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Bt Research 2012, Vol.3, No.1, 1-2
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Research Report Open Access
Ecological Mysteries: is Bacillus thuringiensis a Real Insect Pathogen?
Guillem Marco1 , Manuel Porcar1,2
1. Institut Cavanilles de Biodiversitat i Biologia Evolutiva, Universitat de València. Apartado Postal 22085, 46071 Valencia, Spain
2. Fundació General de la Universitat de València, Spain
Corresponding author email: manuel.porcar@uv.es; Authors
Bt Research, 2012, Vol.3, No.1 doi: 10.5376/bt.2012.03.0001
Received: 17 Jan., 2012
Accepted: 26 Jan., 2012
Published: 29 Jan., 2012
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.
Preferred citation for this article:
Marco et al., 2012, Ecological Mysteries: is Bacillus thuringiensis a Real Insect Pathogen? Bt Research, Vol.3, No.1 1-2 (doi: 10.5376/bt.2012.03.0001)
Abstract Bacillus thuringiensis (Bt) can kill insects and multiply in their bodies, but it can also grow in semi-synthetic media; is
found in environments were insects are absent; and has been reported to require midgut-associated bacteria for toxicity. We propose
here a novel life cycle for Bt combining insect-based and insect-independent life cycles.
Keywords Bacillus thuringiensis (Bt); Insecticidal crystal proteins (Cry); Microbial ecology; Insect pathogenicity
Background
The bacterium Bacillus thuringiensis (Bt ) has been
successfully used as a biopesticide for decades and,
from 1996 onwards, also as a source of genes for the
construction of transgenic plants (Bt crops) protected
against insect pests. However, because of a lack of a
consistent linkage between insects and Bt , the real life
cycle of this well-known bacterium is still unclear. It is
obvious that Bt can kill insects and multiply in their
bodies, but Bt can also grow very well on simple
semi-synthetic media, including LB. Moreover, Bt
strains have been recovered from environments were
insects are scarce or absent; found to be nonpathogenic
towards the species they were originally isolated from
and yet display activity against insects or other invertebrate
species they will certainly rarely encounter (Martin
and Travers, 1989); and, last but not least, reported to
require midgut-associated bacteria to display insecticidal
activity in some cases (Broderick et al., 2006) but not
in others (Johnston and Crickmore, 2009). We believe
that the answer to the question the title of this letter
raises lies on the analysis of the reports on the ability of
Bt to survive and eventually multiply, in the presence or
absence of insects.
Bt usually displays a dual biological activity involving
toxicity and pathogenicity. The parasporal insecticidal
crystal (Cry) proteins synthesized during sporulation
are specifically toxic against a wide range of insects
(see: http://www.glfc.cfs.nrcan.gc.ca/bacillus). The mode
of action of Cry proteins, which involves solubilisation
in the insect midgut, activation by digestive enzymes
and binding to specific receptors located on the host
cell surface, evidences a strong selection pressure
towards insecticidal activity. Moreover, spores are known
to synergize this activity, since they can enter the insect
body and produce septicaemia. The importance of each
of these effects (toxicity and pathogenicity), shows a
large variation depending on the target species (Suzuki
et al., 2004). The toxicity, rather than the pathogenicity,
of Bt has been exploited for six decades through
Bt -based biopesticides that can be sprayed on the field
and have a direct impact on insect populations without
significant bacterial in situ multiplication. But epizootics
(epidemics) in insects caused by Bt are known to occur
on the field, and the most powerful mosquitocidal
strain, Bt var israelensis (De Barjac, 1978), was in fact
isolated from a stagnant pond showing mass mosquito
mortality.
Beyond insects, an alternative insect-free cycle has
been proposed. Indeed, Bt strains have been reported
to multiply on the surface of the leaves and, more
interestingly, even to colonize plantlets from the soil
(Bizzarri and Bishop, 2008). A comparison between
crystal- and non-crystal-producing strains revealed that
both exhibited similar ability in terms of leaf colonization,
indicating that insecticidal parasporal crystals was not