''Photorhabdus'' is a genus of bioluminescent,

gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

bacilli Bacilli is a taxonomic class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as ''Bacillus anthracis'' (the cause of anthrax). ''Bacilli'' are almost exclusively gram-positive ...

which lives symbiotically within

entomopathogenic nematode Entomopathogenic nematodes (EPN) are a group of nematodes (thread worms), that cause death to insects. The term ''entomopathogenic'' has a Greek origin, with ''entomon'', meaning ''insect'', and ''pathogenic'', which means ''causing disease''. Th ...

s, 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 A Biopesticide is a biological substance or organism that damages, kills, or repels organisms seens as pests. Biological pest management intervention involves predatory, parasitic, or chemical relationships. They are obtained from organisms inclu ...

in agriculture.

Life cycle

''Photorhabdus'' species facilitate the reproduction of

s by infecting and killing susceptible insect larvae. Entomopathogenic nematodes are normally found in soil. Nematodes infect larval hosts by piercing the larval cuticle. When the

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...

enters an insect larvae, ''Photorhabdus'' species are released by the nematodes and will produce a range of toxins, killing the host within 48 hours. ''Photorhabdus'' species feed on the cadaver of the insect and the process converts the cadaver into a nutrient source for the nematode. Mature nematodes leave the depleted body of the insect and search for new hosts to infect. During stationary phase growth inside insect larvae, ''Photorhabdus'' species synthesize a molecule called

3,5-Dihydroxy-4-isopropyl-trans-stilbene Tapinarof, also known as benvitimod and sold under the brand name Vtama, is a medication used for the treatment of plaque psoriasis. The medication is applied to the skin. Besides its use in medicine, tapinarof is a naturally occurring compound ...

(ST). It is proposed that ST acts as an antibiotic and protects ''Photorhabdus'' species from competition from other microorganisms, and also helps circumvent the insect's immune system. Benvitimod

''Photorhabdus'' species are essential endosymbionts for ''

Heterorhabditis ''Heterorhabditis'' is a genus of nematodes belonging to the order Rhabditida. All species of this genus are obligate parasites of insects, and some are used as biological control agents for the control of pest insects. ''Heterorhabditis'' nemato ...

'' nematodes.

Genome sequence

The complete genome of ''

Photorhabdus luminescens ''Photorhabdus luminescens'' (previously called ''Xenorhabdus luminescens'') is a Gammaproteobacterium of the family Morganellaceae, and is a lethal pathogen of insects. It lives in the gut of an entomopathogenic nematode of the family Heteror ...

'' was 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 ''Manduca sexta'' is a moth of the family Sphingidae present through much of the Americas. The species was Species description, first described by Carl Linnaeus in his 1763 ''Centuria Insectorum''. Commonly known as the Carolina sphinx moth and ...

'', the tobacco hornworm, gene ''mcf'' that causes

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

in insect

hemocytes A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), a ...

and midgut epithelium, and genes that intervene in the development of insect host. Another important sequence identified is the gene encoding

polyketide Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, discovery, biosynth ...

and

nonribosomal peptide Nonribosomal peptides (NRP) are a class of peptide secondary metabolites, usually produced by microorganisms like bacteria and fungi. Nonribosomal peptides are also found in higher organisms, such as nudibranchs, but are thought to be made by bacter ...

synthases which produce antibiotics to protect against microbial competitors. It is proposed that ''Photorhabdus'' species acquired the toxin genes by

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...

during evolution.

In agriculture

The efficiency of insect-killing nature of ''Photorhabdus'' species and its potential use as

have been studied. Use of ''Photorhabdus'' species alone as biopesticide, independent of its nematode symbiont, against the cabbage white butterfly, ''

Pieris brassicae ''Pieris brassicae'', the large white, also called cabbage butterfly, cabbage white, cabbage moth (erroneously), or in India the large cabbage white, is a butterfly in the family Pieridae. It is a close relative of the small white, ''Pieris ra ...

'', mango mealy bug, ''Drosicha mangiferae'' and the pupae of the diamond back moth, ''

Plutella xylostella The diamondback moth (''Plutella xylostella''), sometimes called the cabbage moth, is a moth species of the family Plutellidae and genus '' Plutella''. The small, grayish-brown moth sometimes has a cream-colored band that forms a diamond along ...

'' has been demonstrated successful. It also has the pathogenic potential to kill the Asian corn borer, a pest of

maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...

in east Asia, in 48 hours.

As disease-causing agent

Three species of ''Photohabdus'' have been found, which are ''

'', ''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 the US state of Texas and the Gold Coast of Australia. The first case of human infection was reported by the

Centers for Disease Control The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...

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.

References

External links

{{Taxonbar, from=Q7187896 Bacteria genera