Aeromonas salmonicida

Aeromonas salmonicida causes furunculosis, a devastating disease of salmon and trout aquaculture. Like Vibrio species, A. salmonicida uses quorum sensing to coordinate its attack, producing virulence factors only when population density indicates that infection has reached a critical threshold. This density-dependent pathogenesis mirrors V. fischeri's density-dependent bioluminescence—both bacteria wait until numbers are sufficient before investing in costly collective activities.

The bacterium has co-evolved with salmonid fish over millions of years, developing sophisticated mechanisms to evade fish immune systems and establish persistent infections. Its A-layer surface protein forms a protective shield against complement killing and phagocytosis, while secreted proteases degrade host immune molecules. Infection creates characteristic boil-like lesions (furuncles) that eventually rupture, releasing bacteria to infect other fish. This transmission strategy requires sufficient bacterial load, hence the utility of quorum sensing.

A. salmonicida devastates aquaculture economically, causing losses exceeding hundreds of millions of dollars annually. The high-density conditions of fish farms create ideal environments for quorum-dependent pathogens—fish crowding ensures bacteria rapidly reach the densities needed to trigger virulence. Intensive aquaculture effectively provides the quorum that natural populations rarely achieve. Understanding this connection between population density and disease has driven changes in aquaculture practice, illustrating how knowledge of microbial communication can inform management of economically important systems.