|M.Sc Student||Cohen Nirrit Dana|
|Subject||The Effect of Microbial Amyloids on Bacteria-Clay|
|Department||Department of Civil and Environmental Engineering||Supervisor||Dr. Adi Radian|
|Full Thesis text|
Interactions between bacteria and clay minerals have enormous impacts on reactions and processes critical to environmental quality and ecosystem health. Clay minerals are key players in the development of microbial communities and can affect their growth, mobility and ability to form biofilm. Many have explored the role of bacterial membranes and extracellular materials in clay-bacteria interactions, however, to date and despite their prevalence in environmental biofilm, the role of microbial amyloids in bacterial adhesion to clay surfaces has not been systematically studied.
Herein, is a systematic study of the effect of microbial amyloids, curli (formed by E. coli), on bacteria-clay surface interactions. Amyloids, such as curli, are strong and stable fibers, ranging from 0.1 to10 µm long with a width of 4-12 nm, a high surface area and a net negative charge. Curli have been known to initiate and support cell-to-cell and cell-to-surface interactions, meaning they are important for biofilm production and may enhance bacterial interactions with soil surfaces such as clay. Consequently, we hypothesized that the production of curli will enhance bacterial interactions with clay minerals, and that biofilm production will be greater when bacteria grow in the presence of clay minerals.
This study aims to elucidate the effect of curli on bacteria-clay interactions and the effects of clays on curli production. For this purpose, aggregation and adsorption of curli-producing E. coli to the abundant clay minerals, montmorillonite and kaolinite, was assessed using zeta potential measurements, LUMISizer measurements, fluorescent spectroscopy and electron microscopy. In addition, curli transcription and overall biofilm formation were monitored as a function of clay type.
The results revealed that curli-producing bacteria (without clay) auto-aggregated into high-density flocs (1.23 g/cm3), ranging in size from 10-50 µm, that settle spontaneously. In contrast, curli-deficient bacteria remained relatively stable in solution as individual cells (1-2 µm, 1.18 g/cm3). The stability of clay suspensions mixed with curli-deficient bacteria depended on clay type and ionic strength, the general trends being consistent with the classic DLVO theory. However, the interactions of curli-producing E. coli alone and with clay did not follow the extended DLVO theory - sedimentation was fast even at low ionic strength. This was probably due to non-DLVO forces such as polymer bridging; the fibers effectively pierce through the energy barrier posed by the electrostatic forces. In addition, curli production and biofilm formation were both enhanced when E. coli were grown in the presence of montmorillonite but not with kaolinite.
The results of this study suggest that natural surfaces can enhance microbial curli production, clay-bacteria interactions and consequently biofilm formation. These findings will help shed a light on important microscale processes at the aqueous-solid soil interfaces, such as biofilm formation, nutrient-cycling, pollutant and pathogen fate in the environment.