In vivo Luminescence of Luminous Bacteria

Overview. In the cultures of luminous bacteria, the bacterial cells are not luminous in their early stages of propagation. The formation of bioluminescence system is controlled by a substance called autoinducer that is produced by the cells of luminous bacteria. 

The autoinducer is a low molecular weight compound that is easily leached from the cells into the culture medium. By the propagation of bacterial cells, the concentration of the autoinducer in the medium increases. When the concentration reaches a certain threshold, the biosynthesis of bioluminescence system begins, and the bacteria become luminescent. The process is also called quorum sensing (Fuqua et al., 1994). 

In the living cells of luminous bacteria, FMNH2 is produced by the reduction of FMN with NADH catalyzed by FMN-reductase. This process is, in effect, the recycling of FMN. In the cells, a long-chain aldehyde is produced by the reduction of the corresponding long-chain acid, which is also a recycling process. 

Another notable feature of the in vivo bacterial luminescence is seen in their emission spectra. Although the emission peak of in vitro bacterial luminescence is normally at about 490 nm, the in vivo emission peaks of various bacterial species and strains are significantly shifted from 490 nm, ranging from the shortest wavelength of 472 nm to over 500 nm. Some expanded notes concerning in vivo bacterial luminescence are given below. 

Energy transfer to fluorescent proteins. There are marked differences among the various bacterial species and strains in terms of the in vivo luminescence spectra. The emission maxima are spread mostly in a range from 472 to 505 nm (Seliger and Morton, 1968), but one of the strains, P. fischeri Y-l, shows a maximum at 545 nm (Ruby and Nealson, 1977), as shown in Fig. 2.3. However, the in vitro luminescence spectra measured with purified luciferases obtained from the various bacterial species and strains are all similar (Amax about 490 nm). The variation in the in vivo luminescence spectra may be due to the occurrence of an intermolecular energy transfer that increases the efficiency of light emission. 

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