Conclusions
After 30 years, the classification of B. thuringiensis strains based on H serotyping remains an effective method for distinguishing strains. This classification is based on stable and distinctive features. Therefore, H serotyping of B. thuringiensis isolates secluded from feces of 20 animal species was performed in laboratory conditions. Of the 190 isolates, 166 represented 14 mammalian species, 20 were obtained from 5 species of reptiles, as well as from 4 species of birds. The obtained bacterial populations were assigned to 8 H serotypes: H31, H6, H3abc, H7, H4ab/43, H5ab/21, H8ab, and H9. The studied populations consisted of 3 atypical, as well as 65 non-verified isolates. Among the 8 serotypes studied, the most common was H3abc (82.0% of cases) out of 122 serotyped samples, less common was H6 (8.5%), the remaining 6 serotypes accounted for no more than 1.5%.
Parasporal inclusions in true fecal isolates can be divided into 4 different groups in terms of morphology. These included bipyramidal, spherical, or irregularly shaped. Of the 190 isolates tested, 125 (65.8%) formed bipyramidal inclusions, and 63 (33.2%) were spherical. All H3abc isolates demonstrated bipyramidal inclusions. The same was true for H8ab and H7 isolates. Fecal isolates of B. thuringiensiscontaining sporulated cultures have been analyzed for oral toxicity against insect larvae to elucidate the properties of the obtained serotypes. Of the 190 isolates tested, 133 (70.1%) showed insecticidal activity, 128 killed B. mori and Aedes sp. Other 3 isolates showed effects only on B. mori , and 2 showed effects only on Aedes sp., proving their biological selectivity against laboratory test objects. All H3abc isolates with bipyramid inclusions were double-toxic to silkworms and mosquitoes. In addition, all H5ab/21 isolates exhibited dual toxicity. Monotoxic B. thuringiensisagainst Aedes sp. were recorded only in cultures of the spherical group. These samples belonged to the H4ab/43 serotype.
Funding. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Data Accessibility. All data generated or analysed during this study are included in this published article.
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Ethics approval . The authors declare that the work is written with due consideration of ethical standards. The study wasconducted in accordance with the ethical principles approved by the Ethics Committee of Federal State Budgetary Educational Establishment of Higher Education “Sechenov First Moscow State Medical University” (Protocol №13 of 13.09.2022).
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Authors’ contributions . Both authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by ES and AA. The first draft of the manuscript was written by ES and AA. All authors read and approved the final manuscript.