Visualization of capsular polysaccharide induction in Acidithiobacillus ferrooxidans

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Extracellular Polymeric Substances (EPS) are of fundamental importance for attachment to metal sulfides, biofilm formation and leaching efficiency of Acidithiobacillus ferrooxidans. In this work we have visualized the capsular polysaccharides (CPS) of A. ferrooxidans ATCC 23270 using the fluorescently-labeled lectin Concanavalin A (ConA) and confocal laser scanning microscopy (CLSM). Ferrous iron-grown cells exhibited low amounts of CPS. A strong induction of CPS production was observed for planktonic and mineral-attached cells 24h after transfer to pyrite (FeS2). Additionally, CPS production was inducible in the presence of metal sulfide degradation products, even when cells had no direct contact to metal sulfide surfaces. Despite being an autotrophic bacterium, it has been found that sugars, presumably serving as polysaccharide building blocks stimulate CPS production in A. ferrooxidans. A strong induction of CPS was found after growth of cells on floating filters in ferrous iron medium supplemented with 2.5mg/mL glucose or galactose, while the presence of 0.25mg/mL casaminoacids did not show effects on induction of CPS. A similar phenotype was also found after growth under phosphate (Pi) limited growth conditions. Our results strongly suggest that CPS expression is inducible by several factors, including bacterial contact with metal sulfide degradation products, while direct mineral contact seems not to be a prerequisite. The molecular mechanisms regulating expression of genes encoding CPS biosynthesis and export systems in A. ferrooxidans remain to be elucidated.