They resemble Actinomyces morphologically: Gram-positive, branching filaments. You can differentiate them from Actinomyces because Nocardia are weakly acid-fast, while Actinomyces are not acid-fast. Nocardia are strict aerobes that can cause pneumonia and subcutaneous granulomas called mycetomas.
Lung infection may present as miliary pneumonia, a lung abscess, a nodule, or an empyema, and it may disseminate systemically. Appropriate samples include pus, sputum, and granules. These can be Gram-stained to show the typical morphology, and they will be weakly acid-fast when stained using a dilute solution of HCl or dilute 1% sulphuric acid. Unlike Actinomyces, Nocardia grow aerobically to form dry, wrinkled colonies.
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