Dazzo, F. B. and S. Gantner. 2013. In situ calling distances and high population independent N-acylhomoserine lactone-mediated communication on plant root surfaces. Pages 785-789 in F. DeBruijn, ed. Molecular microbial ecology of the rhizosphere, Volume 2. J Wiley & Sons, Hoboken, New Jersey.
We performed computer-assisted microscopy at single-cell resolution to measure the in situ spatial scale of N-acylhomoserine lactone (AHL)-mediated cell-to-cell communication of Pseudomonas putida reporter strains colonized on tomato and wheat root surfaces. In situ image analysis indicated that the effective “calling distance” separating activated “AHL-sensor” cells from nearest “AHL-source cells” was most frequent at 4–5 µm, extended out to maximum distances of 37 µm in the root tip/elongation zone and out to 78 µm in the root hair zone. Also, small foci of clustered cells communicated with each other in remote locations that were more than twice the maximum measured calling distance from dense populations. Geostatistical analysis implicated the importance of AHL gradients mediating effective communication between remote attached cells. Thus, bacteria colonized on root surfaces communicate with each other not only within dense populations, but also can do so in small groups separated from dense populations over long-range distances. The results indicate that spatial aggregation readily produces local cell densities that exceed the threshold necessary for quorum-dependent processes. These results challenge the conventional view of a quorum group requirement of high population density for this type of bacterial communication. We propose that AHL-mediated bacterial communication is more common than currently predicted, and is governed more by the in situ spatial proximity of cells within AHL gradients than by a quorum group requirement of high population density.
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