The Effects of Humic Substances and Humic Substance-Based Silver Nanocomposites on the Viability of Rhizospheric Microorganisms

Abstract

The influence of humic substances (HSs) of coals (HS-cl), shale (HS-sl), and mud (HS-md) isolated in Mongolia and silver nanocomposites (NCs) synthesized on their basis on the viability of Rhodococcus erythropolis (gram positive bacteria) and Pseudomonas oryzihabitans (gram negative bacteria) is investigated using methods for measuring the optical density of a bacterial suspension, diffusion into agar, as well as studying the intensity of biofilm formation. It is shown that the effects of HSs and NCs on bacteria in dynamics are different in R. erythropolis and P. oryzihabitans. HS-sl stimulated the growth of R. erythropolis on the first day of observation, and in P. oryzihabitans this effect was noted later, at 2 days of the experiment. Despite the absence of bacteriostatic effects for HSs, a decrease in biofilm formation of bacteria under their influence has been revealed. HS-cl and HS-md reduced the biofilm formation of R. erythropolis, and HS-sl reduced P. oryzihabitans. The addition of HSs to HCs retains the bactericidal effect of NC HS-cl/Ag and NC HS-md/Ag with respect to R. erythropolis. A decrease in biofilm formation of gram-negative bacteria P. oryzihabitans occurred under the influence of NC HS-md/Ag. NC HS-cl/Ag, in contrast, enhanced the reproduction of P. oryzihabitans throughout the observation period. Thus, the reaction of microorganisms to the effects of HSs in the composition of NCs depends on the morphology of the bacteria themselves, as well as on the chemical structure and functional composition of the active substances. The obtained data and previously published results on the positive effect of silver NCs and HSs on plants open the prospects for further investigation of the effects of using NCs of HSs when introduced into the soil in combination with rhizospheric bacteria to increase the resistance of cultivated plants to stress factors.