Impacts Of Dead Bacteria On Physical Properties Of Biofilms

Bacterial biofilms are surface-associated communities of bacteria attached to surfaces or interfaces and embedded into self-produced extracellular polymeric substances?EPS?.Biofilm is more detrimental to public health and industries than planktonic bacteria because of the protection of EPS.Microbicide and antimicrobial surfaces can effectively kill bacteria.However,the killed bacteria or disrupted biofilms are often not removed,but left on the surfaces instead and gradually lay over the initial surfaces.The surface-deposited bacteria casualties significantly vary the chemical and physical properties of the surface,thus impact on the subsequent adhered individual bacteria and biofilm.Nevertheless,studies on the physical properties and microstructures of biofilms upon dead siblings was rarely reported,and how surface-deposited bacteria casualties affect the long-term performances of antimicrobial surfaces and microbicide remains unclear.In this thesis,we cultured Pseudomonas aeruginosa biofilm on their dead siblings,and investigated the effect of dead bacteria on physical properties of upper biofilms by using magnetic force modulation atomic force microscope?MFM-AFM?and digital holographic microscope?DHM?constructed by our laboratory.The details are shown as below:?1?Microrheology of growing biofilms of PAO1?wild strain?and its mutant WFPA800??psl,the psl promoter deletion mutant?upon dead siblings was monitored with MFM-AFM.The dynamic stiffness?kd?is scaled to the disturbing frequency?f?as k_d=k_d,0f^?,indicating the biofilm is glassy.It was found that the biofilms upon the dead bacteria become softer and more fluid-like than those growing upon the alive cells.Meanwhile,as the incubation time increases,the thickness of biofilms upon dead siblings grow slower than that upon alive indicates that the deposited dead siblings slow down the growth of PAO1 biofilm.The enumeration of viable bacteria within PAO1 biofilms further reveal that the smaller population within the biofilm upon the dead siblings is mainly responsible for the softer biofilm.?2?The three-dimensional dynamics of PAO1 swimming near dead siblings and the interactions between them was investigated using DHM.The spatial distribution of planktonic PAO1 upon dead siblings is broader than that upon alive bacteria.The density of PAO1swimming within the distance smaller than 10?m from the bottom surface upon dead bacteria is always less than that on alive siblings.Besides,at 90 min after adding alive PAO1 on to the surface covered with dead bacteria,the number of PAO1 adhered to the dead-bacteria-deposited surface has no significant increase compared with the initial time.This fact illustrates that PAO1 swim and accumulate near the alive-bacteria-covered surface and escape from the dead-bacteria-covered surface.In addition,PAO1 upon dead siblings show less subdiffusive motions compared with the case upon alive siblings,indicating that the PAO1 upon alive bacteria are more likely to undergo irreversible adhesion.