Five mutants of Pseudomonas taiwanensis VLB120ΔCeGFP showed significant autoaggregation when growing on defined carbohydrates or gluconate, while they grew as suspended cells on complex medium and on organic acids like citrate and succinate. Surprisingly, the respective mutations affected very different genes, although all five strains exhibited the same behaviour of aggregate formation. To elucidate the mechanism of the aggregative behaviour, the microbial adhesion to hydrocarbons (MATH) assay and contact angle measurements were performed that pointed to an increased cell surface hydrophobicity. Moreover, investigations of the outer layer of the cell membrane revealed a reduced amount of O-specific polysaccharides in the lipopolysaccharide of the mutant cells. To determine the regulation of the aggregation, reverse transcription quantitative real-time PCR was performed and, irrespective of the mutation, the transcription of a gene encoding a putative phosphodiesterase, which is degrading the global second messenger cyclic diguanylate, was decreased or even deactivated in all mutants. In summary, it appears that the trophic autoaggregation was regulated via cyclic diguanylate and a link between the cellular cyclic diguanylate concentration and the lipopolysaccharide composition of P. taiwanensis VLB120ΔCeGFP is suggested.
Keywords: Autoaggregation; BifA; Carbon source; Cyclic diguanylate; Hydrophobicity; Lipopolysaccharide.