Table of Contents:
Microbes in nature are commonly found in surface-associated communities, now often
referred to as biofilms, leading to drastically altered properties compared to planktonic cells. The
biofilm formation is strongly dependent on the environmental conditions such as nutrient and
oxygen supply. Its developmental program can be divided into several phases. Initial attachment
marks the onset of a bacterial life style switch, where single cells attach to a substratum initially in a
transiently associated manner before becoming permanently immobilized. The permanent
attachment sets the base for subsequent production of extracellular polymeric substances (biofilm
matrix) and biofilm formation. However, signals and regulatory events underlying these initial
processes are still mostly unknown.
The dissimilatory iron-reducing bacterium Shewanella oneidensis MR-1 forms biofilms in
hydrodynamic and static systems, reflecting different natural habitats. Under static conditions,
biofilm development occurs entirely different from hydrodynamically-grown biofilm as the cells
form a flexible sturdy network of cells without the characteristic three-dimensional structures that
are typical for flow-cell cultured biofilms.
To identify genetic requirements for the cellular attachment – as first step in the biofilm
formation – in different environments through transcriptome analyses, a novel system for
harvesting surface-attached cells under hydrodynamic conditions was successfully established. The
microarray analyses of attached cells in static and hydrodynamic environments revealed that
transition between the planktonic compartment and the surface leads to dramatic changes in the
expression profile of the surface-associated cells. While reduction of motility and rapid adaption to
changes in oxygen levels represent an ubiquitous stage-specific genetic requirement, the initial
attachment of the cells can also entail substrate-specific genetic responses. Therefore, the
attachment to the redox-active substratum iron (hydr)oxide results in a drastically reduced synthesis
of cytochromes and transporters, but leads to the induction of stress-dependent sigma factors.
Hence, the genetic changes in initially attached S. oneidensis MR-1 cells improve the adaptation to the
sessile life style.
Following initial attachment, S. oneidensis MR-1 cells start forming biofilm structures and
are encased in a self-produced sticky biofilm matrix with extracellular DNA (eDNA) as a major
component. The eDNA not only serves as structural component in all stages of biofilm formation
under static and hydrodynamic conditions, but is also required for proper attachment of the cells in
different environments. The release of eDNA during early and later stages of biofilm formation is
mediated via cell lysis of a subpopulation of biofilm cells through the lytic activity of the three
S. oneidensis MR-1 prophages, MuSo1, MuSo2 and LambdaSo. However, mutant analyses and
infection studies revealed that only LambdaSo and MuSo2 form infectious phage particles. A
mutant lacking all three prophages is defective in all stages of biofilm formation. Thus, the
prophage-mediated lysis result in the release of crucial attachment- and biofilm-promoting factors,
in particular eDNA.
In addition to the function of eDNA, the role of two extracellular endonucleases, ExeM
and ExeS, was analyzed. While ExeM is involved in degrading external DNA as sole source of
phosphorus, ExeS does not seem to have a function in utilizing DNA in S. oneidensis MR-1.
However, both nucleases are likely to be involved in degrading eDNA within the matrix of
biofilms. This process is essential for detachment events in biofilms giving rise to viable planktonic
cells capable of attaching to non-occupied areas on the surface of the habitat.