This is a high-power view showing thin Gram-negative bacilli of Legionella pneumophila from a buffered charcoal-yeast extract (BYCE) culture. L. pneumophila is the etiological agent of Legionnaire's disease. The disease manifests as a severe, consolidated pneumonia but can affect multiple organ systems. It occurs sporadically, at low endemic levels or epidemic levels in susceptible hosts such as immunosuppressed patients. Legionella can be found naturally in fresh water of streams, lakes and thermal springs. Through man-made systems such as air conditioning, cooling tower water and reservoir waters, Legionella can come into contact with humans and cause disease. Light micrograph.