Which Gram-negative bacillus has been isolated from air-conditioning towers?

Legionella pneumophila is the correct answer because it is well-known for its association with water systems, particularly in man-made environments like air-conditioning towers, cooling towers, and hot water systems. This bacterium thrives in warm aquatic environments and can become aerosolized, leading to inhalation and potential infection, most notably causing Legionnaires' disease, a severe form of pneumonia.

The other options are not typically isolated from air-conditioning systems. Campylobacter jejuni is primarily associated with contaminated food and water, often leading to gastrointestinal infections. Salmonella enterica is commonly found in food sources and can cause foodborne illnesses. Shigella dysenteriae is associated with dysentery, primarily transmitted via contaminated food and water and not by air-conditioning or water systems, further confirming that Legionella pneumophila is the correct choice based on its environmental niche and pathogenicity in relation to air-conditioning towers.