The year is 1918. You lie in a damp bunker severely wounded by shrapnel from a mortar shell. This is it you think as you slowly slip into blackness. Medics rush to your aid and take you to a field hospital at the edge of the battle field. Your wounds are bandaged and you are observed for the next few weeks. As the nurse is changing your bandages after 3 days she notices something unusual. Your wounds are emitting a faint glow. She calls over the doctor who seems relieved at the sight.
So what's wrong with this situation? Were you subjected to secret government tests involving high doses of gamma radiation? Are you an alien from some distant planet? Are you just some mutant freak? Of course not, you simply have a Xenorhabdus luminescens infection.
Xenorhabdus luminescens was first documented by Poinar durring the mid 1970's though it had been observed for hundreds of years. The genus Xenorhabdus is comprised of gram-negative, rod shaped facultative anaerobes. An unusual characteristic of X. luminescens is that it is the only terrestrial member in the three genera of luminescent bacteria while the others are all marine. It is an insect pathogen found as a symbiont of heterorhabdiid soil nematodes. X. luminescens lives in these nematodes stomachs during their non-feeding infective stage. The nematode acts as a vector for the bacteria by entering an insect through their mouth or spiracles and penetrates the gut wall where they release the bacteria and fertilized eggs into the hemocoel. In the hemolymph, the bacteria grow rapidly and begin to emit light after 20 hours which continues to increase in intensity for 4 hours. While growing, the bacteria produces nutrients required to complete the nematode life cycle by using extracellular proteases and lipases. One of the key features of X. luminescens is that it produces antibiotics that inhibit the growth of other bacteria. This prevents the infected insect from putrefying while the nematodes feed and produce larvae. This is why the doctor in the story above was relieved when he saw the glowing wound. There have been many accounts of X. luminescens infecting wounds, with reports that the wounds healed quickly (due to the antibiotic production).
Scientists are not quite sure of why X. luminescens produces light. Scientists once thought that the glowing would attract other insects to the infected insect, thus allowing the continuation of the nematode's life cycle, but by infective stage there is little light produced. The newest suggestion is that it uses the luciferase pathway as a terminal oxidase allowing it to continue aerobic metabolism when oxygen is low. The general equation used for the production of light is FMNH2 + RCHO + O2 ÿ FMN + H2O + RCOOH + hv (490 nm). The substrate is a NADH reduced riboflavin phosphate (FMNH2), and a long chain fatty aldehyde. In the presence of oxygen, these are oxidized by the enzyme and then react with the aldehyde as a monooxygenase forming an excited intermediate which emits light as it decays.
X. luminescens Is often isolated by enrichment technique where insects are incubated in soil. When the dead insect begins to glow the bacteria is isolated and plated directly on agar. It is cultured on a defined medium due to its fastidiousness. The medium needs several required amino acids, malic acid, proline, and a balance of phosphate to NaCl to grow and produce light. An interesting feature of X. luminescens is that unlike other luminous bacteria, it thrives in high (100%) oxygen. Instead of inhibiting it, both its luminescence and luciferase synthesis are enhanced.
Two areas of current research with X. luminescens involve its use in biological pest control and antibiotic produciton. Nematodes are used to find pests since they remain infective for months, and X. luminescens kills the host so quickly that there is no need for the nematodes to adjust to the hosts life cycle. For antibiotic use it has been shown that it is effective against many gram + an - bacteria as well as yeasts.
References
Mr. Brown's Science Classes. 2 April 2000.
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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