LIGHT-EMITTING PROPERTIES OF BACTERIA – BIOLUMINESCENCE

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Bioluminescence is defined as the enzymatic production of visible light by living organisms including some microorganisms. It is the process by which some bacteria emit light. Several species of bacteria emit light through the process of bioluminescence. Most of these bacteria have been classified in the genera Photobacterium, Aliivibrio, and Vibrio, but a few luminous species are also found in Shewanella, a bacterial genus of primarily marine bacteria, and in Photorhabdus, a bacterial genus of terrestrial bacteria. Vibrio, Aliivibrio, and Photobacterium species are marine bacteria; and some species are pathogenic while others are bioluminescent, i.e., they have the ability to emit visible light as aforesaid.  

Most bioluminescent bacteria inhabit the marine environment and can be isolated from seawater, marine sediments, and the external surfaces and gut tracts of marine animals. Some species also colonize specialized organs of certain marine fishes and squids, called light organs, producing light that the animal uses for signaling, avoiding predators, and attracting prey. When livingsymbiotically in light organs of fish and squids, or saprophytically, for example, on the skin of a dead fish or parasitically in the body of a crustacean, luminous bacteria (i.e., bacteria with the capacity to produce visible light) can be recognized by the light they produce.

Because some pathogenic strains of Vibrioare also luminous, such as certain strains of Vibrio cholerae and V. vulnificus, care should always be taking when handling luminous bacteria. Although Photobacterium, Aliivibrio, and Vibrio isolates are facultative aerobes, they are bioluminescent only when O2 is present. Luminescence in bacteria is catalyzed by bacterial luciferase, which used O2, a long-chain aliphatic aldehydes (for example, tetradecanal), and reduced flavin mononucleotide (FMNH2) as substrates. The primary electron donor is NADH, and the bacterial luciferase reaction is as follows:

FMNH2 + O2 + RCHO – FMN + RCOOH +H2O + Light

The light-generating system constitutes a bypass route for shunting electrons from FMNH2to O2, without involving other electrons carriers such as quinones and cytochromes. The evolutionary origins of bacterial bioluminescence are obscure, but one possibility is that a primitive luciferase arose in a lineage of bacteria that give rise to modern day species of vibrionaceae as a means of detoxifying oxygen as Earth’s atmosphere became progressively more oxic. However, many species and strains of Vibrio, Aliivibrio, and Photobacterium lack luxCDABE, the genes necessary for producing luciferase and the enzymes for synthesis of the long-chain aldehydes. Apparently, these genes have been lost from many Vibrio, Aliivibrio, and Photobacterium lineages over evolutionary time.

The regulation of bioluminescence in bacteria

The expression of luminescence in many luminous bacteria is induced at high population density. The enzyme luciferase and other proteins of the bacterial luminescence system exhibit a population density-responsive induction, called autoinduction, in which transcription of the luxCDABE gene is controlled in Aliivibrio fischeri and related species by a regulatory protein, LuxR, and an inducer molecule, acylhomoserine lactone. During growth, cells produce autoinducer, which can rapidly cross the cytoplasmic membrane in either direction, diffusing in and out of cells. Under conditions in which a high local population density of cells is attained, as in a test tube, colony on a plate, or in the light organ of a fish or squid, autoinducer can accumulate.

Luminous bacteria growing in test tube

When it reaches a threshold concentration in the cell (equal to the concentration in the cell’s local environment), the autoinducer interacts with LuxR, forming a complex that activates transcription of luxCDABE, and cells become strongly luminous. In contrast, when cells of luminous bacteria are at lower population densities, such as in seawater where the autoinducer can diffuse away, luminescence is not induced. The primary autoinducer in Aliivibrio fischeri has been identified as N-3-oxo-hexanoylhomoserine lactone; other luminous species produce other kinds of compounds, resulting in highly specific autoinduction mechanisms. This gene regulatory mechanism is also called quorum sensing because of the population density-dependent nature of the phenomenon.

In saprophytic, parasitic, and symbiotic habitats, a rationale for population density-responsive induction of luminescence may be that luminescence develops only when sufficiently high population densities are reached to allow the light produced to be visible to animals. The bacterial light can then attract animals to feed on the luminous material, thereby bringing the bacteria into the animals nutrient-rich gut tract for further growth, or serve as the source of light in symbiotic, light organ associations. The genetics of quorum sensing using bioluminescence as a model experimental system has been actively explored; this form of regulation has also been found to be a general feature of many different nonluminous bacteria, including several animal and plant pathogens. Quorum sensing in these bacteria controls activities such as the production of extracellular enzymes and expression of virulence factors for which a high population density is thought to be beneficial for the bacteria.

Further reading

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

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