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.