Vibrio parahaemolyticus

Vibrio parahaemolyticus shares quorum sensing machinery with V. fischeri but uses it for opposite purposes: causing seafood-borne illness rather than beneficial symbiosis. This pathogen coordinates virulence factor production through population-density sensing, ensuring that toxins are produced only when bacterial numbers are sufficient to overwhelm host defenses. The same communication logic that enables V. fischeri's cooperative relationship with squid enables V. parahaemolyticus's assault on human intestines.

V. parahaemolyticus causes the majority of seafood-associated gastroenteritis worldwide, particularly from raw or undercooked shellfish. The bacterium lives on chitinous surfaces—crab shells, shrimp exoskeletons, oyster tissues—reaching high densities in summer warmth. Quorum sensing coordinates the transition from benign environmental bacterium to pathogen: at low densities during cold months, the bacterium persists harmlessly; at high densities in warm conditions, it expresses the Type III secretion system that causes disease.

The bacterium demonstrates phenotypic plasticity through phase variation—switching between different cell types optimized for different environments. Opaque colonies specialize in biofilm formation and environmental persistence; translucent colonies specialize in host infection. This bet-hedging creates mixed populations ready for whatever conditions arrive. The same individual genotype produces different phenotypes depending on environmental signals, illustrating how organisms can maintain multiple strategies simultaneously rather than committing to one approach.