Effect of rhamnolipids on initial attachment of bacteria on glass and octadecyltrichlorosilane-modified glass

Bacterial attachment on solid surfaces has various implications in environmental, industrial and medical applications. In this study, the effects of rhamnolipid biosurfactants on initial attachment of bacteria on hydrophilic glass and hydrophobic octadecyltrichlorosilane (OTS) modified glass were evaluated under continuous-flow conditions. The bacteria investigated were three Gram-negative species Pseudomonas aeruginosa, Pseudomonas putida, and Escherichia coli, and two Gram-positive species Staphylcoccus epidermidis and Bacillus subtilis. Rhamnolipids, at 10 and 200 mg/l, significantly reduced the attachment of all but S. epidermidis on both glass and OTS-modified glass. For S. epidermidis rhamnolipids reduced the attachment on OTS-modified glass but not on glass. Studies were further done to identify the mechanism(s) by which rhamnolipids reduced the cell attachment. The following potential properties of rhamnolipids were investigated: inhibition of microbial growth, change of cell surface hydrophobicity, easier detachment of cells already attached to substratum, and modification of substratum surface properties. Results showed that rhamnolipids were ineffective for the latter two effects. Rhamnolipids, up to 200mg/l, inhibited the growth of B. subtilis, S. epidermidis and P. aeruginosa PAO1 but not the growth of E. coli, P. putida and P. aeruginosa E0340. Also, rhamnolipids tended to increase the hydrophobicity of P. aeruginosa PAO1 and E. coli, decrease the hydrophobicity of P. putida and S. epidermidis, and have no clear effect on the hydrophobicity of B. subtillis. These trends however did not correlate with the observed trend of cell attachment reduction. The responsible mechanism(s) remained unknown.