Curiously, we NQ 301 observe quorum NSC 663284 sensing induction is not simply a population-based event on these semi-solid surfaces. Large populations of cells, seemingly sufficient for a quorum, are present on all surfaces examined. Thus, the mere presence of a threshold quorum sensing population does not yield tendrils and increased swarming on hard agar. The influence of quorum sensing upon swarm motility has previously been shown to be conditional with changes to the carbon source. These results showing decreased fluorescence of transcriptional reporters for rsaL and rhlA on hard agar suggest a limitation in quorum sensing induction for hard agar-grown populations. Our results for tendril-forming soft agar swarms are largely in agreement with another study that showed upregulation of quorum sensing genes under swarming conditions. It is not yet clear, however, why quorum sensing is limited on harder agar. Quorum sensing limitation during swarming has been used to explore the role of ����cheaters���� that bring about a ����collapse���� of swarming when quorum sensing is not sufficient. A potential explanation for our results and others may be surface variable activity of RsmA, a protein shown regulate quorum sensing and rhamnolipid production. Limitations to quorum sensing induction seemed not to be explained by limited exposure of bacteria to AHL signals on hard agar. Based on a study by Dulla and Lindow that investigated P. syringae aggregation upon plant leaves, one might expect quorum sensing to initiate more quickly with limited diffusion and limited surface water. Here we observe the opposite: cell aggregates that develop on hard agar show limited quorum sensing induction. Further, the addition of exogenous AHL does not lead to increased tendril swarming. This attempt to artificially induce quorum sensing with exogenous signal for cells growing on hard agar plates did not promote tendril formation. Lastly, rhamnolipid tendril swarms were not affected with changes to the carbon substrate concentration, and conversely, these changes did not stimulate tendril formation on hard agar. There is clearly a need to understand the development of subpopulations that influence swarming. Additionally, the recent report by Glick et al. of the importance of rhamnolipid to type IV pili-mediated twitching motility may point to a more general response of P. aeruginosa to increase rhamnolipid and surface motility under conditions of micronutrient limitations that were not examined here.