Effect of electrical charges on potential of fibers for transport of microbial particles in dry and humid air

Abstract

Coexistence of natural, synthetic or semi-synthetic fibers and microbial particles in aerosol state has not received so far a proper attention. In the scientific literature, there is almost no research devoted to fibrous aerosols as carriers of microbial particulates. The aim of this study was to assess the potential of natural and man-made fibers for airborne transport of electrically charged bacterial and fungal particles in different humidity conditions. The abilities of 4 natural (cotton, hemp, dog and horse hair) and 2 man-made (polyester and viscose) fibers to transport 7 microbial strains (Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Streptomyces albus, Cladosporium cladosporioides, Aspergillus versicolor and Penicillium melinii) in dry (RH < 30%) and humid (RH > 60%) air were analyzed in laboratory conditions using newly built aerosolization chamber. Each tested microbial aerosol was generated (for 10min) into the chamber from water suspension by Collison nebulizer (bacteria) or from contaminated agar surface using tube generator (fungi) and electrically charged or neutralized (–4500V, −3000V, −1500V, −500V, 0V, +500V, +1500V, +3000V, +4500V) using Topas electrostatic aerosol neutralizer. Bioaerosol concentrations were measured with Grimm optical particle counter. In the chamber, each tested microbial strain was mixed (for 10min) with respective studied fibers (their concentrations were controlled using 7400AD real-time fiber monitor) and such mixture was sampled with IOM cassette (for 5min at 5Lmin−1) on polycarbonate filter. After sampling, randomly selected samples were checked using scanning electron microscope to confirm the microbial presence on fibers. Then, all collected fibers carrying microbial particles were eluted by vortexing from the filter using 0.9% NaCl with Tween 80. Spread plate method was applied to assess culturable bacterial or fungal concentration in the resulted suspension.The quantitative assessment of electrically charged culturable microbial particles revealed that fibers were able to transport in the air up to 100 bacterial vegetative cells/spores or fungal conidia per 1 fiber. The most effective in this case were animal (dog and horse) and plant (cotton) fibers. Both tested man-made fibers were significantly less efficient than natural ones (p < 0.05). The study also showed that fraction of culturable microorganisms generated into the air and subsequently transported on fibers may reach 50% of total bioaerosol particles. In the environment of low air humidity (RH < 30%), the percentage of culturable microorganisms was always higher than those in high (RH > 60%) humidity conditions (p < 0.05). Moreover, the highest percentage of culturable microorganisms was always noted when the particles (bacterial vegetative cells or spores as well as fungal conidia) were electrically neutralized (0V). Each time when microbial particles were electrically charged (± 500–4500V), such process decreased the percentage of culturable bacteria and fungi (p < 0.05). This phenomenon can be explained by a disruptive influence of electrical conduction on viable microorganisms in the environment in which water is relatively easily available.