Biophysical investigation of bacterial aggregation

In nature, bacteria usually exist as aggregates, in order to withstand changes in
environmental conditions. Bacterial aggregation is of great significance in the field of
biotechnology, environmental studies and medicine. Bacterial aggregation is thought to
be governed by physical forces such Van der Waals and electrostatic interaction.
However, extracellular polymeric substances (EPS), and the ability. of bacteria ·to
participate in cell-to-cell communication via quorum sensing molecules have also been
implicated in the bacterial aggregation. process. Despite the wealth of knowledge
available, a detailed understanding of bacterial aggregation still remains unclear. The
overall aim of this work therefore, is to understand bacterial aggregation at the cellular
and sub-cellular level using existing colloidal characterisation techniques and post
genomic methods. This will enable both the biological and the physical aspects of
aggregation to be studied together.
E.coli strains (AB1157, MG1655 and MG1655 luxS) were cultivated in Luria-Bertani
(LB) medium at 30°C supplemented with or without 0.5 w/v (%) glucose at the
beginning of growth phase. Depletion aggregation studies were carried out using E.coli
AB1157 and E.coli MG1655 harvested at different growth phases using a nonadsorbing
polymer, sodium polystyrene sulphonate (SPS) and biological produced
extracellular polymeric substances (EPS) respectively. The content of EPS produced by
E.coli MG1655 during their growth in different media w~s quantified and characterized
using Fourier transformation infrared spectroscopy (FTIR), SDS-PAGE and an
electrospray ionization-tandem mass spectrometry. The changes in cell surface
properties of E.coli strains during growth, changes in media composition and quorum
sensing were elucidated using potentiometric titration, FTIR and electrophoretic
mobility.
Neither quorum sensing, nor the addition of 0.5 w/v (%) glucose affected the growth
pattern for the strains. However, the addition of 0.5 w/v (%) glucose to the medium
affected the measurable amount of quorum sensing molecule present in the supernatant.
Aggregation of E. coli was found to be dependent on the concentration and type of
polymer used, as well as the surface chemistry of the cell. The cell surface functional
groups, such as such as, hydroxyl, phosphoryl, amines and carboxylate groups varied
with respect to different growth phase and changes in media. The protein content of
free-EPS was found to significantly increase due to changes in growth phase and media
composition. The growth phase, changes in media and quorum sensing all affected the
cell surface properties and hence played a role in the aggregation capability of E.coli.