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photorhabdus

Photorhabdus(Draft)
Photorhabdus is a genus of bioluminescent, gram-negative bacilli which lives symbiotically within entomopathogenic nematode, hence the name photo (which means light-producing) and rhabdus (rod-shape) . Photorhabdus is known to be pathogenic to a wide range of insects and has been used as biopesticide in agriculture.

Life cycle
Photorhabdus species facilitates the reproduction of entomopathogenic nematode by infecting the larvae of susceptible insects larvae and kill the larvae. entomopathogenic nematodeis normally found in soil which looks for insects larvae. When entomopathogenic nematode enters an insect larvae, Photorhabdus species is released by the nematodes which produces a range of toxins that kills the host insect within 48 hours. Photorhabdus species feeds on the cadaver of the insect and the process converts the cadaver into nutrient source for the nematode. Mature nematodes leave the depleted body of insect and search for new host insects.

During stationary phase growth inside insect larvae, Photorhabdus species synthesize a molecule called 3,5-Dihydroxy-4-isopropyl-trans-stilbene (ST). It is proposed that 3,5-Dihydroxy-4-isopropyl-trans-stilbene (ST) acts as an antibiotic and protect Photorhabdus species from competition from other microorganisms and also help penetrate insect's immune system.

Thus Photorhabdus species has been studied in detail how it is able to act as pathogen in insect host while being a mutualist to nematode host.

Genome sequence
The complete genome of Photorhabdus luminescens has been sequenced in 2003. The DNA sequence of Photorhabdus contains a number of toxin-encoding genes that are essential for killing the insect after infection. This includes genes encoding toxins that kill Manduca sexta, the tobacco hornworm, gene mcf that causes apoptosis in insect hemocytes and midgut epithelium, and genes that intervene in the development of insect host.

Another important sequence identified is the gene encoding polyketide and nonribosomal peptide synthases which produce antibiotics to protect against microbial competitors.

It is proposed that Photorhabdus species acquires the toxin genes by horizontal gene transfer during evolution.

Photorhabdus in agriculture
The efficiency of insect-killing nature of Photorhabdus species and its potential use as biopesticide have been studied. Use of Photorhabdus species alone as biopesticide, independent of its nematode symbiont against the cabbage white butterfly, Pieris brassicae, mango mealy bug, Drosicha mangiferae and the pupae of the diamond back moth, Plutella xylostella has been demonstrated successful.

Photorhabdus as disease causing agent
Three species of Photohabdus have been found, which are Photorhabdus luminescens, Photorhabdus temperata and Photorhabdus asymbiotica. P.asymbiotica has been shown to be infectious to human, but the cases are mostly non-fatal and are restricted to Texas, USA and the Gold Coast of Australia.

The first case of human infection was reported by the Centers for Disease Control in the United states in 1989.

In 1999, a study reported another four cases of Photorhabdus luminescens infection in South Eastern Australia, one in 1994 and three in 1998.