Like the Greeks inside the Trojan Horse, certain bacteria live inside the gut of nematodes. When the nematodes invade caterpillars and other insect larvae in search for food, the Photorhabdus luminescens bacteria are released into the insect's blood system. Once inside, the bacteria start producing toxins that kill the insect within 24 hours.

Larvae of the tobacco hornworm Manduca sexta. The caterpillar on the left is infected with Photorhabdus bacteria. The makes caterpillars floppy gene causes loss of body turgor; the insect becomes floppy and dies within 24 hours.



These toxins have been used as pesticides against many insects and are a bacterial alternative to chemicals. Now, British researchers have identified a single large Photorhabdus gene that causes the insect to die. The gene is called makes caterpillars floppy (mcf), alluding to the fact that within 12 hours after the bacteria's invasion a caterpillar becomes floppy and loses its body vigor before it eventually dies. The team led by Richard ffrench-Constant, an insect toxicologist at the University of Bath, U.K. discovered the toxin gene while studying the unusual lifecycle of P. luminescens. The bacteria go through both symbiotic and pathogenic stages. They are symbiotic with the nematode, but as soon as the nematode invades an insect, they turn into killers and help the nematode overcome the insect's immune system. After the insect's death, the bacteria and nematodes reproduce quickly, feeding on the tissue of the cadaver before emerging and seeking new victims. In the study, the researchers used the tobacco hornworm Manduca sexta as an insect host because of its large size and well-studied immune system. The Photorhabdus genome may contain a large number of novel genes associated with pathogenicity, symbiosis and the switch between these two states, according to the study reported in Proceedings of the National Academy of Sciences. Soon, scientists will have the bacterium's genome, which is to be completely sequenced by the end of 2002. The sequencing is done by researchers at the French Institut Pasteur's Laboratory of Genomics of Microbial Pathogens (GMP) in Paris. "We expect the complete sequence to show homologs of mcf and multiple copies of toxin complex genesorally active toxins that are being engineered into insect-resistant crops," says ffrench-Constant, whose team is not involved in the sequencing at GMP. "Toxins, like Mcf, which act on both the gut and the insect immune system represent a promising, yet underexploited, avenue for future insecticide development," the researchers write in the paper. One potential application lies in transferring the toxin-producing genes from the bacteria to crop plants. Next, says ffrench-Constant, his lab plans to prove that mcf affects apoptosis, or programmed cell death, and at what point in the pathway. Scientists have compared Photorhabdus luminescens to another insect-killing bacterium, Bacillus thuringiensis or Bt, which also produces toxins. Bt has been widely used in biological pest control, and some insects have begun to show resistance to its toxin. Having another bacterium available will help in the fight against insect pests. . . .

Daborn, P.J. et al. A single Photorhabdus gene, makes caterpillars floppy (mcf), allows Escherichia coli to persist within and kill insects. Proc Natl Acad Sci USA 99, 10742-10747 (August 6, 2002).

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