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Title: Electrophoretic studies of surface charge on unicellular bacteria
Authors: Lee, Poh Foong
Keywords: Electrophoretic mobility
Peptidoglycan layer
Surface charge
Unicellular bacteria
Electrical double layer
Issue Date: Apr-2009
Publisher: University Malaya
Abstract: Electrophoretic studies of surface charge on unicellular cell were carried out. The unicellular bacteria selected for this research consisted of two different strains, which are gram-positive S.aureus and gram-negative E.coli. Colloidal particles, (Titanium dioxide) TiO2, and liposome, were used as non-living counterparts for comparison. The studies have focused on the effects of different physical and chemical conditions, including variations of temperatures, applied field, and time intervals between measurements. We did calculations of the Donnan potential and also investigated the polarizability of the bacteria. Methodology used in this research was laser Doppler velocimetry electrophoresis. Surface charge of bacteria cells is measured with electrophoresis due to development of net charge at the particle surface affects the distribution of ions in the surrounding interfacial region. This process increases concentration of counterions (ions of opposite charge to that of the particle) close to the surface. Zeta potential is considered to be the electric potential of this inner area including this conceptual "sliding surface". The electrophoretic mobility ( μ ) is the migration rate of the charged particles to the electrode when electric field is applied. In various pH, dead cells of both E.coli and S.aureus exhibited lower electrophoretic mobility than live cells. This was mainly due to dead cell had lost the metabolic activity to give charge balance to surface charges. Besides, it was found that increase in the ionic strengths of NH4Cl in the buffer reduced the electrophoretic mobility of samples. In contrary, increased ionic strengths of NaCl obtained the opposite result for the bacteria. This might be due to the increase of influx Na+ into cells, which increases the anionic lipid to balance the charges in the cells. Results indicated that both approaches fitted well at greater ionic strength of NaCl with the Donnan potential approximation. The electrophoretic mobility of live E.coli displayed a mild increase at higher temperatures. On the other hand, results indicated only a slight increase in negative electrophoretic mobility of live S.aureus as the temperature increased. Increase in applied field increased the electrophoretic mobility of bacteria and colloidal particles due to the stronger attraction field. Different time intervals between measurements with unchanged conditions showed that higher zeta potential was measured for time intervals more than 10s between measurements. This indicated that the particles still swirled around when the next measurements started. In addition, live bacteria recorded greater polarizability compare to colloidal particles. This may be attributed to the semipermeable cell membrane of live cells which hasten the ions exchange between the cell interior and the external environment. A greater polarization of E.coli was obtained, which possessed thinner cell wall peptidoglycan layer compared to S.aureus. The conductivity and permittivity of cells increased as the frequency increased. However, the permittivity of cells were shown constant at higher frequency (>200Hz).
Description: Thesis (PhD) -- Faculty of Science, University of Malaya, 2009.
URI: http://dspace.fsktm.um.edu.my/handle/1812/592
Appears in Collections:PhD Theses : Science

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