FIGURE 3 | Clonal relationship and intraspecies signalling.

a | For most quorum-sensing bacteria, intraspecies cell?cell signalling predominates in clonal populations of cells. The role of cell?cell signalling is hypothesized to function most significantly early during colonization of a new site. Microcolonies will form from single cells and will therefore be clonal in nature. Where cell?cell signalling would provide a competitive advantage, quorum-sensing-positive (QS+) cells will form microcolonies more efficiently. Even if cheaters accumulate within a maturing community, these cells will be at a disadvantage in subsequent colonization events. For example, in Pseudomonas aeruginosa it has been shown that quorum sensing has an important role in colonization and virulence in cystic fibrosis, and in most patients it seems that infection is established clonally⁹¹. Isolates from patients are predominantly N-acyl homoserine lactone (AHL) producers, however isolates that are defective in AHL production are also found⁹². This behaviour is not restricted to adhered populations, as illustrated, but could occur effectively in non-attached populations of aggregates or cells that grow in static conditions in which local concentrations of signals (and extracellular enzymes) could accumulate around cells. b | For bacteria that undergo fruiting-body formation and sporulation, such as Myxococcus species^{52, 53, 54, 55, 56}, colony formation is fundamentally different. The initial event involves the aggregation of cells (triggered by a signalling pathway called A-signalling, in the case of Myxococcus xanthus) followed by the formation of a fruiting body (involving a contact-dependent signalling pathway called C-signalling). Last, a fraction of cells within the fruiting body develop into spores (thick black outline) and will seed the next generation of free-living cells. Because the initial event is an aggregation of cells within a local area, there is no guarantee of clonality. The pathway on the left would prevail if there was a mechanism to prevent non-signalling cells from aggregating, and the pathway on the right would occur if cheating occurs.

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