Combined effect of ethoxylated nonylphenol and copper oxide nanoparticles (CuO) on Rhodococcus ruber biofilms

Background. Ethoxylated nonylphenol (ENP) is a surfactant characterized by high toxicity and endocrine-disrupting activity that persists after partial biodegradation. In nanobioremediation, sublethal concentrations of nanoparticles can induce a hormetic effect, stimulating biofilm formation and enhancing the catabolic potential of microorganisms. Copper oxide nanoparticles (CuO NPs) at sublethal concentrations modify cell surface properties and stimulate biofilm formation, thereby enhancing the biocatalytic potential of bacteria of the genus Rhodococcus in bioremediation processes. However, the mechanisms of Rhodococcus biofilm adaptation to organic xenobiotics and nanometals remain poorly understood.

Aim. To evaluate the effect of combined exposure to ENP and sublethal concentrations of CuO NPs on the morphology, surface properties, and biofilm formation of Rhodococcus ruber.

Materials and methods. Strains R. ruber IEGM 1263 and IEGM 71 from the Regional Specialized Collection of Alkanotrophic Microorganisms were used. The minimum inhibitory concentration (MIC) was determined under exposure to ENP (0.16–100 g/L) and CuO NPs (0.0007–0.5 g/L). Biofilms were cultivated on polypropylene discs. The ζ-potential of cells was measured by dynamic light scattering, while cell viability and surface morphology were analyzed using combined confocal laser scanning and atomic force microscopy (CLSM-AFM).

Results. Combined exposure to ENP and CuO NPs reduced rhodococcal viability by 20–80 %, whereas the overall toxic effect was mainly determined by copper oxide nanoparticles. Biofilms of the resistant strain R. ruber IEGM 1263 grown in the presence of sublethal concentrations of ecotoxicants (1–10 g/L ENP and 0.001 g/L CuO NPs) were characterized by decreased adhesion to the carrier (2.6-fold), cell coaggregation, a more negative ζ-potential (−34.2 mV), and increased cell surface roughness (up to 4.8 nm) compared with the control.

Conclusion. Morphophysiological adaptations of Rhodococcus ruber cells promoting biofilm formation and increasing bacterial resistance to complex chemical stress were identified. The obtained data confirm the potential of using Rhodococcus biofilms for bioremediation of ecosystems contaminated with organic xenobiotics and nanometals.

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