Bacterial infections present as pneumonias or abscesses. Acute pneumonia and lung abscesses are characterized by a neutrophilic exudate. Many bacteria, but not all, can be seen with Routine stains as well as with Gram stain.
A specific identification of Legionella pneumophila can be made in BAL samples by immunofluorescent antibody staining.
Infection by Mycobacterium tuberculosis results in granulomatous inflammation and cytologic specimens may contain clusters of epithelioid histiocytes, lymphocytes and Langhans` giant cells. Necrosis may be or not be evident. In immunocompromised patients there may be very little reaction, with few granulomas, and abundance of acid-fast organisms (demonstrated by Ziehl-Neelsen stain). Granulomas can be seen in other conditions, such as sarcoidosis and fungal infections.