Empyema thoracis is most commonly caused by which organism

Empyema thoracis is collection of pus in the pleural cavity.It is commonly seen in post-pneumonic patients and in tuberculosis.Streptococcus pneumoniae is the most common causative agent.Empyema is the most common focal complication of pneumococcal pneumonia,occurring in <5% of cases.When fluid in the pleural space is accompanied by fever and leukocytosis after 4-5 days of appropriate antibiotic treatment for pneumococcal pneumonia,empyema should be considered. Reference:Harrison's medicine-18th edition,page no:1156.