Major issues of bacterial cooperativity are illustrated by the symbiosis between the bobtail squid Euprymna and the luminous bacterium Vibrio fischeri. Each morning, the squid discharges most of the Vibrio from its light organ and grows a new uncontaminated culture by nightfall. The squid uses the light produced by that culture to erase its shadow on the sea floor as it forages for food under moonlight. Vibrios grow in the sea, and when a Vibrio cell releases its signaling molecule, called autoinducer (AI) into the open ocean, the AI concentration remains low because there are only 10 Vibrio cells per ml sea water. The levels of luminescence enzymes and their oxidizable substrate depends on the concentration of AI, so V. fischeri is not luminescent in its free-living state in the sea. AI allows V. fischeri to discriminate between the free living, low density state and the squid-associated, high-density state when it fills the animal’s pair of light organs. Luminescence genes are activated only when the host is there to pay the high energy price of light production. Microbiologists speak of the minimum number of bacteria necessary for high level luminescence in the light organ as a "quorum", and the regulation as “quorum sensing”. Specific interactions that depend on contact between Vibrio and the squid during the development of a light organ will also be discussed.
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