Phoretic Bacteria (Nematode symbionts)
Xenorhabdus and Photorhabdus spp.
Summary: Two genera of bacteria serve as symbionts in entomopathogenic nematodes (EPN), one (Xenorhabdus) in the EPN Family Steinernematidae, and the other (Photorhabdus) in the EPN Family Heterorhabditidae. Both genera are, in general, harmless to humans and other mammals, making them and the EPN that carry them, excellent agents for biocontrol of insects. Scroll down to read full text of of article. Home...
Bacteria of the genera Xenorhabdus and Photorhabdus live in a phoretic relationship with certain nematodes and as pathogens in the insects the nematodes invade. The definition of the bacteria-nematode relationship is both phoretic (with respect to the bacteria, which use the nematode as its vehicle to gain access to an insect's blood supply, or hemocoel) and symbiotic (with respect to both the nematode and the bacteria, each of whom benefits from their close relationship). Nematodes need the bacteria to survive, as they feed on the bacterial mass that the infection produces. The insects invaded by the nematodes soon die from infections caused by the bacteria the nematodes bring with them.
Species of these bacteria are rod-shaped, facultative, anaerobic, Gram-negative members of the family Enterobacteriaceae. Members of the Enterobacteriaceae that serve as symbionts in entomopathogenic nematodes (EPN) are normally harmless to humans and their pets. Medical records from the American Civil War and World War I even attest that when a certain species (now known as Photorhabdus luminescens) contaminated human wounds, the wounds--which glowed faintly in the dark--often healed faster than normal, presumably because the bacteria secreted broad-spectrum antibiotics that inhibited the development of harmful microbes.
Though similar to other Enterobacteriaceae, species of Xenorhabdus and Photorhabdus tend to be bigger and, unlike most other members of this family, do not reduce nitrates to nitrites. Some emit light during portions of their life cycles, a feature that makes them unique in a number of ways. Only three genera of bacteria are known to luminesce. Two are found in sea water and marine organisms, but the terrestrial bacteria with this property are all members of the genus Photorhabdus (recently reclassified from a single species, Xenorhabdus luminescens to this new genus, originally believed to contain a single species but now recognized as comprised of several.)
Microbiologists studying the DNA of various strains from these genera
are beginning to find evidence that they have been around for a long
time. For example,
Patrick Tailliez, et al., after analyzing 76 different
strains from 27 species of Steinernema nematodes collected in 32
countries, found strong evidence that the common ancestor of the genus
emerged between 250-500 million years ago.
The specificity of the bacteria/nematode association appears to be an important safety feature regarding all EPN. As a result of this specificity, the risk that any nematode species whose symbiotic bacteria does not, today, produce morbidity in humans, will become a human pathogen in the future, appears to be comfortably remote.
The safety of EPN with respect to mammals and non-target organisms is a well-documented fact. Ralf-Udo Ehlers, an authority on bio-control, describes in a paper written in 2003 the history of the use of these biological control agents:
"Since the first use of the entomopathogenic nematode Steinernema glaseri against the white grub Popilla japonica in New Jersey (USA) (Glaser and Farrell, 1935), not even inferior damages or hazards caused by the use of EPN to the environment have been recorded. The use of EPN is safe for the user. EPN and their associated bacteria cause no detrimental effect to mammals or plants (Poinar et al., 1982; Boemare et al., 1996; Bathon, 1996; Akhurst and Smith, 2002)."
None of the EPN presently in use as agents for the control of insects, or their associated phoretic bacteria, are harmful to mammals. Indeed, as mentioned elsewhere, they must be coaxed, cajoled, and pampered to make them work. They comprise, after all, a package engineered by mother nature through millions of years of natural selection to kill only enough of a narrow range of organisms to survive. Man can use them to nullify termite colonies only by repeatedly inoculating specially-designed food sources where the members of a termite colony specifically targeted for elimination have been enticed to feed.
When used in that manner, they are efficient servants, doing man's bidding. They do their work without exposing the human applicator, or those who live in or at the site where they are used, to anything harmful, including such things as toxic, chemical termiticides.
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