Light micrograph (LM) of a lung tissue specimen from a Knoxville patient with fatal pneumonia due to Legionnaire's disease; H&E stain. Legionella pneumophila are Gram-negative bacteria. These organisms, if present in the specimen, stain a pink or red color. Note how the alveolar spaces are so congested with a leukocytic infiltrate in response to the infection. Magnification unknown.