Adhesion of Acinetobacter venetianus to diesel fuel droplets studied with in situ electrochemical and molecular probes

The adhesion of a recently described species, Acinetobacter venetianus VE-C 3 (F. Di Cello, M, Pepi, F, Baldi, and R, Fani, Res. Microbiol. 148:237-249 , 1997), to diesel fuel (a mixture of C-12 to C-28 n-alkanes) and n-hexadec ane was studied and compared to that of Acinetobacter sp, strain RAG-I, whi ch is known to excrete the emulsifying lipopolysaccharide, emulsan, Oxygen consumption rates, biomass, cell hydrophobicity, electrophoretic mobility, and zeta potential were measured for the two strains. The dropping-mercury electrode (DME) was used as an in situ adhesion sensor. In seawater, RAG-1 was ;ydropphobic, with an electrophoretic mobility (mu) of -0.38 x 10(-8) m (2) V-1 s(-1) and zeta potential (zeta) of -4.9 mV,while VE-C3 was hydrophi lic, with mu of -0.81 x 10(-8) m2 V-1 s(-1) and zeta of -10.5 mV,The microb ial adhesion to hydrocarbon (MATH) test showed that RAG-1 was always hydrop hobic whereas the hydrophilic VE-C3 strain became hydrophobic only after ex posure to n-alkanes, Adhesion of VE-C3 cells to diesel fuel was partly due to the production of capsular polysaccharides (CPS), which were stained wit h the lectin concanavalin A (ConA) conjugated to fluorescein isothiocyanate and observed in situ by confocal microscopy, The emulsan from RAG-I, which was negative to ConA, was stained with Nile Red fluorochrome instead. Conf ocal microscope observations at different times showed that VE-C3 underwent two types of adhesion: (i) cell-to-cell interactions, preceding the cell a dhesion to the n-alkane, and (ii) incorporation of nanodroplets of n-alkane into the hydrophilic CPS to form a more hydrophobic polysaccharide-n-alkan e matrix surrounding the cell wall. The incorporation of n-alkanes as nanod roplets into the CPS of VE-C3 cells might ensure the partitioning of the bu lk apolar phase between the aqueous medium and the outer cell membrane and thus sustain a continuous growth rate over a prolonged period.