History of Science and Engineering Annotation << Back THE MAIN TRENDS IN SCIENTIFIC INVESTIGATIONS OF LABORATORY OF CYTOLOGY OF MICROORGANISMS. RESULTS OF SCIENTIFIC ACHIEVEMENTS OVER THE PAST FIVE YEARS (2015–2019) V.I. Duda The investigations of laboratory of microbial cytology were focused on exploring mainly two important issues in general theoretical microbiology like biology of ultramicrobacteria (UMB) and the nature of intracellular polysporogenesis of bacteria. The relevance of these studies is associated not only with the high scientific significance of the problems to be solved for fundamental microbiology, but also with new prospects for the use of UMB and polysporogenous prokaryotes in biotechnology. In this study, for the first time we succeeded in isolating 50 different strains of free-living ultrasmall bacteria with cell volumes between 0.02–1.3 μm3 from various extreme natural biotopes (permafrost soil, oil sludge, soil, lake silt, moss of thermal bogs and the skin of an clawed frog Xenopus laevis). Fifteen isolates with cell sizes (volumes) <0,1 μm³ and with the genome size from 1,5 to 2,4 Mb of these 50 strains are affiliated with UMB belonging to different phylogenetic groups (Alphaproteobacteria, Bacteroidetes, Actinobacteria) and genera (Kaistia, Chryseobacterium, Microbacterium, Leucobacter, Leifsonia, and Agrococcus) of the domain Bacteria. They are free-living mesophilic heterotrophic aerobic bacteria. The cytophysiological features of UMB isolated from the skin of the clawed frog Xenopus laevis, which determine the epibiont mode of their existence, are described in detail. FORMULA for ultramicrobacteria is proposed, now it is in the international “Encyclopedia of Life Science” (2020). The nature and factors determining the intracellular polysporogenesis of bacteria have been investigated. The research results make a signifi cant contribution to understanding the nature of prokaryotic cells, providing insight into the microbial diversity and help to develop techniques to use ultramicrobacteria in biotechnology. Keywords: ultramicrobacteria, biodiversity of ultramicrobacteria, intermicrobial parasitism, inhibition of bacterial spore formation, cell ultrastructure, UMB formula, cell polarity, reversal of cell polarity, proliferative and generative cell poles, twin spores. DOI: 10.25791/intstg.06.2020.1190 Pp. 42-50.