Discussion
According to several studies, strains of B. thuringiensis tend to
be associated with the feces of animals living in national parks or kept
in zoos (Swiecicka, et al., 2002; Noda et al., 2009; Djenane et al.,
2017). In particular, this organism was quite frequently revealed in the
feces of animals that are herbivores. Besides, daily plant-based food
intake implies consumption of B. thuringiensis in high
concentrations. The authors analyzed the pathogenic activity and
distribution of antigenic H serotypes in B. thuringiensisbacteria, samples of which were obtained from feces.
The data showed that fecal populations of B. thuringiensis in
terms of serology were diverse and represented by at least 9 H-serotypes
with an undefined serogroup/H serogroup. A typical feature of results
obtained in this study is that serotype H3abc was identified
predominantly in fecal samples belonging to 12 herbivore animal species.
In particular, the presence of this serotype was extremely high in the
feces of chimpanzees, gorillas, tapirs, two kinds of bears (white and
black), as well as rabbits. Hence, all 39 isolates of chimpanzees were
serologically assigned to this serotype. Similarly, 25 isolates from
polar bears were identified as H3abc. Noteworthy is that 2-3 serotypes
could sometimes be detected even in a single fecal sample. Thus, a fecal
sample isolated from a rabbit contained 3 serotypes (one H3abc serotype,
one H6, and one non-typeable).
Researchers have previously reported that B. thuringiensisbacteria can be found on the phyllosphere of different plants
(Swiecicka, 2008; Monnerat et al., 2009; Dubey et al., 2017). Serotype
H3abc has also been identified as a typical natural flora member of H
serotype on phylloplane (Jeong et al., 2017). Hence, the obtained H3abc
isolates may originate from natural populations of phylloplane. More
likely, however, that the source of some isolates under study is
insecticides extracted from microbes. These insecticides can be applied
to various plant crops (vegetables, agricultural crops). Thus,
insecticides based on H3abc serotype of B. thuringiensis are
popular in controlling agricultural insect pests (Mendoza-Almanza et
al., 2020). Most B. thuringiensis toxins identify their specific
target through bounding of specific cell membrane receptors. Cry
proteins are the best-known toxins representing B. thuringiensis,with numerous related studies having been published. Cry is cytotoxic to
insect larvae, affecting important crops by recognizing certain types of
plant membranes using specific receptors such as cadherin,
aminopeptidase-N, and alkaline phosphatase. These toxins mainly affect
mosquitoes that are vectors of human diseases such as Anophelesspp (malaria), Aedes spp (dengue, Zika, and chikungunya) andCulex spp (Nile fever and Rift Valley fever), respectively
(Mendoza-Almanza et al., 2020).
Previous research reported that B. thuringiensis strains without
a pronounced insecticidal activity outperformed insecticidal isolates in
natural environments in several countries (Lone et al., 2017). Other
laboratory study reports have concluded that B. thuringiensisisolates with non-insecticidal Cry proteins overperform insecticides in
natural ecological niches, comprising over 90% of natural populations
in soils and phylloplane (Aboul-Soud et al., 2019). These data contrast
with the present findings that insect pathogenic activity was detected
in 70.1% of fecal samples. Another important result is that the
majority of pathogenic isolates belonged to serotype H3abc.
Global efforts are currently focused on discovering local B.
thuringiensis isolates with unique anticancer properties. Thus,
parasporins are a group of non-insecticidal crystalline proteins with
potential and specific antitumor activity in vitro (Aboul-Soud et
al., 2019). However, despite the significant therapeutic potential of
PS-producing B. thuringiensis strains, knowledge on the effects
of these proteins remains limited. Thuringiensis has been found
to have unique biological activities. Among them are cytotoxicity
specific to certain human cancer cells (Mizuki et al., 2000; Aboul-Soud
et al., 2019; Mendoza-Almanza et al., 2020), lectin activity against
mammalian red blood cells (Torres-Quintero et al., 2018; Onofre et al.,
2020), and activity against trichomonads (Lee et al., 2017). Future
research will include tests of parasporal proteins, e.g., animal feces,
which may affect indicators of biological activity unrelated to
pathogenicity.