1 Introduction
Detrimental biocorrosion or microbially influenced corrosion
(MIC) [1–13] on the one hand and corrosion inhibition by
microorganism [14–23] on the other hand represent two faces of
the same medal – the ubiquitous presence of microorganisms
and their ability to quickly adapt to changes of environmental
conditions. Whereas obvious in the field of medical application,
the important role of microorganisms and related microbial
biofilms in particular for MIC has been questioned for long time
but is nowadays widely accepted. The corrosive properties of
some microbial biofilms are indeed exploited for the extraction of
metals, e.g. in the bioleaching process [24–26]. As for these
processes optimized biofilms are of need studies of biofilm
formation and properties are matter of research (e.g. [27, 28]).
This knowledge is also important for the prevention of biofilm
formation. Among the various factors that contribute to biofilm
formation, quorum sensing (QS) or microbial intercellular
communication is of special interest as comparably simple
chemical substances like acyl-homoserine lactone (AHL) can be
involved [28]. Disturbing or even blocking the communication
pathways by application of appropriate substances could offer an
effective mechanism for biofilm prevention.
Besides living biofilms also extracellular polymeric substances
(EPS) as biofilm key-constituents are discussed to be
involved in biocorrosion and MIC as well [1, 3, 29–31]. EPS –
mainly composed of polysaccharides (phospho-)lipids and
proteins – are of high significance for cell adhesion and biofilm
formation as they decrease electrostatic repulsion between cell
and substrate. In addition, the reversible adsorption of the cell as
very first step of biofilm formation is facilitated by these
substances. The properties of EPS strongly depend on microbial
origin and growth conditions as well e.g. availability of nutrients
and hydrodynamics. In addition, the release of EPS is found to be
controlled by QS mentioned above [28]. Several fractions of EPS
can be distinguished, e.g. capsular EPS wrapping the single cell
and EPS of the biofilm. One even must discuss differences
between microbial EPS of planktonic state cells or sessile state
cells. Due to this complexity of EPS in general, its analysis
strongly depends on how EPS are extracted from the biofilm as
methods too harsh will destroy cell walls and also introduce cell
material into the medium. Some bacterial EPS exhibit high ability
for complex-binding metal ions and thus are discussed to
promote corrosion, e.g. in the case of EPS extracted from
sulphate-reducing bacteria (SRB) [31, 32].
Some of the harmful microbes are found in the group of the
SRB that anaerobically metabolize sulphate as electron acceptor
and low-chain organic substances as carbon source [13]. The
metabolically produced hydrogen sulphide is known to attack
unalloyed steel. In addition, enzymatic consumption of cathodically
formed hydrogen is discussed to speed up corrosive
processes. This cathodic depolarization usually implies well
adhered bacteria cells but recent results show that the adsorption