| Summary: | 碩士 === 國立臺灣大學 === 微生物學研究所 === 89 === Streptococcus mutans is the primary cause of dental caries and opportunistic infective agent of infective endocarditis. We established an experimental endocarditis animal model to examine the virulence of S. mutans in causing infective endocarditis (IE). Initially, we found that histopathologic findings were different between catheter damaged rat valves with or without bacterial infection, although vegetations could be found in both cases. Colonized bacteria in infective vegetation were confirmed by Gram stain and these bacteria induced pronounced inflammatory reactions in underlying myocardium. Using this experimental rat model, we tested the virulence of S. mutans serotype C strains of different origins in inducing endocarditis. S. mutans strains included blood isolates from patients with IE, oral isolates from healthy young adults, and laboratory strains. Firstly, we found the laboratory strain GS-5 was less virulent than clinical isolates from blood culture in terms of ID50. Secondly, the infectivity and colonized bacteria in vegetations were also different when rats were challenged intravenously with different clinical isolates. A positive correlation was found between the infective dose of various strains and severity of infection, including colonized bacteria in vegetation, bacteremia, and spleen infection. In addition, we found that in vivo passage of an oral isolate, 410-4 strain increased the virulence in inducing infective vegetation.
To analyze further the factors that were related to the virulence of different strains, we examined the ability of each strain in inducing platelet aggregation. We found that strains of higher infectivity and virulence in vivo exhibited higher tendency in inducing platelet aggregation in vivo, suggesting such phenotypical characteristics are important in the virulence of S. mutans induced experimental endocarditis
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