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2.4.1 Ag43 expression and motility
The importance of Ag43 expression in bacteria attachment to quartz grain surfaces was evident
from the high positive
r
-value between Ag43 and
strai
Α
in both DI and AGW and from the high
positive
r
-values between the sticking efficiency and Ag43 expression at the short travel
distances (0.13 m and 0.33 m; in both solutions, DI and AGW) and reducing as travel distance
increased. The Ag43 adhesin is known to establish autoaggregation of cells through Ag43-Ag43
interactions by a kind of intercellular handshake mechanism (Klemm et al, 2004; Hasman et al.,
2000) resulting in the retardation of cell movement (Yang, 2005). In our case, such mechanism
might have played a role, but, more importantly, we found that Ag43 also played a crucial role in
the initial attachment of bacteria cells to the quartz grain surfaces. Ag43 is composed of two
proteinaceous subunits,
4
Β (Henderson et al., 1997). Of these,
Α is surface expressed
and is bound to the cell surface through an interaction with
4
Β
, itself an integral, outer
membrane protein. Although the mechanism involved in the contact between bacteria cells and
grain surface is unknown, because of the predominantly negatively charged quartz surface, we
surmise that in our case
Α and
43
43
Α
was positively charged, and responsible for promoting
43
Α
mediated
43
favorable attachment between bacteria cell and quartz surface.
The reduction in
r
values between
Α
and Ag43 expression with increasing transport distance
can probably be attributed to intra-population heterogeneity. There is the possibility that not all
cells within an Ag43 expressing strain indeed express the adhesin, which may lead to the
Pearson's correlation coefficient,
r
, with distance to preferential attachment of cells expressing
the Ag43 adhesin. Also, Danese et al. (2000) reported that 43% of wild type
E. coli
cells grown
in glucose minimal medium synthesize Ag43 and hypothesized the possibility of the remaining
population expressing a partially redundant adhesin.
We found high positive r-values between motility and
strai
Α
in DI water and low positive
r
-
values in AGW. In addition, the correlation between motility and
Α
reduced with increasing
transport distance. Motility might have increased the rate of diffusion of motile bacteria to the
surfaces of the quartz grains. Motile bacteria can propel themselves to the collector surface,
thereby overcoming repulsive forces (Pratt and Kolter, 1998), and thus increasing the rate of
collisions. However, it remains difficult to explain the low
r
-value in DI at 0.13 m (
Fig. 2.3
)
. The
high
r
values in DI water compared to the low
r
-values in AGW can be attributed to two factors:
(1)
In DI, non-motile cells were unable to overcome the repulsive electrostatic energy
barrier between bacteria cell and quartz surface.
(2)
In AGW, the electrostatic repulsive barrier was low, and therefore both motile and
non-motile cells could reach the surface of the quartz grains.
An increase in the rate of initial collisions between bacteria cells and collector surfaces, and a
reduction in bacteria transport rates has also been attributed to flagella mediated motility (van
Loosdrecht et al. 1989, Becker et al., 2004; Shemarova and Nesterov, 2005; de Kerchove and
Elimelech, 2008). Their finding also explains our assertion that motility increases the probability
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