Contents - Faperta

Genetic Engineering of Entomopathogenic Microbes for Pest Management 263
media could be developed for producing effective NPVs, this would greatly enhance their
utility in IPM programs. Sequencing of entire genomes of NPVs will throw more light on
the structure-function relationships of baculoviruses, which will facilitate their production
with better safety. The promise of baculoviruses for pest control is yet to be realized,
because of low speed of kill, long incubation period in the host, and failure to reduce insect
damage. The availability of selective and fast-acting viral insecticides would certainly
reduce overdependence on synthetic pesticides.
Entomopathogenic Bacteria
Several entomopathogenic bacteria belonging to Bacilliaceae, Micrococcaceae, Lactobacillaceae,
and Pseudomonaceae have been identifi ed, of which Bacillus sphaericus Meyer
and Neide, B. thuringiensis, B. thuringiensis var. israeliensis, B. thuringiensis var. kurstaki,
B. subtilis, (Ehrenberg) Gohn, Paenibacillus (Bacillus) lentimorbus Dutky, and Paenibacillus
(Bacillus) poppillae Dutky have been studied in considerable detail as biological control
agents (Miller, Lingg, and Bulla, 1983; Charles, Neilsen-Leroux, and Delécluse, 1996).
Bacillus poppillae and Serratia entomophila Grimont et al. have been used for the control of
white grubs, while B. sphearicus has been used for the control of mosquito larvae (Klein
and Jackson, 1992; Klein and Kaya, 1995). Bacillus sphaericus is more persistent in polluted
habitats, recycles under certain conditions, but has a narrow host range (Nicolas, Regis,
and Rios, 1994; Charles, Nielsen-Leroux, and Delécluse, 1996). The bacterium Photorhabdus
luminescens (Thomas and Poinar) Boemare et al., which lives in the gut of entomophagous
nematodes, produces a toxin, which consists of four native complexes encoded by tca,
tcb, tcc, and tcd. Both tca and tcd encode complexes with high oral toxicity to tobacco
hornworm, Manduca sexta L., and therefore are potential alternatives to Bt (Bowen et al.,
1998). Genes from P. luminescens that encode insecticidal toxins have recently been described
from other insect-associated bacteria such as S. entomophila, an insect pathogen, and Yersinia
pestis KIM, the causative agent of bubonic plague, which has a fl ea vector (Waterfi eld
et al., 2001).
Bacillus thuringiensis
Bacillus thuringiensis, which has been exploited extensively for insect control, is an aerobic,
Gram-positive endospore-forming bacterium, and is widespread in natural environments
in the soil. Ishiwata discovered this bacterium in 1901 from diseased silkworm, B. mori
larvae. Berliner (1915) isolated it from diseased larvae of Ephestia kuhniella (Zeller), and
designated it as B. thuringiensis. Its genome size is 2.4 to 5.7 million bp (Carlson, Caugant,
and Kolsto, 1994). Bacillus thuringiensis isolates have several extra chromosomal elements;
some are circular and others linear. Proteins comprising the parasporal crystal are generally
encoded by large plasmids (González, Dulmage, and Carlton, 1981). Sequences hybridizing
to cry gene probes occur commonly among B. thuringiensis chromosomes (Carlson
and Kolsto, 1993), but the degree to which these chromosomal homologs contribute to
production of crystal proteins is not clear. Bacillus thuringiensis harbors a large number
of transposable elements, including insertion sequences and transposons. Transposable
elements are probably involved in amplifi cation of cry genes in the bacterial cell. The cry1A
genes are fl anked by two sets of inverted repeat sequences (Kronstad and Whiteley, 1984),