Bioluminescence

Adapted from Microbiology: Diversity, Disease, and the Environment by A. A. Salyers and D. D. Whitt, page 479-480 and a minireview entitled An Exclusive Contract: Specificity in the Vibrio fischeri-Euprymna scolopes partnership by Karen L. Visick and Margaret J. McFall-Ngai, Journal of Bacteriology, Vol. 182, No. 7., page 1779.

Members of several bacterial genera, including Vibrio and Photobacterium, have the unusual ability to emit blue-green light, a phenomenon referred to as bioluminescence.  Luminescent bacteria often live within special organs of marine animals such as some squids and fish, in a symbiotic relationship by which both organisms benefit.  A fascinating example of this symbiotic relationship is described below.  

Euprymna scolopes is a small (~1 inch) squid that lives in the shallow warm ocean waters off the coast of Hawaii.  E. scolopes is a night feeder and swims above the bottom of the shallow shore waters in search of food.  To predators on the bottom, which might want to feast on a succulent E. scolopes, the squid should be a clear target because of the shadow it casts against the twilight or starlit sky.  However, the squid uses a novel strategy to hide itself from predators. 

The squid has a special organ, called a light organ, which acts as a kind of low-wattage flashlight.  The light organ is found in the center of the squid’s body cavity and contains small spaces called crypts that are flanked with reflector, lens and ink sac tissues.  These tissues direct the emitted light out of the light organ and in a downward direction.  In this way, the squid produces a low level of light, similar to that from the moonlit or starlit sky, so that it disappears against this dimly lit background.  The source of the light is the light-producing (luminescent) bacterium Vibrio fischeri that colonizes the crypts.  Luminescent microbes are widespread in nature and use the same strategy for luminescence seen in fireflies, glowworms, and some jellyfish. 

The biochemistry of bacterial luminescence has been fairly well worked out.  The components required in the reaction are FMN (flavin mononucleotide), oxygen, a reduced fatty acid, and an electron donor, NADH.  The reaction is catalyzed by an enzyme complex, luciferase.  (Luciferase derives its name from Lucifer, which means "morning star").  The luciferase reaction responsible for bioluminescence is shown below. 

Molecular oxygen is consumed in the reaction, reminiscent of part of an electron transport system in aerobic respiration, except that instead of serving as the final electron acceptor, oxygen interacts with the enzyme luciferase and FMNH2 to generate light.  The importance of oxygen in the luminescence reaction is visibly demonstrated by using a culture of V. fischeri, which has been grown in broth culture without shaking and has thus depleted the medium of oxygen.  This culture is turbid, but in the dark it exhibits no luminescence.  If the culture is stirred very quickly, allowing the air to pass through it, the culture suddenly starts to glow.

Just a few last comments:

The interaction between the squid and the bacteria is more complex than this description of luminescence in the laboratory makes it sound.  Amazingly, the light organs of juvenile squids begin to glow only hours after hatching.  Perhaps even more stunning is the fact that the light organ contains a pure culture of V. fischeri. This is very uncommon in nature, especially for an animal exposed to the thousands of bacterial species found in seawater.  Although the details of this mutualistic relationship are still being researched, the following provides some explanation of how the relationship is established and the mechanisms of specificity.

As E. scolopes swims, seawater containing the potential symbiont V. fischeri, passes in and out of its body cavity.  Within the body cavity, the squid’s light organ has two ciliated epithelial fields that surround three pores leading to the interior of the organ.  These fields appear to promote colonization of the light organ, perhaps simply by trapping the bacteria in the correct vicinity for entrance.  Once the bacteria have entered one of the pores, they must overcome both physical and chemical barriers.  In addition, the correct receptor-ligand interactions must take place enabling an intimate association of the bacterial cells with the crypt epithelial cells.  The first barrier after entering the pore is a physical barrier.  Bacteria must swim upstream through a mucus-filled duct.  V. fischeri cells are able to navigate their way through the duct, and once in the crypts, the bacteria encounter macrophage-like defense cells.  These cells are likely responsible for removal of bacteria other than V. fischeri.  It is thought that at this point, a very specific binding between V. fischeri adhesins and squid glycans occurs allowing for the symbiotic assocation to become more intimate.  Finally, the squid secretes an active halide peroxidase, similar to that found in human phagocytic cells, that kills most bacteria except for V. fischeri.  Thus, incoming bacteria must master a series of hurdles that seems tailor-made to select for V. fischeri.  It is important to note that it is not only the squid that is actively involved in the specific selection process; the bacterial cells themselves appear to play a role.  For example, V. fischeri cells that are non-flagellated are unable to colonize the light organ.  Recent findings also show that it is the bacteria that induce the host (through initial interaction and the use of chemical messengers) to secrete mucus onto the ciliated fields, assisting with the initial “trapping” of the bacterial cells.

Once in the light organ, V. fischeri benefits by receiving amino acids and possibly other organic carbon and energy sources from the squid.  The bacterial cells grow and divide rapidly until the cell density reaches approximately 1011 cells/mL.  It is only at this concentration and with the presence of molecular oxygen (presumably provided by the squid) that luminescence occurs (see quorum sensing in lecture notes).  

For most bacteria, the relationship with one squid is transient, because E. scolopes expels 90% of the crypt contents every morning.  The remaining bacteria divide and repopulate the light organ throughout the course of the day.  Ejection from the light organ is presumably not a problem for V. fischeri as it can live freely in the environment until it colonizes another new hatchling.

Rachel Watson, M.S.
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Cell: 307-760-2942
rwatson@uwyo.edu