A mechanism of pathogenicity of "Streptococcus milleri group" in pulmonary infection: synergy with an anaerobe.

The relationship between Streptococcus constellatus, one of the species of the "Streptococcus milleri group", and Prevotella intermedia was studied in a model of pneumonia in mice and in vitro to elucidate mechanisms of pathogenicity in "S. milleri group"-associated pulmonary infection. Acute pneumonia with or without empyema and lung abscess in mice with mixed infection resulted in 60% mortality rate, but there was only 10% mortality and mild pneumonia in each separate infection. Bacterial clearance of organisms, especially S. constellatus, in mixed infection was delayed. Enhancement of growth of S. constellatus was demonstrated when cultured with P. intermedia; growth was also stimulated by a culture filtrate of P. intermedia which also inhibited bactericidal activity of human neutrophils. In an examination of infectivity and bacterial clearance of S. constellatus with P. intermedia culture filtrate in vivo, there was 20% mortality and delayed clearance of S. constellatus, although the infection was not as severe as that produced by the combination of both organisms. These results suggest that P. intermedia may act with S. constellatus in the production of pulmonary infections by stimulating its growth and suppressing bactericidal activity of the host.