Assessment of bacterial biosurfactant production through axisymmetric drop shape analysis by profile

Summary

Axisymmetric drop shape analysis by profile (ADSA-P) is a technique developed in colloid and surface science to simultaneously determine the contact angle and liquid surface tension from the profile of a droplet resting on a solid surface. In this paper is described how ADSA-P can be employed to assess bacterial biosurfactant production. Nine Streptococcus mitis strains, two of which are known to produce biosurfactants, and two S. salivarius strains, which do not produce biosurfactants, were suspended at two concentrations in a 10-mm potassium phosphate buffer, pH 7.0. Subsequently, a 100-μl droplet of each suspension was put on a fluoroethylenepropylene surface and the profile of the droplet determined with a contour monitor as a function of time up to 2 h. The surface tension of these suspensions was then calculated from the droplet profiles with ADSA-P. The surface tension of suspensions of the two non-producing strains remained stable within 4 mJ·m⁻², whereas the surface tension of suspensions of five out of the nine S. mitis strains employed, including those of the known producer strains, decreased significantly (up to 26 mJ·m⁻²). This decrease was, in addition, concentration dependent. From these observations, we decided that all strains for which these concentration-dependent decreases were observed, could be regarded as biosurfactant producers. In order to rule out the possibility that the surface tension decreases observed were due to the collection of cells at the suspension-air interface, we investigated whether there was a relationship between surface tension decrease and hydrophobicity of the cells, as assessed by contact angle measurements and bacterial adhesion to hydrocarbons. Since no such a relationship was found, it can be concluded that ADSA-P is an excellent technique, based on using small amounts of cells to rapidly determine whether or not a bacterial strain produces biosurfactants.