Galactose as inducer of the production of extracellular polymeric substances by Acidithiobacillus ferrooxidans

摘要

The presence of extracellular polymeric substances (EPS) is important for the formation of biofilms on mineral surfaces, increasing the bioleaching activity, as well as protecting the cells from adverse environmental conditions. The objective of this work was to study the effect of galactose for EPS production by Acidithiobacillus ferrooxidans. The work was performed in shake flasks of 250 mL at 30°C, 200 rpm and at an initial pH of 1.8. In order to establish the natural tolerance of the strain, its growth behaviour was evaluated at high ferric iron concentrations by adding consecutively the equivalent of 9 g/L of ferrous iron each time after oxidation to the broth. Cell growth stopped once ferric iron concentration increased up to 38 g/L. In order to determine the optimal conditions for EPS production, experiments were run in a chemostat of 0.5 L, operated at a constant dilution rate of 0.03 h~(-1). Different steady states were obtained varying feeding concentrations of galactose (0.15%; 0.25% and 0.35%) and carbon dioxide (180 ppm and 360 ppm). Cells grown in the chemostat at optimum operational conditions were used as inoculum to determine oxidative capacity of the microorganisms overproducing EPS. The EPS was quantified using confocal laser scanning microscopy (CLSM), labelling the cells with propidium iodide and EPS carbohydrates with wheat germ agglutinin (WGA). The high volume production of EPS was observed in cells grown using 360 ppm of CO_2; 0.35% of galactose. Also it was observed a size increase of cells, compared to cells grown in culture medium having 9 g/L of ferrous iron, where presence of EPS was not detected. The results revealed that EPS overproducing At. ferrooxidans showed a tolerance to ferric iron concentration almost 9.5 g/L higher than the natural tolerance of cells grown in absence of galactose. Presence of galactose in culture medium stimulated the EPS production.