DETECTION AND QUANTIFICATION OF VIBRIO-FISCHERI AUTOINDUCER FROM SYMBIOTIC SQUID LIGHT ORGANS

Vibrio fischeri is the specific light organ symbiont of the sepiolid s quid species Euprymna scolopes and Euprymna morsei. Both species of sq uid are luminescent by virtue of their bacterial symbionts, but the na tural symbionts of E. scolopes do not produce visible luminescence in laboratory culture. The primary cause of this depressed luminescence b y E. scolopes symbionts in culture was found to be the production of r elatively low levels of V. fischeri autoinducer, a positive transcript ional coregulator of the lux regulon, identified as N-(3-oxohexanoyl) homoserine lactone, Concentrations of autoinducer activity produced by these symbionts in culture were quantified and found to be at least 1 .0-fold lower than those produced by E. morsel isolates (which are vis ibly luminous outside the association) and perhaps 10,000-fold lower t han those of the brightest V. fischeri strains. Despite the difference s in their symbiont strains, the intact light organs of the two specie s of squid contained comparable amounts of extractable autoinducer act ivity (between 100 and 200 pg per adult animal). The chromatographic b ehavior of this autoinducer activity on reverse-phase high-performance liquid chromatography was consistent with its presumptive identificat ion as V. fischeri autoinducer, Within the 5-mu l volume of the epithe lial core of the light organ in which the symbiotic V. fischeri strain s are housed, these amounts would result in an effective autoinducer c oncentration of at least 100 nM. Because these levels are over 40-fold higher than the concentration needed for the induction of luminescenc e of bacteria in culture, we conclude that the inherent degree of auto inducer production by strains of V. fischeri may not influence their e ffectiveness as light organ symbionts. Furthermore, this study provide s the first direct evidence that the phenomenon of cell density-depend ent autoinduction, discovered and described first for laboratory cultu res of V. fischeri but believed to be a general phenomenon in many spe cies of host-associated symbionts and pathogens, is in fact a conseque nce of bacterial colonizations of host tissue.