Depressed light emission by symbiotic Vibrio fischeri of the sepiolid squid Euprymna scolopes

Abstract

Bioluminescent marine bacteria of the species Vibrio fischeri are the specific light organ symbionts of the sepiolid squid Euprymna scolopes. Although they share morphological and physiological characteristics with other strains of V. fischeri, when cultured away from the light organ association the E. scolopes symbionts depress their maximal luminescence over 1,000-fold. The primary cause of this reduced luminescence is the underproduction by these bacteria of luciferase autoinducer, a molecule involved in the positive transcriptional regulation of the V. fischeri lux operon. Such an absence of visible light production outside of the symbiotic association has not been previously reported among light organ symbionts of this or any other species of luminous bacteria. Levels of luminescence approaching those of the E. scolopes bacteria in the intact association can be restored by the addition of exogenous autoinducer to bacteria in laboratory culture and are affected by the presence of cyclic AMP. We conclude that some condition(s) specffic to the internal environment of the light organ is necessary for maximal autoinduction of luminescence in the symbionts of this squid-bacterial association.The emission of visible light by the marine bacterium Vibriofischeri is dependent upon the enzymatic activity of an inducible luciferase. Its synthesis is regulated at the transcriptional level by a diffusible, sensory autoinducer molecule (7,22,28). The autoinducer, which is believed to be species specific (7,13,22), has been chemically identified as N-(3-oxohexanoyl)homoserine lactone (8). Synthesis of this molecule appears to be continuous (22), and, because it is freely diffusible across the cell membrane, its internal concentration is directly related to the extent of its external accumulation (20). Thus, the autoinducer acts as the sensory component of the autoinduction process, enhancing transcription of the bacterial luminescent system only when V. fischeri cells achieve a high density in a confined environment (19,26,33). This self-regulation of light production results in visible luminescence when the cells exist either as colonies on the surfaces of decaying organic matter or as symbionts within species-specific light-emitting structures of certain marine fishes and cephalopods (13). Conversely, in seawater and other environments where V. fischeri concentrations are rarely more than a few cells per milliliter, autoinducer does not accumulate to a sufficient level for visible luminescence to be expressed (27).The symbiotic luminescent bacteria that are responsible for light emission by the marine sepiolid squid Euprymna scolopes are acquired by the animal early in its development (37). The bacterial culture that develops is maintained in a pair of internal light organs within the mantle cavity (17, 21), where it produces an easily visible luminescence. However, as we demonstrate herein, once removed from the animal, the bacteria become essentially nonluminous. This condition is in marked contrast to the bright lumin...

Larvae of Pocillopora damicornis (Anthozoa) settle and metamorphose in response to surface-biofilm bacteria

“…Each sample was serially diluted to 10%, 1%, and 0.1% of the original concentration. Following dilution, 100 µl of each diluted sample was spread onto an agar plate consisting of halfstrength seawater-tryptone (½SWT) (Boettcher & Ruby 1990) for bacterial colonies to develop. The different colonies that developed on the agar were separated based on morphological characteristics and streaked onto a new plate for further isolation.…”

Physical and functional maps of the luminescence gene cluster in an autoinducer-deficient Vibrio fischeri strain isolated from a squid light organ

“…The specific light organ symbiont of the squid Euprymna scolopes has recently been isolated and characterized as V. fischeri (2). Unlike previous isolates of V. fischeri, including the Japanese pinecone fish symbiont V. fischeri MJ1, the E. scolopes symbiont (represented by strain ES114) is essentially nonluminous when grown under standard laboratory conditions.…”

“…Nonetheless, transconjugants of V. fischeri ES114 that contain multicopy plasmids bearing the ES114 lux genes synthesize sufficient autoinducer to induce luminescence. These results suggest that V. fischeri ES114 does not lack a functional luxI, nor is it deficient in the ability to synthesize metabolic precursors for autoinducer synthesis.Vibrio fischeri is a luminescent bacterium that occurs in marine environments both as free-living cells and as symbionts within the specialized light organs of a number of animal species (2,12,(29)(30)(31)(32). Luminescence of these bacteria is regulated by the accumulation of autoinducer [N-(3-oxohexanoyl) homoserine lactone], a species-specific signalling compound that is required for the transcriptional activation of the luminescence genes (14,16).…”

“…Vibrio fischeri is a luminescent bacterium that occurs in marine environments both as free-living cells and as symbionts within the specialized light organs of a number of animal species (2,12,(29)(30)(31)(32). Luminescence of these bacteria is regulated by the accumulation of autoinducer [N- (3-oxohexanoyl) homoserine lactone], a species-specific signalling compound that is required for the transcriptional activation of the luminescence genes (14,16).…”

Complete genome sequence of Vibrio fischeri : A symbiotic bacterium with pathogenic congeners

“…Nevertheless, important questions remain concerning the genetic and metabolic mechanisms by which V. fischeri and other symbiotic bacteria adjust to the special environment of host tissues. To better understand these symbiotic activities in V. fischeri, and to begin to identify features common to beneficial and pathogenic bacteria, we sequenced the genome of strain ES114, the model light-organ symbiont of the squid Euprymna scolopes (9).…”