Mechanical properties of hexadecane-water interfaces with adsorbed hydrophobic bacteria

This study focuses on how intact, hydrophobic bacteria in their stationary (i.e., non-dividing) phase could adsorb onto the hexadecane-water interface and alter its mechanical properties. The two strains of bacteria used in forming the interfacial films were Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c. Using the dynamic pendant drop technique, the film interfacial tension was monitored as the surface area was made to undergo transient changes. Under static conditions, both types of bacteria had no effect on the interfacial tension. When subjected to transient excitations, however, the two bacterial films exhibited clear and qualitatively similar rheological properties: they responded as two-dimensional Maxwellian materials when the interfacial areas were dilated suddenly, but appeared to be purely elastic upon rapid area compression. Such rheological behaviours are "non-linear" in that the responses of the tension to area dilation and contraction are not mirror images of one another. Despite their qualitative similarities, the two types of film had very distinct film elasticities and relaxation times. The most striking difference between the two bacterial films was revealed under continuous reduction of area, when the A. venetianus RAG-1 system displayed a "paper-like" interface, whereas the interface of the R. erythropolis 20S-E1-c system was "soap film-like". These macroscopic observations could be explained by the surface ultrastructures of the two cell strains determined using transmission electron microscopy.