3.5.1 Microbial Adhesion and Biofilms
Microbes rely on surface adherence processes, a feature that becomes important
when it comes to the colonization of substrates and eventually the creation of
biofilms (Hall-Stoodley et al. 2004). Through surface tension, water molecules
create a hydrosphere around sediment grains which is a prerequisite for all microbial
life, thus creating a dependency of microbes on moistened surfaces (Stoodley 2016).
Extremely small epipsammic diatoms that grow attached to the surfaces of sand
grains (sometimes at densities of ~100 cells per individual grain) know this all too
well (Mann et al. 2017).
On evolutionary terms, surfaces may have provided a protective niche in which
attached cells could create a localized homeostatic environment (Stoodley et al.
2002). Thus, the stage for what may be regarded as the two-dimensional world of
biofilms was set. In fact, not only did biofilms appear early in the fossil record
(reviewed by Hall-Stoodley et al. 2004), but concomitantly, it has been hypothesized
that complex interactions within prokaryotic communities evolved in surfaceassociated biofilms (Stoodley et al. 2002).
The term “biofilm” was coined more than 30 years ago (Costerton et al. 1987) and
refers to aggregated microbial cells that are adhered to a biological or
non-biological surface and have secreted a gelatinous matrix of EPS. Cells in
biofilms grow in matrix-enclosed microcolonies separated by a network of open
water channels (Stoodley et al. 2002). Prokaryotic biofilms are highly structured
multispecies communities that may take over 10 days to reach structural maturity
and in which metabolic activities are integrated.
This coordination in ecophysiological responses among biofilm microbes calls
for signalling or communication among its members. The mechanism has recently
been discovered, and it implies a cluster of organisms pertaining to a microbial
biofilm population that produces diffusible chemical signals in a coordinated manner; then, the signal concentration builds up until the microbial population reaches a
critical “quorum” level from which the signals act as a “switch” to synchronize the
behavior of other individuals in the population; these switches are used to regulate
various functions (Stoodley 2016). The EPS matrix favors this intercellular communication within biofilms by keeping clusters of organisms within relative proximity,
more effectively allowing their quorum sensing to occur (Flemming and Wuertz
2019). Quorum sensing has been observed even in hard rock interfaces, making
viable effective microbial communication, which would otherwise be impossible in
a liquid-only world.
Biofilm anchorage to the sedimentary substrate may seem relatively weak when
extrapolations are made to coastal marine environments. In intertidal mudflats where
the microphytobenthos is dominated by diatoms, patches of biofilm of significant
size can be resuspended in the water when water flows at high tide (Saint-Béat et al.
2014). This may be regarded as a drawback of microbial association into larger
biofilms, although it may also represent an opportunity to disperse and colonize new
niches. In sum, there is consensus that the aggregation into biofilms (1) confers a
76
J. Pan
Microbes rely on surface adherence processes, a feature that becomes important
when it comes to the colonization of substrates and eventually the creation of
biofilms (Hall-Stoodley et al. 2004). Through surface tension, water molecules
create a hydrosphere around sediment grains which is a prerequisite for all microbial
life, thus creating a dependency of microbes on moistened surfaces (Stoodley 2016).
Extremely small epipsammic diatoms that grow attached to the surfaces of sand
grains (sometimes at densities of ~100 cells per individual grain) know this all too
well (Mann et al. 2017).
On evolutionary terms, surfaces may have provided a protective niche in which
attached cells could create a localized homeostatic environment (Stoodley et al.
2002). Thus, the stage for what may be regarded as the two-dimensional world of
biofilms was set. In fact, not only did biofilms appear early in the fossil record
(reviewed by Hall-Stoodley et al. 2004), but concomitantly, it has been hypothesized
that complex interactions within prokaryotic communities evolved in surfaceassociated biofilms (Stoodley et al. 2002).
The term “biofilm” was coined more than 30 years ago (Costerton et al. 1987) and
refers to aggregated microbial cells that are adhered to a biological or
non-biological surface and have secreted a gelatinous matrix of EPS. Cells in
biofilms grow in matrix-enclosed microcolonies separated by a network of open
water channels (Stoodley et al. 2002). Prokaryotic biofilms are highly structured
multispecies communities that may take over 10 days to reach structural maturity
and in which metabolic activities are integrated.
This coordination in ecophysiological responses among biofilm microbes calls
for signalling or communication among its members. The mechanism has recently
been discovered, and it implies a cluster of organisms pertaining to a microbial
biofilm population that produces diffusible chemical signals in a coordinated manner; then, the signal concentration builds up until the microbial population reaches a
critical “quorum” level from which the signals act as a “switch” to synchronize the
behavior of other individuals in the population; these switches are used to regulate
various functions (Stoodley 2016). The EPS matrix favors this intercellular communication within biofilms by keeping clusters of organisms within relative proximity,
more effectively allowing their quorum sensing to occur (Flemming and Wuertz
2019). Quorum sensing has been observed even in hard rock interfaces, making
viable effective microbial communication, which would otherwise be impossible in
a liquid-only world.
Biofilm anchorage to the sedimentary substrate may seem relatively weak when
extrapolations are made to coastal marine environments. In intertidal mudflats where
the microphytobenthos is dominated by diatoms, patches of biofilm of significant
size can be resuspended in the water when water flows at high tide (Saint-Béat et al.
2014). This may be regarded as a drawback of microbial association into larger
biofilms, although it may also represent an opportunity to disperse and colonize new
niches. In sum, there is consensus that the aggregation into biofilms (1) confers a
76
J. Pan