Natural antimicrobial compounds immobilised on silica microparticles as filtering materials: Impact on the metabolic activity and bacterial viability of waterborne microorganisms

Abstract

The aim of this work was to assess the capability of filtering materials based on silica microparticles functionalised with essential oil components (EOCs) to remove waterborne bacteria from water, and to elucidate the mechanism of action of the inhibitory effect of the filtering materials on the metabolic activity and viability of the studied pathogens. Different silica microparticles (25, 50, 75, 200 or 375 μ m ) were functionalised with carvacrol, eugenol, thymol and vanillin to obtain filtering materials which removal capability was evaluated using distilled water inoculated with Escherichia coli, Helicobacter pylori, Legionella pneumophila or Pseudomonas aeruginosa (10 4 –10 7 cells/mL). Water samples were filtered through different layer thicknesses (0.5, 1 or 1.5 cm) of the filtering materials and the microbial load retained was determined by plate count. In addition, fluorescent viability staining, determination of cellular ATP content, direct viable count–fluorescent in situ hybridisation (DVC–FISH) and propidium monoazide-quantitative polymerase chain reaction (PMA-qPCR) analyses were performed to prove the materials’ antimicrobial properties. The results exhibited that EOC-functionalised supports were capable of eliminating waterborne microorganisms from water with log reduction values falling within the 3–5 range, whereas the non-functionalised materials did not present relevant inhibitory capacity. The irreversible effect of the EOC-functionalised supports on the viability and metabolic activity of treated bacteria was confirmed by fluorescent staining (absence or red stained cells) and DVC–FISH (no elongated cells). Cellular ATP content was significantly reduced after filtering the inoculated water samples through the EOC-functionalised supports (cATP values below 10 pg/mL). Similarly, the concentration of viable bacteria determined by PMA-qPCR showed the inhibitory effect of the developed materials with negative quantification values for H. pylori and values of 7.98 ⋅ 10 1 –6.07 ⋅ 10 3 GU/mL for L. pneumophila water samples filtered with the EOC-functionalised supports. Thus, the use of the functionalised filtering materials led to loss of bacterial viability of the treated microorganisms with irreversible morphological and metabolic alterations, which confirms their potential use as filtering aids with additional properties for the biological control of water. • Filters based on essential oil components immobilised on silica supports were made. • The developed filters were used to remove and inactivate waterborne bacteria. • As proof of concept study, removal capacity fulfilled water quality requirements. • Viability assays confirmed the antimicrobial properties of filtering materials.