Increase in fitness of streptococcus pneumoniae is associated with the severity of necrotizing pneumonia

Background: The incidence of necrotizing pneumococcal pneumonia has increased during the past 2 decades. We hypothesized that increased pneumococcal load or augmented inflammatory cytokine production might lead to destructive pneumococcal lung disease. Methods: This study enrolled prospectively 0- to 18-year-old children with a diagnosis of community-acquired pneumonia with pleural effusion admitted to 6 medical centers from March 2010 to April 2012. Children were diagnosed with pneumococcal empyema if the pleural fluid tested positive for quantitative pneumococcal (lytA) detection by real-time polymerase chain reaction. Pneumococcal empyema cases were further divided into 4 groups according to necrosis severity: (0) nonnecrosis, (1) mild necrosis, (2) cavitation and (3) bronchopleural fistula. Nasopharyngeal and pleural pneumococcal load, as well as levels of proinflammatory cytokines (TNF-α, IL-1β, IL-6, IL-8), Th1-(IL-2, IFN-γ), Th2-(IL-4, IL-10) and Th17-cytokines (IL-17), in the pleural fluid was measured. Results: Serotypes 19A and 3 accounted for 69.4% and 12.5%, respectively, of 72 cases of pneumococcal empyema. Pleural pneumococcal load was significantly higher in serotypes 19A and 3 infection than in the other strains causing infection (P = 0.006). There was a correlation between nasopharyngeal and pleural pneumococcal load (ρ = 0.35; P = 0.05). In multivariate ordinal logistic regression analysis, pleural pneumococcal load (adjusted odds ratio: 1.79; 95% confidence interval: 1.03-3.06) and IL-8 (adjusted odds ratio: 2.64; 95% confidence interval: 1.21-5.75) were independent factors associated with the severity of lung necrosis. Conclusions: Evolution of Streptococcus pneumoniae toward increased fitness in their interaction with host and exaggerated IL-8 expression may be responsible for the increase of necrotizing pneumococcal pneumonia.