| In aqueous solution,complex interactions exist between charged particles and electric fields.Common characterization techniques examine the effects of electric fields on particles in a statistic way.Bacteria and microbubbles,two typical charged particles,are ubiquitous in nature,daily life and both susceptible to surrounding environments.Utilizing a homemade digital holographic microscopy(DHM),we explored the 3D individual behaviors of Pseudomonas aeruginosa(PAO1)and microbubbles under varied electric fields.For PAO1,using DHM and RNA sequencing techniques,we reveal the impacts and related mechanism of the cathodic direct-current(DC)electric fields on the near-surface individual behaviors as well as bacterial adhesion and detachment from the surface.For microbubbles,the surface polarization and electric field intensity show significant influence on their size distribution and movements,thus affecting their stability in solution.(1)Using DHM,the 3D behaviors and adhesion of Pseudomonas aeruginosa PAO1 exposed to short-term DC electric fields were monitored.The results indicate that cathodic electrical stimulation with short durations(for minutes)can induce the detachment of PAO1(2.5 V,detachment ratio: 40-50%)which were already adhered to the surface.Interestingly,with polarization duration ?t ≤ 10 min,bacterial detachment on cathodic surface mainly occurs after the electric fields were removed.The detachment can last for 10 min.In the initial stage of cathodic polarization(0-5 min),the proportion of subdiffusive bacteria increases significantly,indicating that some bacteria undergo irreversible adhesion.RNA-sequencing results reveal that the application of electric fields causes the up-regulation of the Dgc M gene as well as the down-regulation of Csr A gene related to bacterial adhesion and motility and the down-regulation of the Rhl B gene related to bacterial motility,which confirm that the bacteria can adaptively respond to the electric fields,in the manner of increased flicking frequency,declined 3D swimming speeds and inhibition of oscillating motion.When the electric fields were removed,the oscillating of the adhered bacterial cells gradually becomes violent,which can lead to detachment from surface.However,when the bacteria were exposed to electric field for over 20 min,five genes related to Psl polysaccharide biosynthesis(Psl B,Psl G,Psl H,Psl L)are all significantly up-regulated,which benefits the adhesion of bacteria instead of detachment.(2)3D behaviors of bulk microbubbles prepared by mixing alcohol and water under DC electric fields were examined by DHM.The size and its distribution of microbubbles in the solution depend on the intensity and spatial distribution of the DC electric fields.Namely,the averaged size(～ 1 μm)of the microbubbles near the anode surface slightly changes,while that near the cathode surface increases by 21-24%.With a polarization voltage smaller than 2.5 V,the 3D velocity and motion patterns of the microbubbles are slightly changed.In contrast,a cathodic polarization voltage of 3 V can increase the percentage of microbubbles in superdiffusive motion by 15%.The anodic electric fields have little influence on the distribution of microbubbles near the interface probably due to the repulsive force between bubbles,but the cathodic electric fields can significantly reduce the density of microbubbles near the cathode.Utilizing DHM,3D individual behaviors of P.aeruginosa and microbubbles under DC electric fields were examined.It was shown that the adhesion and detachment of P.aeruginosa can be tuned by the exposure time of cathodic electrical stimulation.On the other hand,the size and density distribution of microbubbles can be altered by the intensity and polarization direction of DC electric fields.This paper quantitatively assesses the interactions between electric fields and bacteria as well as microbubbles which provide inspiration for the design of highly-efficient,energy-saving antifouling strategies based on electric fields. |
