Role of biopolymers on bacterial adhesion and mineral beneficiation

Abstract

The formation of biofilms on solid surfaces is a universal bacterial strategy for survival and optimum positioning with respect to available nutrients. Bacterial adhesion is an important initial step in biofilm formation, which is controlled by the production of variable amounts of biopolymers. The formation of these extracellular polymers solely depends on the growth condition and the substrate type. In this paper, the mechanisms of bacterial adhesion on different minerals are discussed on the basis of results obtained using a multipronged approach involving zeta potential, contact angle, FTIR and TEM measurements. Paenibacillus polymyxa has been used as a model bacterium, and quartz and calcite have been used as model minerals. Bacterial interaction was found to make the calcite surface more negative, while the same bacterial treatment makes the quartz surface more positive. Furthermore, it was seen from the contact angle measurements that the bacterial treatments make quartz surfaces more hydrophobic and calcite surface more hydrophilic. Interestingly, the bacteria showed different cell-surface composition depending on the minerals in their vicinity. FTIR results revealed that Paenibacillus polymyxa grown in the presence of quartz produced more surface proteins, while the same bacterium grown in the presence of calcite produced more surface polysaccharides. From the TEM study it was found that a slime layer surrounded the Bromfield-grown cells, whereas the calcite-grown cells were surrounded by well-structured capsule consisting of polysaccharides. The quartz-grown cells, on the other hand exhibited neither a capsule nor a slime layer on their surfaces. From the flotation and flocculation studies it was found that both bacterial cells and their metabolites enhanced the flocculation of calcite, whereas similar biotreatments promoted the dispersion of quartz. Studies with Thiobacillus ferroxidans have also shown marked differences in their effect on flotation of sulfide minerals such as sphalerite and galena. These selective effects of bacterial treatments on different mineral substrates are potentially useful in the development of environmentally benign and efficient bioprocessing technologies to extract low-grade valuable minerals in a cost-effective way.