Pathogenicity of the Streptococcus milleyi Group in Pulmonary Infections

The pathogenicity of Streptococcus constellatus in pulmonary infections was investigated in mice in correlation with the phagocytic killing of the microorganisms by human polymorphonuclear neutrophils (PMNs). The histological inflammation score of the lungs in mice inoculated intratracheally with 10(8) cfu/mouse of the virulent strain S. constellatus RZYK001 obtained from bronchoalveolar fluid of a patient with pneumonia, was significantly higher than in mice inoculated with the avirulent strains S. constellatus RT4303 and RT6002 obtained from saliva of healthy adults (p < 0.001). The bactericidal activity of human PMNs against Staphylococcus aureus ATCC 25923 was not inhibited at all by the culture filtrate of the virulent strain, but the killing index against RZYK001 was 36.4% after 90-min incubation whereas that against RT4303 and RT6002 was 94.7% and 99.4%, respectively (p < 0.001). Therefore, these results suggest a possible pathogenic mechanism of S. constellatus apparently mediated by a structural component of the organisms so that a virulent strain is less likely to be killed than an avirulent one, predisposing it to survive in the infection sites.