Abstract
Bacterial-bioluminescence regulation is often associated with quorum sensing. Indeed, many studies have been made on this subject and indicate that the expression of the light-emission-involved genes is density dependent. However, most of these studies have concerned two model species, Aliivibrio fischeri and Vibrio campbellii. Very few works have been done on bioluminescence regulation for the other bacterial genera. Yet, according to the large variety of habitats of luminous marine bacteria, it would not be surprising to find different light-regulation systems. In this study, we used Photobacterium phosphoreum ANT-2200, a piezophilic bioluminescent strain isolated from Mediterranean deep-sea waters (2200-m depth). To answer the question of whether or not the bioluminescence of P. phosphoreum ANT-2200 is under quorum-sensing control, we focused on the correlation between growth and light emission through physiological, genomic and, transcriptomic approaches. Unlike A. fischeri and V. campbellii, the light of P. phosphoreum ANT-2200 immediately increases from its initial level. Interestingly, the emitted light increases at much higher rate at the low cell density than it does for higher cell-density values. The expression level of the light-emission-involved genes stays constant all along the exponential growth phase. We also showed that, even when more light is produced, when the strain is cultivated at high hydrostatic pressure, no change in the transcription level of these genes can be detected. Through different experiments and approaches, our results clearly indicate that, under the tested conditions, the genes, directly involved in the bioluminescence in P. phosphoreum ANT-2200, are not controlled at a transcriptomic level. Quite obviously, these results demonstrate that the light emission of the strain is not density dependent, which means not under quorum-sensing control. Through this study, we point out that bacterial-bioluminescence regulation should not, from now on, be always linked with the quorum-sensing control.
Highlights
Quorum sensing is the regulation of gene expression in response to fluctuations in cell-population density as defined by Miller and Bassler (2001)
In P. phosphoreum ANT-2200, and in the majority of other bioluminescent Photobacterium species, the luxF gene is present between luxB and luxE (Mancini et al, 1988; Soly et al, 1988)
To date, in any Aliivibrio, Vibrio or other bioluminescent bacteria, the luxF gene appears to be confined to the genus Photobacterium (Mancini et al, 1989; Ast and Dunlap, 2004)
Summary
Quorum sensing (in short designed as QS) is the regulation of gene expression in response to fluctuations in cell-population density as defined by Miller and Bassler (2001). It results that the light seems to be emitted only from a certain cell density allowing an important amount of these autoinducers to be produced Since these studies, the whole genetic mechanism for A. fischeri has been well described as summarized thereafter. Referenced to as “autoinduction” (Nealson et al, 1970), the term quorum sensing was coined later by Fuqua et al (1994) It is common knowledge among microbiologists that QS systems, analogous to the one described above, regulate gene expression in a great variety of gram-negative Bacteria (Hastings and Greenberg, 1999), as well as in gram-positive Bacteria (Miller and Bassler, 2001) and more recently in Archaea (Zhang et al, 2012). See reviews about QS (Whitehead et al, 2001; Waters and Bassler, 2005; Li and Nair, 2012; Rajput et al, 2015)
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