Reinterpreting quorum sensing as positional sensing in bacterial communication

Background and Aims The phenomenon of bacterial cell-to-cell communication via n-acyl homoserine lactone molecules was examined in a simple series of direct microscopy tests aimed at verifying the postulates of the current quorum sensing working model. Methods Using a wild type strain of Rhizobium leguminosarum, we first devised a signal production system in which cell number, cell density and cell distance could be varied at leisure while maintaining the physiological status and preventing further cell divisions. The model was subsequently verified by epifluorescent microscopy in a tri-dimensional set up in LB agar, using an rfp-tagged AHL producing strain of Pseudomonas aeruginosa and a sensor Escherichia coli bearing a gfp-fused AHL reporter gene. Results Data obtained by measuring 697 individual cell-to-cell communication distances using the CMEIAS image analysis software showed that indeed quorum sensing can be reduced to an interaction involving as few as two individual cells, with a mean calling distance of 37.4 um and a maximum calling distance extended as far as 123.6 um. Direct microscopy showed that a single bacterium can produce sufficient signal in situ to activate a population of 180 bacterial cells. Conclusions The results indicate that quorum sensing does not comply with the paradigm of a mechanism simply depending on cell density. A novel concept of positionally-releated sensing, via mixing concentration gradients, is presented.