Role of adherence in growth of Acinetobacter calcoaceticus RAG-1 on hexadecane

The high affinity of Acinetobacter calcoaceticus RAG-1 for liquid hydrocarbons permitted the isolation of a spontaneous nonadherent mutant, MR-481. Strain MR-481 exhibited no significant affinity for three test hydrocarbons, yet resembled the wild type in many properties, including production of the extracellular emulsifying agent emulsan. To study the role of adherence in growth on hydrocarbons, RAG-1 and MR-481 were compared for growth on hexadecane under conditions of limited agitation and at low initial cell densities. Adherent RAG-1 cells were able to grow rapidly under these conditions, whereas nonadherent MR-481 cells failed to grow for at least 54 h. However, the addition of emulsan either initially or at various times after inoculation enabled the nonadherent MR-481 cells to grow on hexadecane. Growth was not the result of reversion of MR-481 from nonadherent to adherent cells. The data demonstrate that adherence is a crucial factor in the growth of A. calcoaceticus RAG-1 on hexadecane in the absence of extracellular emulsification of the substrate.