Insect Control: Biological and Synthetic Agents - Index of

9 Insecticidal Toxins from Photorhabdus and
Xenorhabdus
R H ffrench-Constant, N Waterfield, and
P Daborn, University of Bath, Bath, UK
ß 2010, 2005 Elsevier B.V. All Rights Reserved
9.1. Introduction 313
9.1.1. The Biology of Photorhabdus and Xenorhabdus 313
9.1.2. The Need for Alternatives to Bt 314
9.2. The Toxin Complexes 314
9.2.1. Discovery of the Toxin Complexes 314
9.2.2. Homologs in Other Bacteria 317
9.2.3. Molecular Biology of the toxin complex Genes 318
9.3. The Makes Caterpillars Floppy Toxins 319
9.3.1. Discovery of Makes Caterpillars Floppy 319
9.4. Toxin Genomics 321
9.4.1. Microarray Analysis 321
9.5. Conclusions 323
9.1. Introduction
9.1.1. The Biology of Photorhabdus and
Xenorhabdus
9.1.1.1. Bacteria, nematodes, and insects
Photorhabdus and Xenorhabdus bacteria live in
association with nematodes from the families
Heterorhabditidae and Steinernematidae, respectively
(Forst et al., 1997). Both genera bacteria are
members of the Enterobacteriaceae, and are found
as symbionts within the guts of the infective juvenile
nematodes (ffrench-Constant et al., 2003). These
infective juvenile nematodes seek out and invade
insects, whereupon the bacteria are released from
the mouth of the nematode directly into the insect
blood system or hemocoel (Forst et al., 1997). The
bacteria then multiply rapidly within the insect, apparently
unaffected by the insect immune system
(Silva et al., 2002). In the case of Photorhabdus,
the bacteria first colonize the gut and grow rapidly
in the hemocoel only, later colonizing all the other
insect tissues (Silva et al., 2002). The nematodes also
multiply within the insect cadaver, feeding off both
the bacteria and the degraded insect tissues (Forst
et al., 1997). Finally, a new generation of infective
juvenile nematodes reacquires the bacteria and
leaves the insect in search of new hosts.
During the course of this infection process the
insect dies and the bacteria are inferred to secrete a
range of toxins capable of killing the insect or attacking
specific tissues (ffrench-Constant et al., 2003).
For the purposes of this chapter, it is important to
note that during this process the bacteria lie within
the hemocoel of the insect or within the insect tissues.
Therefore, any toxin action on the gut in vivo
is likely to be from the hemocoel side of the gut. The
existence of toxins acting from the lumen side of
the gut is therefore, unexpected from the biology of
the organism (Bowen et al., 1998). In this respect,
this chapter is divided into sections on the orally
active Toxin complexes (Tc’s) and on the injectably
active Makes caterpillars floppy (Mcf) toxins.
9.1.1.2. Bacterial nomenclature As this chapter
involves the description of toxins from a wide range
of Photorhabdus and Xenorhabdus strains, some
description of current bacterial taxonomy is necessary
to clarify subsequent discussion. The genus
Xenorhabdus contains a number of species including
X. nematophilus, X. beddingii, X. bovienii, and
X. poinarii (Forst et al., 1997). The taxonomy of the
genus Photorhabdus, however, is more complicated
and has recently been revised. In this revision
(Fischer-Le Saux et al., 1999), the genus was recently
split into three species, P. luminescens, P. temperata,
andP. asymbiotica. Two of these genera have
also been subdivided into new subspecies. Thus, the
P. luminescens group has three subspecies, luminescens,
akhurstii,andlaumondii, and the P. temperata
group has one subspecies, temperata. The third species
group, P. asymbiotica, which consists of isolates
from human wounds recovered in the apparent absence
of a nematode vector (Gerrard et al., 2003),
has not yet been subdivided. Most of the genetic
analyses have been performed on a limited number
of strains: P. luminescens subsp. akhurstii strain