###### Abstract  
Quorum sensing is a density-dependent mechanism using chemical signal molecules termed autoinducers to regulate diverse biological processes in bacteria, including bioluminescence. However, the correlation between growth and light emission of two typical luminescent bacteria, _Photobacterium phosphoreum_ T3 and _Vibrio ginghaiensis_ sp.-Q67, is still unclear. This study investigates the variations of bioluminescence and the light-emission-involved gene expression of the above two strains, respectively, showing that bioluminescence is population density-dependent. Furthermore, the effect of crude extracts (175, 350, 700 and 1750 mg/L) from the bacterial culture that contains the potential autoinducers on the bioluminescence is explored. At the exponential and the early stationary growth phase, T3 did not exhibit an obvious light intensity and cell density change after adding crude extracts at 175 and 350 mg/L, while the light intensity decreased at 700 and 1750 mg/L, showing a luminescence inhibition. For Q67, the light intensity increased dramatically with crude extract concentration. These results suggest that the bioluminescence process of both T3 and Q67 is controlled by quorum sensing. Furthermore, the different response modes of these two strains to autoinducers imply that the two strains could be applied to different compounds for toxicity assesses. quorum sensing; quorum sensing; _Photobacterium phosphoreum_ T3; _Vibrio ginghaiensis_ sp.-Q67; bioluminescence; gene expression; toxicity  
17 January 2022  
Accepted: 15 February 2022  
16 February 2022  
1  
Footnote 1: email: _Applied_  
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**1. Introduction**  
Quorum sensing (QS) is a microbial cell-cell communication process that depends on the secretion of diffusible chemical molecular-termed autoinducers to regulate associated gene expression in response to cell density [1, 2]. When the accumulated autoinducers (in and out of the cell) gradually reach a significant threshold concentration the expression of related genes will be induced, leading to various microbial behaviors, including bioluminescence production and virulence secretion, biofilm formation and so on [3]. Meanwhile, various studies have demonstrated that acyl-homoserine lactone (AHL) is one of the most important autoinducers that mediates the QS-dependent gene expression in Gram-negative bacteria [4, 5]. Since QS was first discovered in two marine luminescent bacteria species, _Vibrio fischeri_ (_Aliivibrio fischeri_) and _Vibrio harveyi_, over 45 years ago, much effort has been devoted to illustrating the physiological and genetic mechanisms in these two typical microorganisms. One of the well-defined QS systems in _V. fischeri_ is LuxI/LuxR, in which _luxI_ encodes a protein of 193 amino acids that catalyzes the synthesis of autoinducer N-3-oxo-hexanoyl-homoserine lactone (3-oxo-C\({}_{6}\)-HSL), and LuxR is an intracellular receptor which binds with 3-oxo-C\({}_{6}\)-HSL to form an initiation complex resulting in the expression of light gene _lux_ICDABE. Later, a second QS system named AinS/AinR was reported in _V. fischeri_. The AinS/AinR functioned like the LuxM/LuxN in _V. harveyi_, relying on the autoinducer N-3-octanoyll-homoserine lactone (C\({}_{8}\)-HSL) [6]. Besides the abovementioned LuxM/LuxN system, there are two other kinds of QS systems in _V. harveyi_, LuxS/LuxPQ and CqsA/CqsS [7]. These three parallel regulatory systems form multiple feedback loops that regulate the bioluminescence elaborately in _V. harveyi_. The coupling of light-emission-involved gene expressions with that of cell population provides the theoretical basis for characterizing quorum sensing by gene expression changes [8]. Alternatively, the diversity of QS systems helps to illustrate QS in the context of bacterial sociality, which has aroused great attention and interest [9].