Linking Micro-physical And Chemical Processes With Bacterial Surface Attachment And Aggregation Behaviors Under Multiple Stresses

Facing environmental pressure,bacteria commonly regulate their physiological behaviors as aggregates or biofilms to adapt to a harsh environment.Nevertheless,the environmental stresses-shaped bacterial surface attachment and aggregation process along with associated driving forces remain sketchy,especially for drinking water bacteria,which are often subject to disinfection,nutrient limitation and hydrodynamic disturbance.Accordingly,this study coupled an experimental investigation and individual-based modeling simulation to understand how chlorination,nutrient limitation and hydrodynamic disturbance conspire to form biofilm using wild-type Pseudomonas aeruginosa and its flagellum deletion mutant((?)fli C)and excess alginate secreted mutant((?)muc A).This study would offer the insights into the interactions between micro-physical,chemical processes and bacterial physiological and aggregation behaviors,thus providing theoretical guidance and novel ideas for the bacterial pollution control of drinking water systems and other aqueous environments.Firstly,this paper examined the impacts of disinfectants on bacterial movement,extracellular polymeric substances(EPS)production and surface adhesion behaviors.Results showed that the addition of disinfectant(1.0 mg/L chlorine)promoted the surface adhesion of wild-type strains,and its surface coverage increased to 3.17 times as compared to that in the absence of chlorine.Accompanying it,the cell swimming velocity(20 min)and EPS(6 h)secretion rate in the biofilms augmented.Compared to the (?) fliC mutant,the initial attachment advantage of the wild-type strain was obvious,while its advantage was gradually replaced by the (?)mucA mutant over time course,demonstrating that bacterial flagella movement and EPS production are the main driving factors in the process of initial attachment and subsequent biofilm formation,respectively.The IBMbased simulation analysis at micro scales indicated that the disinfectant level and its spatial distribution collectively determined bacterial chemotaxis and extracellular polymers formation,thus regulating surface attachment and subsequent biofilm formation.Secondly,the processes of bacterial movement,EPS production,attachment and aggregation under the dual stresses of oligotrophic and disinfection conditions were studied.The results showed that limiting nutrient further enhanced the promoting effect of disinfectants(especially at 1.0 mg/L chlorine)on bacterial movement,EPS production and surface adhesion(the surface coverage increased by 10.06 times as compared to that with sufficient nutrient supply).Moreover,limiting nutrient encouraged the attachment of the two mutant strains((?)fliC and (?)mucA)by 4.81 and 17.35 times respectively,which further proved that nutrient limitation enhanced the promoting effect of disinfectant on bacterial aggregation behavior,while EPS posed a major factor driving bacterial surface attachment and subsequent aggregation processes.The IBM-based simulation analysis at micro scales revealed that the spatial distribution of nutrient and disinfectant levels jointly determined bacterial movement behavior and EPS secretion,strengthening the cell-cell and cell-surface interactions and aggregation,and thereby promoting bacterial surface attachment and subsequent biofilm formation.Finally,the effects of disinfectant and nutrient limitation on bacterial surface attachment and aggregation under various hydrodynamic disturbances were explored.The results showed that hydrodynamic disturbance was not conducive to bacterial adhesion and biofilm formation without disinfection and nutrient limitation stresses.In the face of oligotrophic conditions,hydrodynamic disturbance promoted the bacterial attachment and aggregation,especially at the speed of 80 RMP with the surface coverage increased by 32.85% compared with 0.2% of eutrophication.Addition of disinfectant further promoted its adhesion and aggregation behavior(the coverage increased by58.73%).The IBM-based simulation analysis indicated that hydrodynamic disturbance encouraged the nutrient absorption of bacteria and the cell-cell,cell-surface interactions while the resulting shear force was not conducive to cell attachment and aggregation.The dual stresses of disinfectant and nutrient limitation promoted the formation of bacterial EPS,which exacerbated cell-cell and cell-surface interactions and thereby formation of biofilms on solid surfaces.