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

The adhesion of a new, recently described species, Acinetobacter venetianus strain VE-C3 (Di Cello, F., M. Pepi, F. Baldi, and R. Fani, 1997. Res. Microbiol. 148:237-249), to diesel fuel (mixture of n-alkanes C12-C28) and n-hexadecane was studied and compared to that of Acinetobacter sp. RAG-1, which 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 strain was hydrophobic with an electrophoretic mobility, ?= -0.38x10-8 m2 V-1 s-1 and zeta potential, ?= -4.9 mV, while VE-C3 strain was hydrophilic, with ?= -0.81x10-8 m2 V-1 s-1 and ?= -10.5 mV. The microbial adhesion to hydrocarbon test (MATH) showed that RAG-1 strain was always hydrophobic whereas the hydrophilic VE-C3 strain became hydrophobic only after the exposure to n-alkanes. Adhesion of VE-C3 cells to diesel fuel was partly due to the production of capsular polysaccharides (CPS), which were stained with lectin concanavalin-A (Con-A) conjugated with fluorescein isothiocyanate and observed in situ using confocal microscopy. The lipopolysaccharide emulsan from RAG-1 strain, which was negative to Con-A, was stained with Nile Red fluorochrome instead. The confocal microscope observations at different times showed that VE-C3 strain underwent two types of adhesion: (1) cell-to-cell interactions, preceding the cell adhesion to the n-alkane and (2) incorporation of nanodroplets of n-alkane into the hydrophilic CPS to form a more hydrophobic polysaccharide/n-alkane matrix surrounding the cell wall. The incorporation of n-alkanes as nanodroplets into the CPS of VE-C3 cells might ensure the partitioning of the bulk apolar phase between the aqueous medium and the outer cell membrane and thus sustains a continuous growth rate over a prolonged period of time.

Acinetobacter venetianus; diesel fuel droplets; in situ electrochemical and molecular pobes

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