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.