Role of Interfacial Tensions in the Translocation of Rhodococcus erythropolis during Growth in a Two Phase Culture

Rhodococcus erythropolis PR4 is an alkane-degrading bacterium, which grows well in media containing high concentrations of alkanes. These properties give the organism potential in the bioremediation of various environments contaminated by alkanes. In this study, we report the translocation of R. erythropolis PR4 from an aqueous phase to an alkane phase during growth in a two phase culture medium. When the alkane chain length was between C10 and C12, PR4 was located at the aqueous-alkane interface, but when the alkane chain length was above C14, PR4 translocated into the alkane phase. Complete translocation into alkane phase was accompanied by normal growth, whereas interfacial localization hampered growth, indicating that localization among other possible factors, play an important role in the growth of R. erythropolis PR4 in two phase cultures. The PR4 cell surface was physico-chemically characterized in terms of its cell surface charge and surface free energy. Contact angles were measured on bacterial lawns, followed by thermodynamic analyses of Gibbs free energies for localization of PR4 in the aqueous or alkane phase or at the interface. Although entry into the alkane phase of PR4 grown in the presence of both C12 and C19 was thermodynamically favorable, translocation from the inside of the alkane phase to the interface was only favorable for PR4 grown in the presence of C12. In line with these thermodynamic analyses, two phase partitioning showed that PR4 grown in the presence of C12 and C19 were more hydrophobic than PR4 grown in the presence of lower alkanes, while C12 grown bacteria were less lipophilic than C19 grown bacteria. In conclusion, the localization of R. erythropolis PR4 in a two phase culture medium is thermodynamically driven to facilitate its optimal growth.