Impacts Of Electric Fields On Three-Dimensional Dynamics Of Bacteria

Bacterial biofilm refers to a spatial structure formed by bacterial communities encapsulated by extracellular polymeric substances(EPS)after adhesion to the surface.Biofilms cause serious problems such as bacterial infectious diseases and biofouling.Adhesion onto a surface is the preliminary step for biofilm formation.Therefore,inhibition of bacterial adhesion is critical to control the development of biofilm.It was demonstrated that electric fields can minimize surface adhesion of bacteria,but how they affect the near-surface bacterial dynamics has remained largely overlooked.In this thesis,the three-dimensional motions of Escherichia coli(E.coli)HCB1(wild-type)and mutant HCB1414((35)cheY(35)cheZ,non-tumbling)under direct current(DC)and alternating-current(AC)electric fields were monitored with digital holographic microscope(DHM)constructed by our laboratory.The underlying mechanisms governing the bacterial motions induced by electric fields were explored.Besides,controllable surface adhesion of bacteria under electric fields was investigated by utilizing patterned electrode.The main results are as follows:(1)The period(T)of AC fields exhibited profound effects on near-surface bacterial behaviors and thus affected the adhesion of E.coli.When T ? 1 s or under DC fields,the bacterial density near the surface varies due to galvanotaxis which is highly related to the initial polarization of the surface.It was found that HCB1 tumble more frequent,and both two strains increasingly undergo subdiffusive motions compared to the case without electric fields.For shorter periods(T ? 0.1 s),the application of electric fields reduces the near-surface bacterial density by 10-20 % with the surface as either an anode or a cathode.We revealed that the AC fields directly disturb the intrinsic bacterial wobbling.The bacterial body exhibits strong wobbling at T ? 1 s.However,this wobbling motion was suppressed with decreasing T,thus reduces the collisions between E.coli and the surface and leads to declining bacterial density.(2)Selected bacterial adhesion on patterned electrode surfaces utilizing electric fields.By applying DC fields(positive or negative)or low-frequency AC fields on a patterned electrode(strip),the near-surface bacteria upon different regions show distinguished difference in distribution,which are driven by galvanotaxis and the electrophoretic force.However,the bacterial density distribution exhibits slight difference with the application of high-frequency(1KHz ? f ? 10 Hz)AC fields.In particular,the application of 1 KHz AC fields reduces the near-surface bacterial density significantly by about 30 %,thus inhibit the bacterial adhesion.