Gram staining of the sputum of the patients with pneumonia is diagnostic with all these pathogens, EXCEPT:

Gram's staining may identify ceain pathogens (e.g., S. pneumoniae, S. aureus, and gram-negative bacteria) by their characteristic appearance. A sputum sample must have 1. >25 neutrophils 2. <10 squamous epithelial cells per low-power field, to be adequate for culture. Ref: Harrison, E-18, P-2133