The development of biofilms on mild and stainless steel surfaces in pure and mixed batch cultures of the bacterial species Pseudomonas fluorescens and Desulfovibrio desulfuricans and the role of these biofilms in corrosion of steel has been investigated. Early events leading to the formation of biofilms have been elucidated by studying the attachment of bacterial cells to steel using epifluorescence microscopy. To identify the nature of the bacterial surface components involved in the initial adhesion to mild steel, lectins, their sugar inhibitors and saccharolytic and proteolytic enzymes have been employed. Polyclonal antibodies have been raised against bacterial lipopolysaccharides (LPS) and their influence on bacterial adhesion assessed. LPS have been analysed chemically by gas-chromatography (GC-FID) and gas chromatography-mass spectrometry (GC-MS) to determine their carbohydrate composition and fatty acid content. On the basis of the results obtained the involvement of glucose and N-acetylglucosamine, present in O-antigenic fractions of LPS, in the initial attachment of the two bacterial species to mild steel is suggested. Both types of carbohydrates are likely to be involved in early attachment of Pseudomonas to mild steel, whereas only a polymeric form of N-acetylglucosamine seems to participate in adhesion of Desulfovibrio. The subsequent biofilm development on steel surfaces and their accompanying corrosion has been monitored by scanning electron microscopy (SEM). SEM studies reveal very different patterns of bacterial biofilms on mild and stainless steel and show varied degrees of corrosion occurring on these surfaces. Thin and patchy Pseudomonas biofilms are accompanied by little corrosion whilst thick. more continuous, Desulfovibrio biofilms are associated with higher levels of corrosion. Energy dispersive X-ray analysis (BOAX) of corrosion products present on steel surfaces indicates ferrous sulphides as the major components in Desulfovibrio biofilms. The corrosion of steel in bacterial cultures has also been investigated by kinetic polarisation measurements. The results obtained from cathodic and anodic polarisation curves, combined with SEM and EDAX analyses confirm the SEM observation. Stainless steel is not subjected to any great degree of fouling or corrosion under the chosen experimental conditions. The EPS associated with biofilms and released into the liquid phase of the culture media (free EPS) has been characterised. Proteins and carbohydrates in these polymers are detected colorimenically and by SDS-gel electrophoresis. Uronic acids, found in biofilm-bound BPS. are not detected in free EPS. The GC-MS and GC-FIO analyses have aided in establishing types and quantities of neutral carbohydrates present in bacterial exopolymers and show that the neutral sugar composition of free and surface-associated BPS is not identical for a given bacterial culture. The biofilm-bound BPS are believed not to playa major role in corrosion of mild steel but to provide additional mechanisms in its facilitation. No correlation between levels of free BPS and corrosion of steel is found.
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