Legionella pneumophila

Legionella pneumophila caused panic when it first emerged in 1976, killing 29 American Legion convention attendees in Philadelphia. The culprit proved to be an entirely novel ecological phenomenon: a bacterium that had evolved to parasitize freshwater amoebae was now thriving in air conditioning systems. The same intracellular survival mechanisms that allow Legionella to grow inside amoebae enable it to survive inside human macrophages—our immune cells become incubators rather than killers.

Legionella's relationship with biofilms illustrates ecological complexity in engineered systems. The bacterium cannot grow in pure water; it requires biofilm communities that provide nutrients and protective niches. Pseudomonas aeruginosa and other biofilm-forming bacteria create the conditions Legionella needs. Within biofilms, Legionella infects resident amoebae, replicating to high numbers before lysing host cells and releasing infectious progeny. This cycle can amplify Legionella populations by orders of magnitude within cooling towers, hot tubs, and plumbing systems.

The Legionella story reveals how human infrastructure creates novel evolutionary opportunities. Cooling towers, hot water systems, and decorative fountains maintain temperatures (25-42°C) ideal for Legionella growth. Biofilms in these systems resist disinfection, providing refugia where Legionella persists between amplification events. Periodic aerosolization—from cooling tower drift, shower heads, or fountain sprays—delivers infectious droplets to human lungs. We inadvertently created ideal habitat for a pathogen that had no historical relationship with humans. Legionella thus exemplifies how technological systems generate emergent biological risks.