A gram-negative, fastidious bacillus that has been isolated from air-conditioning towers is

Legionella pneumophila is the correct answer because it is a gram-negative, fastidious bacillus that is commonly associated with water environments, particularly in man-made water systems like air-conditioning towers. This organism is known to thrive in warm water and can form biofilms, making it particularly resilient in such settings. Its association with outbreaks of pneumonia, known as Legionnaires' disease, is well-documented.

In contrast, the other organisms listed are not primarily linked to air-conditioning systems. Proteus vulgaris is known for causing urinary tract infections and is not typically considered fastidious or isolated from air-conditioning systems. Brucella abortus, while a gram-negative bacterium, is primarily associated with zoonotic infections and livestock, and does not exhibit the same habitat preferences as Legionella. Edwardsiella tarda is often linked to aquatic environments but is not specialized to air-conditioning systems in the same way Legionella is. The unique environmental niche and the clinical significance of Legionella pneumophila make it the best choice for this question.