On the left is the lung of a 63 year old with a history of viral pneumonia (influenza). He was treated supportively and showed improvement. However, during his convalescence he developed a cough (with sputum), severe pleuritic pain, and fever. Exam reveals widespread crackles and blood cultures are positive for Staph Aureus.

Compare this patient's lung to a normal example and a lobar pneumonia caused by Strep Pneumonia (using the buttons above).

How do the X-rays differ? Which appears more localized? more severe?

How does the gross pathology differ. Is it patchy or diffuse?

How about the histology? Which bacteria appears more agressive and destructive?

If the patients survive, what do you think their lungs would look like in 2 months and why?

Is the diagnosis of influenza clinically relevant?

S Aureus is a far more destructive pathogen than S Pneumonia, in particular because it produces a number of cell membrane-damaging toxins (including alpha and beta toxins). S Aureus also produces gamma toxin which lyses erythrocytes. As a result, rapid tissue necrosis and the resulting exuberant inflammatory response can result in abscess formation.

In contrast to the localized area of consolidation in the lobar pneumonia, the X-ray of the S. Aureus pneumonia shows patchy consolidation as well as a pneumatocele (with an air-fluid level) in the right lower lobe. A pneumatocele (or pneumatocyst) is a thin-walled air-filled cyst that occur as a sequela of acute pneumonia. Severe inflammation can cause bronchiolar wall ruptures through which air escapes and forms intraparenchymal air-filled cysts. Another consequence of of S. Aureus (not seen here) is the development of empyema (frank pus in the pleural space)..

Grossly the S Aureus pneumonia shows a patchy, multifocal pattern of tissue destruction and inflammation, typical of bronchopneumonia. The lobar pneumonia shows diffuse consolidation.

Histologically, S Aureus destroys the alveolar walls, promotes abscess formation and alveolar hemorrhage. In contrast to S Pneumonia infections which can resolve completely, the damage caused by S Aureus usually means the patient will have significant scarring, possibly including cavitation and pleural retraction.

The prior infection with influenza likely resulted in compromise of the mucociliary apparatus which pre-disposed the patient to a subsequent bacterial infection.