A rise in hospital-acquired pneumonia with suspected tap water as the source points to infection with which gram-negative organism?

• A. Legionella spp.
• B. Pseudomonas aeruginosa
• C. Acinetobacter baumannii
• D. Klebsiella pneumoniae

Answer: (A)

Legionella bacteria are a classic cause of pneumonia linked to water systems in buildings, including hospital taps, showers, and cooling towers. Infection occurs when people breathe in aerosolized water containing the organism, which makes hospital water sources a prime suspect in a rise of hospital-acquired pneumonia. Among the Gram-negative organisms listed, Legionella is the best fit because its primary environmental reservoir is tap water, and transmission is via inhalation of contaminated aerosols rather than person-to-person spread. The other organisms can cause hospital pneumonia, but they are not as specifically associated with tap water as a source—Pseudomonas, Acinetobacter, and Klebsiella are more often linked to contaminated equipment, surfaces, or patient-to-patient transmission.