INVOLVEMENT OF BACTERICIDAL FACTORS FROM THROMBIN-STIMULATED PLATELETS IN CLEARANCE OF ADHERENT VIRIDANS STREPTOCOCCI IN EXPERIMENTAL INFECTIVE ENDOCARDITIS

Platelets activated with thrombin release bactericidal factors. We stu died 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 adh erence and the development of IE, were assessed for 10 viridans strept ococcal strains differing in their susceptibilities to releasate (mate rial released) from thrombin-activated platelets. Six strains were sus ceptible and four strains were resistant to these releasates. The numb ers of vegetations (VGs) colonized at 5 min and 48 h after intravenous challenge with 10(4) CPU were determined. At 5 min after challenge, s ignificantly more VGs were colonized with bacteria of the six platelet releasate-susceptible strains than with bacteria of the four releasat e-resistant strains (P < 0.005). In the reIeasate-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 decre ase was not observed with the releasate-resistant strains. As a result , the final developments of IE due to releasate-susceptible and -resis tant strains were not significantly different. The releasate-susceptib le strain I and the releasate-resistant strain 2 were selected for mor e 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 determi ned. The 90% infective dose for each strain was 10(5) CFU. At low inoc ulum concentrations (10(3) and 10(4) CFU) a larger proportion of the i noculated bacteria of both strains was found to be adherent on VGs tha n at higher challenge doses. The number of culture-positive VGs as wel l as the number of adherent bacteria per VG decreased rapidly in the f irst 30 min after challenge with strain 1 but not after challenge with strain 2. Additional experiments with the platelet releasate-suscepti ble strain S224 and the platelet releasate-resistant strain S182 confi rmed the data obtained with strains 1 and 2 and indicated that releasa te-susceptible strains disappeared from the VGs with time, whereas rel easate-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 bactericid al activity or surface phagocytosis by polymorphonuclear cells. Bacter ial cells of strain I adherent on excised VGs were rapidly cleared by exposure to fresh clotting blood or to releasates from thrombin-stimul ated platelet suspensions. In contrast, strain 2 bacteria adherent on VGs were hardly affected by these treatments. These data strongly indi cate that bactericidal factors released from platelets upon thrombin s timulation 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 to res ist the activity of platelet-released bactericidal factors.