Bt Research 2015, Vol.6, No.4, 1-12
2
bacterium directly into the genotype of the plant
(Pinto-Zevallos e Zarbin, 2013). In the early 80s,
Schnepf and Whiteley (1981) conducted the first
cloning and sequencing of the first genes encoding
these proteins, obtained through an isolated from
Bt
subsp.
kurstaki
strain and expressed it initially in
tobacco and tomato plants, which were the first
transgenic-
Bt
plants to be marketed (Ali et al., 2010;
Rodríguez et al., 2012). Thus, it was possible the
introduction of
Bt
genes in other plants to introduce
resistance to insect pests, such as corn and cotton
cultivars which are the two major
Bt
crops grown
worldwide (Ali et al., 2010).
The main toxins produced by
Bt
presenting entomopa-
thogenic activity are the δ-endotoxins (Cry and Cyt)
and parasporins, which are intracellular proteins; besides
the α-exotoxins; thuringiensins (β-exotoxins); toxins
VIPs (vegetative insecticidal proteins); S-layer proteins
(SLP) and exoenzymes (lipases, proteases, chitinases
and phospholipase C), which are extracellular macro-
molecules enabling virulence (Arora et al. 2013;
Bravo et al. 2011; Vu et al. 2012). As biolarvicides,
Bt
proved to be innocuous in tests that evaluated the
potential toxicity of the most of their toxins in
mammalian cells, and organisms non-target (Thomas
and Ellar, 1983). All of these toxins have been studied
for its potential use in biological control practices.
However, not all can be used in pest control, such as
the thuringiensin, which has a broad spectrum of
biological toxicity to a variety of non-target species,
including mammals. Therefore, these present review
discuss current knowledge about characteristics, types,
genetic determinants, biosynthesis, mode of action,
insecticide spectrum, safety assessment and procedures
to identification of thuringiensins in
Bt
strains.
1 Features of Thuringiensins
Thuringiensins are secondary metabolites, nonproteic
and soluble in water. It is also a heat-stable exotoxin
that maintains their bioactivity to 121
℃
for 15 min
(Farkas et al., 1969). It is known that are more stable
at pH 7.0, but their stability decreases with increasing
temperature (Zhou et al., 2013). As in other extracellular
insecticides toxins, their production occurs during the
vegetative growth phase of some strains of
Bt
, when
are secreted in the culture medium where the bacteria
is inoculated (Liu et al., 2010; Obeidat et al., 2012).
However, according to (Argôlo-Filho et al., 2014), the
secretion of this secondary metabolite varies temporally,
thus the knowledge of the temporal pattern of secretion
or activity in the culture medium is necessary to avoid
the lack of identification of this undesirable exotoxin. The
different toxicity scale of thuringiensin, highly dependents
of the time of cultivation of the producing strain, may
also be explained by different volumes of culture, and
the variable conditions of the culture medium (aeration
level; pH value) (Argôlo-Filho et al., 2014). (Jing-Wen et
al., 2007) reported that the pH and the concentration
of glucose had an important effect on the synthesis and
efficiency of thuringiensin. Physiological differences
between strains can also result in a variation of the
secretion of toxins, even when it is inoculated the
same quantities of cells, generating toxicity profiles
temporally distinct for each strain, and, consequently,
a potential wrong classification of producer isolates
(Argôlo-Filho et al., 2014).
The exotoxin thuringiensin was discovered by
McConnell and Richards (1959), which describe it as
a substance thermostable toxic to insects. Heimpel
(1967) proposed the name β-exotoxin to designate it,
but over time, this term was considered inappropriate,
due to the structure of this toxin. Instead, several
authors suggested the thuringiensin synonym that is
currently used (Kim and Huang, 1970; Pais and De
Barjac, 1974; Farkas et al., 1977).
The chemical formula of the thuringiensin is
C
22
H
32
N
5
O
19
P (De Rijk et al., 2013). Initially, (Farkas
et al., 1969) reported that the structural formula of
thuringiensin consisted of adenosine, glucose, a
phosphoric acid and a gluconic diacid. Šebesta and
Horska (1970) suggested that exotoxin is composed of
adenine, aleric acid phosphorylated and a sugar
moiety formed by D-ribose and D-glucose linked by
an ether unusual. Currently, it is known that the
thuringiensin has a unique structure and, as a polymer
of monosaccharides, has asymmetric carbon atoms
(Liu et al., 2010) (Figure 1).
Many
Bt
strains belonging to different serotypes
excrete the toxin (Jing-Wen et al., 2007), as well as
some strains of
B. subtilis
, and
B. megaterium
(Pinto
et al., 2010). Some studies reported that strains of
B.
cereus
also are able of producing thuringiensins
(Carlberg 1986; Krieg and Lysenko 1979; McConnell
and Richards 1959; Ohba et al., 1981). Perchat et al.,
