Lytic Exoenzymes of Soil Strains of Bacillus Representatives and Manifestations of their Biological Activity

Abstract

Bacteria of the genus Bacillus make up a significant (8–12%) part of the soil microbiome. Manifestation of their biological activity, in particular, the antagonistic and lytic activity against other microorganisms directly depends on their exometabolites. According to the literature, such properties of soil bacteria of the genus Bacillus thus can be related to their various lytic exoenzymes. Aim. To evaluate the role of lytic exoenzymes of the studied soil bacteria strains of the genus Bacillus in the manifestation of their biological (antagonistic, lysing) activity. Methods. The antagonistic activity of bacteria strains of the genus Bacillus against phytopathogenic micromycetes was determined by the method of double culture in Petri dishes on potato-glucose agar. For qualitative analysis of the presence of extracellular enzymes, strains of bacteria of the genus Bacillus were plated on Petri dishes with solid mineral-salt medium and a suitable substrate inducer. The ratio of the diameter of substrate hydrolysis zone to the diameter of the colony was taken as the relative enzymatic activity of the culture. Bacteriolytic activity of the studied strains was determined by the change in optical density of living cells of phytopathogenic bacteria suspension at 540 nm. Results. Six strains of bacteria of the genus Bacillus were selected by the results of preliminary screening, with at least five types of lytic activity, namely proteolytic, chitinase, amylolytic, cellulase, and xylanase of different levels (low, average, high). Analysis of the antagonistic activity of the selected strains of bacteria of the genus Bacillus to the main groups of phytopathogenic bacteria (six test cultures) singled out the strain Bacillus sp. 41 for a careful study of the nature and spectrum of its antagonism. Analysis of the level of antagonistic activity of the selected Bacillus strains against the phytopathogenic micromycetes showed that the minimum decrease of antagonism (the decrease of growth inhibition zones) during the observation period (at the 3rd and 7th days) was in Bacillus sp. 41 strain. Therefore, only this strain showed a stable and relatively wide range of antagonistic activity against phytopathogens of bacterial and fungal etiology. The nature of this antagonism is probably complex and conditioned by the participation of various biochemical mechanisms, in particular, the synthesis of a complex of lytic exoenzymes. To assess the lysing activity of Bacillus strains, three strains with the highest proteolytic and cellulolytic activity of exoenzymes were taken from the six previously chosen. Only Bacillus sp.1913 strain showed high (70%) lytic activity against gram-negative polyphagous phytopathogen Pseudomonas syringae pv. syringae UCM B-1027T. Such activity of the strain did not manifest against the rest of the phytopathogenic test cultures. The high lytic activity of Bacillus sp. 1913 strain may be associated with high activity of exogenous proteases and cellulases of the lytic complex, which is quite consistent with the literature data on the lytic activity of bacteria of the genus Bacillus. Conclusions. The spectrum and activity of lytic exoenzymes of strains of the studied soil bacteria of the genus Bacillus indicate the indirect participation of these enzymes in the manifestation of biological activity (antagonistic and lytic).