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The Streptococcus milleri group as a cause of pulmonary infections
1First Department of Internal Medicine, University of the Ryukyus, Okinawa, Japan.
Abstract:
Streptococci that colonize the mouth and upper respiratory tract tend to be considered harmless commensals. In 45 cases of acute pneumonia and/or pulmonary abscess and 25 cases of thoracic empyema, the predominant species recovered were anaerobic bacteria and the Streptococcus milleri group, which encompasses the oral species Streptococcus anginosus, Streptococcus constellatus, and Streptococcus intermedius. The isolation of most S. milleri organisms along with oral anaerobes indicated synergy between these groups. Studies in a mouse model of pneumonia demonstrated this synergy; mortality was higher, histopathologic abnormalities were more marked (reflecting acute pneumonia followed by pulmonary abscess or empyema), and viable bacteria were more numerous in the lungs of mice with mixed infections caused by the S. milleri group and anaerobes than in the lungs of those with monomicrobial infection. In vitro studies elucidated a possible mechanism of this synergistic effect: anaerobes may enhance the growth of the S. milleri group and/or inhibit the bactericidal activity of the host. We conclude that the S. milleri group is more important in pulmonary infections than has previously been recognized.
Insights
The Streptococcus milleri group, often found in the mouth, plays a significant role in severe lung infections like pneumonia and empyema, especially when combined with anaerobic bacteria.
Area of Science:
- Microbiology
- Infectious Diseases
- Pulmonary Medicine
Background:
- * Oral streptococci, including the Streptococcus milleri group (Streptococcus anginosus, Streptococcus constellatus, Streptococcus intermedius), are typically considered harmless commensals.
- * Pulmonary infections such as pneumonia, lung abscess, and thoracic empyema can have complex etiologies.
- * The role of specific bacterial groups in polymicrobial lung infections requires further elucidation.
Purpose of the Study:
- * To investigate the prevalence and significance of the Streptococcus milleri group in cases of acute pneumonia and thoracic empyema.
- * To explore the synergistic interactions between the Streptococcus milleri group and anaerobic bacteria in pulmonary infections.
- * To elucidate the mechanisms underlying this synergy.
Main Methods:
- * Retrospective analysis of bacterial isolates from 45 cases of pneumonia/pulmonary abscess and 25 cases of thoracic empyema.
- * In vivo studies using a mouse model of pneumonia to assess the impact of mixed infections (S. milleri group + anaerobes) versus monomicrobial infections.
- * In vitro experiments to investigate potential mechanisms of synergy, including effects on bacterial growth and host immune response.
Main Results:
- * The Streptococcus milleri group and anaerobic bacteria were predominant in severe pulmonary infections.
- * Mixed infections with the S. milleri group and anaerobes resulted in higher mortality, more severe lung pathology, and increased bacterial load in the mouse model compared to single-organism infections.
- * In vitro studies suggest anaerobes may promote S. milleri group growth or inhibit host defenses.
Conclusions:
- * The Streptococcus milleri group is a significant contributor to severe pulmonary infections, including pneumonia, lung abscess, and empyema.
- * A synergistic relationship exists between the Streptococcus milleri group and oral anaerobes, exacerbating infection severity.
- * These findings highlight the underestimated importance of the Streptococcus milleri group in the pathogenesis of pulmonary infections.