analysis of the proteins from the EPS indicates that
approximately 35–40% of the extracellular proteins, identified
in both strains, had a predominance of basic amino
acids and therefore these proteins had a basic isoelectric
point (pH≥7). This implies that at physiological pH values
(pH 6.5–7.4), these extracellular proteins would be positively
charged (Tables 3, 4, 5 and 6). This suggests that
attractive electrostatic forces between charged proteins in
the EPS could impart cohesive stability to the biofilm
matrix. This lends credence to previous reports that have
identified electrostatic forces to be prevalent within biofilms
(Mayer et al. 1999). But such electrostatic interactions
were thought to be mediated mainly by divalent cations
(Chen and Stewart 2002). Our study indicates that, in
addition to divalent cations, electrostatic interactions between
charged proteins are also present within the biofilm
in both ATCC 10987 and ATCC 14579. Additionally, the
more hydrophilic nature of the cell surface of the biofilm
cells of both the strains facilitates electrostatic interactions
between the cell surface and the EPS proteins (van
Loosdrecht et al. 1987a). This enables both ATCC 10987
and ATCC 14579 to form biofilms on hydrophilic glass
surfaces. This suggests that B. cereus follows the polymer
bridging model of flocculation (Neu and Marshall 1990;
Wilén et al. 2003) during biofilm formation, where
adhesive and cohesive properties of the biofilm are
mediated by the EPS constituents. Therefore, planktonic
cells reversibly bound to surfaces undergo changes in the
surface chemistry and produce EPS resulting in irreversible
attachment of the bacterial cells to the surfaces. Consequently,
biofilm formation proceeds favourably in these
conditions. This process conforms to the accepted model of
biofilm formation developed by Costerton et al. (1995).
In order to confirm the electrostatically induced stability
of B. cereus biofilms, biofilm formation, using the glass
wool was tested in unbuffered LB and in LB buffered with
10 mM HEPES at pH 6.8. Preliminary results suggested
that the pH of the unbuffered LB medium became more
basic with the growth of the culture while the pH of the LB
medium buffered with HEPES stayed relatively constant
over the same period of time (data not shown). The results