Our results show a striking parallel between the metabolic down-modulation induced by contact with the synthetic surface of a particle and that induced by cell-cell contact, suggesting that the molecular details of the contact play no determining role in signaling. However, there is no clear-cut conclusion concerning the fundamental difference between cellartificial surface contact and cell-cell contact in the context of bacterial adhesion. Due to the presence in the cell culture Ro 67-7476 supernatant of a variety of surface active molecules such as proteins and polyelectrolytes, the description of cell/synthetic surface Ro 15-4513 interactions according to a physico-chemical model is hazardous, and the hypothesis of specific interactions between substrate-adsorbed biopolymers and cell surface receptors cannot be ruled out. Again, since experimental data are often the result of biofilms established over several hour or day time scales, at a time when cell-substrate interactions are masked by the predominance of cell-cell interactions, any particular influence of the substrate upon development of the adhesive community is difficult to detect. Here, since we individually and simultaneously monitored each type of contact in the same sample, we were able to demonstrate that formation of cell-cell contact induced a cell response similar to the formation of cell-synthetic surface contact. Recently, based on growth rate, tolerance to antibiotics and extracellular matrix property criteria, Alhede and co-workers pointed out that Pseudomonas aeruginosa floating aggregates and surface-attached biofilm shared most phenotypes, suggesting that the substrate per se does not play a determining role in establishment of biofilm properties. These behaviors might, of course, depend on the nature of the adhesive surface, although this point remains difficult to clarify in the context of bacterial adhesion. In our experiments, the polystyrene surface of the particles is expected to be neutral; in fact, it exhibited a slight negative charge when suspended in cell culture minimal medium. In previous work, we showed that the particle surface was quickly converted to being negatively charged in the presence of cell culture medium independently of the initial physico-chemical properties of the particles, explaining the similar colonization kinetics observed using different particles. This indicates that the substratum initial properties are often masked by a conditioning film in the presence of bacterial cell culture products. The similarity of the cell response to a single contact with another cell and with a synthetic surface also enables hypothesizing surface sensing mechanisms driven by physical forces.