic patients without IE and among oral VS isolates. Serum from rabbits vaccinated with human platelet sonicate supernatants neutralized human and rabbit platelet-released microbicidal activity and had antibodies recognizing microbicidal proteins thrombocidin-1 and -2 and other human platelet proteins. In the 2 rabbit IE models, vaccination increased the susceptibility to experimental IE due to platelet releasate-susceptible VS. Thus, platelet-released microbicidal activity is an important host defense factor against IE due to VS.

Platelets activated with thrombin release bactericidal factors. We studied the role of the susceptibility of viridans streptococci to these bactericidal factors in the development of infective endocarditis (IE). By using the experimental endocarditis rabbit model, the initial adherence and the development of IE were assessed for 10 viridans streptococcal strains differing in their susceptibilities to releasate (material released) from thrombin-activated platelets. Six strains were susceptible and four strains were resistant to these releasates. The numbers of vegetations (VGs) colonized at 5 min and 48 h after intravenous challenge with 10 4 CFU were determined. At 5 min after challenge, significantly more VGs were colonized with bacteria of the six platelet releasate-susceptible strains than with bacteria of the four releasate-resistant strains (P < 0.005). In the releasate-susceptible group of strains, the number of colonized VGs decreased significantly between 5 min and 48 h after intravenous inoculation (P < 0.001). Such a decrease was not observed with the releasate-resistant strains. As a result, the final developments of IE due to releasate-susceptible and-resistant strains were not significantly different. The releasate-susceptible strain 1 and the releasate-resistant strain 2 were selected for more detailed experiments. Rabbits were killed at 5 and 30 min and 2, 4, and 48 h after inoculation. The number of culture-positive VGs as well as the number of adherent bacteria on the individual VGs were determined. The 90% infective dose for each strain was 10 5 CFU. At low inoculum concentrations (10 3 and 10 4 CFU) a larger proportion of the inoculated bacteria of both strains was found to be adherent on VGs than at higher challenge doses. The number of culture-positive VGs as well as the number of adherent bacteria per VG decreased rapidly in the first 30 min after challenge with strain 1 but not after challenge with strain 2. Additional experiments with the platelet releasate-susceptible strain S224 and the platelet releasate-resistant strain S182 confirmed the data obtained with strains 1 and 2 and indicated that releasate-susceptible strains disappeared from the VGs with time, whereas releasate-resistant strains persisted. In vitro studies with VGs excised 5 min after challenge with strain 1 or 2 showed that clearance of the releasate-susceptible strain 1 was not caused by complement bactericidal activity or surface phagocytosis by polymorphonuclear cells. Bacterial cells of strain 1 adherent on excised VGs were rapidly cleared by exposure to fresh clotting blood or to releasates from thrombin-stimulated platelet suspensions. In contrast, strain 2 bacteria adherent on VGs were hardly affected by these treatments. These data strongly indicate that bactericidal factors released from platelets upon thrombin stimulation are involved in the clearance of bacteria early after their adherence to VGs. Therefore, development of IE is the combined result of the abilities of viridans streptococci to adhere to VGs and ...

Major predisposing conditions for infective endocarditis (IE) are the presence of a cardiac platelet-fibrin vegetation and of circulating bacteria with relatively low susceptibility to microbicidal activity of blood platelets. The influence of proinflammatory conditions on development of IE is unknown. We studied the effects of the presence of a catheter, inserted to induce platelet-fibrin vegetations, and of the proinflammatory cytokine interleukin-1␣ in rabbit experimental IE. Leaving the catheter in place after challenge with viridans streptococci predisposed for experimental IE. IE susceptibility rapidly decreased between 0 to 6 h after catheter removal. The catheter did not predispose for IE by providing a site for bacterial adherence, as almost all explanted catheters were culture negative. To mimic the proinflammatory influence of the catheter, rabbits were injected with interleukin-1␣ at 24 h after catheter removal and at 0, 1, and 3 h before bacterial challenge. Interleukin-1␣ injected 3 h prior to challenge significantly increased IE incidence due to a platelet releasatesusceptible Streptococcus oralis strain, with rapidly increasing numbers of bacteria within the vegetations. IE due to the Streptococcus sanguis strain less susceptible to platelet releasate was not enhanced. We conclude that proinflammatory stimuli, either a catheter or interleukin-1␣, enhanced susceptibility to IE due to the platelet releasate-susceptible S. oralis. As with rabbits, temporary intravascular proinflammatory conditions may predispose for IE in humans at risk for this serious infection.Infective endocarditis (IE) denotes infection of the endocardial surface of the heart (3). The presence of sterile vegetations at the damaged surface of the endocardium (1) and bacteria and fungi circulating in the bloodstream that are able to adhere to and to colonize the vegetation (3,9,13,29,35) are prerequisites in the pathogenesis of IE. A great diversity of microorganisms cause IE, with viridans streptococci (VS) as the predominant microorganisms encountered in native valve IE (3,14,16,24,27,30,33,34).Another major determinant for the development of IE is the susceptibility of the circulating microorganisms to platelet microbicidal activity (2, 7-9) due to cationic proteins such as thrombocidin or platelet microbicidal proteins, released after thrombin stimulation (7,9,26,36,39). Neutralization of this defense mechanism enhanced susceptibility of rabbits to IE due to platelet releasate-susceptible VS (8). Platelet microbicidal activity is very likely to operate in humans as well, as VS and Staphylococcus aureus isolated from vegetations of endocarditis patients are generally less susceptible than oral isolates (7,8,38). Despite an apparent selective advantage, the VS isolates from IE patients are presumably still subject to the platelet defense system, since these isolates are only slightly less susceptible to purified thrombocidins in vitro (9, 41).In the rabbit model of experimental IE, a catheter inserted retrograde across the aort...