Photorhabdus luminescens

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Classification

Domain: Bacteria, Phylum: Proteobacteria, Class: Gammaproteobacteria, Order: Enterobacteriales, Family: Enterobacteriaceae, Genus: Photorhabdus, Species: luminescens

Species

Figure 1(A) shows the bioluminescence of Photorhabdus luminescens. It was taken with film 72 hours after the bacteria infected Galleria mellonella (waxworms).(Image courtesy Todd Ciche/California Institute of Technology),Figure 1(B) shows GFP-labeled Photorhabdus luminescens in the intestine of the Heterorhabditis bacteriophora nematode.(Image courtesy Todd Ciche,Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI 48824 USA)

Photorhabdus luminescens

Description and Significance

Photorhabdus luminescens also known as Xenorhabdus luminescens is a bioluminescent microbe (Figure 1). Bioluminescence is an energy costly process and as yet no good explanation is given for this process. Theories include some unknown biochemical role, a warning to scavenging nocturnal mammals or even that it serves as a lure to temp fresh insect victims into range. The most important function of this microbe is the symbiotic relationship with soil entomopathogenic nematodes of the family Heterorhabditidae and pathogenic to a wide range of insects (Figure 2).When the nematode infects an insect, P. luminescens is released into the blood stream and rapidly kills the insect host (within 48 hours) by producing toxins. It also secretes enzymes which break down the body of the infected insect and convert it into nutrients which can be utilized by both nematode and bacteria. In this way, both organisms gain enough nutrients for further replication and reproduction. P. luminescens is the only organism that is known to exhibit dual phenotype (symbiotic within one insect, and pathogenic with another) (Figure 3).During the process P. luminescens produces antibiotics to prevent invasion of the insect by bacterial or fungal competitors and it becomes visibly luminescent due to the bioluminescence of P. luminescens. There have been number of reported cases of human infection by Photorhabdus luminescens.

Figure 2:Photorhabdus luminescens on an insect midgut under the collagen sheath. Photorhabdus luminescens produces toxins that could potentially be used as insecticides. (http://staff.bath.ac.uk/bssnw/photorhabdus_luminescens.htm)

Genome Structure

Figure 4: Circular representation of the Photorhabdus luminescens genome. The outer scale is marked in megabases.(Duchaud et al. 2003, Nature, 21, 1307-1313)

The complete genome sequence of Photorhabdus luminescens, strain TT01 is 5,688,987 base pairs (bp) long and contains 4,839 predicted protein-coding genes (Figure 4). It encodes a large number of adhesins, toxins, hemolysins, proteases and lipases, and contains a wide array of antibiotic synthesizing genes. These proteins play role in the elimination of competitors, host colonization, invasion and bioconversion of the insect cadaver, making P. luminescens a promising model for the study of symbiosis and host-pathogen interactions. Comparison with the genomes of related bacteria reveals the acquisition of virulence factors by extensive horizontal transfer and provides clues about the evolution of an insect pathogen. Pathogenic functions are encoded within a number of pathogenicity islands (PAIs) contained on the bacterial chromsome. These include a large number of genes that code for secreted toxins and enzymes, as well as genes that encode products for the production of antibiotics and bacteriocins. Secretion of these products occurs by an array of systems including type I, type II, and type III secretion systems. The type III system is closely related to the Yersinia plasmid-encoded type III system. Genes that promote symbiotic relationships are also encoded on genomic islands on the chromosome including some that affect nematode development. Now that the complete genome has been sequenced, the next challenge is to identify genes involved in symbiosis that could be used to increase the production of the worms for the biological control of insects. In tests, Photorhabdus luminescens reduced Colorado potato beetles and sweet potato whitefly by 100 percent in lab conditions. The potato beetle is notorious for developing resistance to insecticides, so scientists are seeking non-chemical controls as possible natural insecticides.

Cell Structure, Metabolism and Life Cycle

The life cycle of Photorhabdus luminescens is dependent on its symbiotic relationship with Heterorhabditis nematodes (Figure 3).Photorhabdus luminescens can be thought of as starting in the intestine of these nematodes. Once the nematode finds a suitable host insect the bacteria are released into the insect. While there they reproduce as do the nematodes. Once the infection stage is complete the bacteria find their way back into the intestine of the nematodes and the nematodes with their bacteria counterparts leave the old host in search of a new host to repeat the cycle. Photorhabdus luminescens exists in two distinct phases called phase I and phase II variants. The two variants have several differences in their cell expression. Phase I variants occur in infective-stage nematodes while Phase II variants occur after sustained growth in vitro. The phase I variant of Photorhabdus luminescens produces an extracellular protease, an extracellular lipase, antibiotic substances, and intracellular protein crystals while the phase II variant does not produce these substances. The phase I variant also has the remarkable trait of bioluminescence. The intracellular protein crystals are expressed as a result of the cipA and cipB genes which are present in the genome. The phase II variants do not express the cipA or cipB gene.Photorhabdus luminescens can be classified as an anaerobic organoheterotroph. The chemical processes responsible for metabolism occur in the absence of oxygen and organic carbon is used as the food source.

Figure 3: The representation of complex life cycle of Photorhabdus luminescens. (Williamson et al. 2003, Nature, 21, 1294-1295)

Ecology and Pathogenesis

The Photorhabdus luminescens bacterium is involved in a complex symbiotic relationship with certain entomopathogenic nematodes. But before the symbiosis can be understood the parasitic nature of the nematodes must be stated. These nematodes are parasitic for many types of insects, so much so that they are often used as a form of insect control across the world. Insects infected with these nematodes usually do not survive. But the nematode could never be lethal or proliferate on its own. It owes its continued existence to the symbiotic relationship it has with Photorhabdus luminescens which live in the intestine of these nematodes. Once inside the insect the nematodes systematically release Photorhabdus luminescens cells into the insect. Releasing as few as 10 cells into the insect is usually fatal. The process by which these bacteria avoid the insects natural defense mechanisms are currently unknown. The Photorhabdus luminescens cells inside the insect produce toxins and proteins which damage the host insect. One such toxin is the Mcf(makes caterpillars floppy) toxin. The Photorhabdus luminescens bacteria also are able to produce antibiotics which reduce competition between other bacteria. While inside the insect both the nematode and bacteria reproduce. After the infection is complete nematodes emerge once again living with Photorhabdus luminescens and the process can repeat with new insect hosts.

References

Saux, M. F et al. Polyphasic classification of the genus Photorhabdus and proposal of new taxa: P. luminescens subsp. luminescens subsp. nov., P. luminescens subsp. akhurstii subsp. nov., P. luminescens subsp. laumondii subsp. nov., P. temperata sp. nov., P. temperata subsp. temperata subsp. nov. and P. asymbiotica sp. nov, International Journal of Systematic Bacteriology, 49, 1645-1656 (1999).

Duchaud, E. et al. The genome sequence of the entomopathogenic bacterium Photorhabdus luminescens. Nature Biotechnology 21, 1307-1313 (2003).

Liu, D. et al. Insect resistance conferred by 283-kDa Photorhabdus luminescens protein TcdA in Arabidopsis thaliana. Nature Biotechnology 21, 1222-1228 (2003).

Williamson et al. Sequence of a symbiont. Nature Biotechnology 21, 1294 - 1295 (2003)

Blackburn, M. B. et al. The broadly insecticidal Photorhabdus luminescens toxin complex a (Tca): Activity against the Colorado potato bettle, Leptinotarsa decemlineata, and sweet potato fly Bemisia tabaci, Journal of Insect Science, 1-11 (2005)

http://staff.bath.ac.uk/bssnw/photorhabdus_luminescens.htm

http://www.wormbook.org/chapters/www_genomesHbacteriophora/genomesHbacteriophora.html

http://www.genomenewsnetwork.org/articles/10_03/toxic_glow.shtml

http://www.ebi.ac.uk/2can/genomes/bacteria/Photorhabdus_luminescens.html

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Author

Page authored by Ahsan Munir & Brian Charles Mcmillen ,student of Prof. Jay Lennon at Michigan State University.