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#5 The expansion of Pseudomonas aeruginosa

Swarming is an important mechanism for bacterial colonization. Many hypothesis has been proposed to explain the phenomenon of swarming such as quorum sensing [1] or nutrient diffusion coupled with reaction-diffusion model [2]. However, the authors of the suggested reading believe that the swarming of Pseudomonas aeruginosa (a deadly pathogen) and other bacteria is accomplished by the gradient of surface tension, or the Marangoni flow. (Please check it out with the suggested video link below.)
Let’s watch the suggested video. The surface tension is greater in water than in soap water. As a drop of soap water fall upon the water surface, the water will “pull” the soap water. Assume area of Petri dish A, surface tension of water γ1, surface tension of soap water γ2. γ1>γ2. Pulling of soap water film by water could change the free energy of the system by (γ2-γ1)A < 0 and it is therefore favorable.

What does this have to do with the swarming of Pseudomonas? Pseudomonas could produce a substance decreasing surface tension called rhamnolipid. The more rhamnolipid produced, the lower the surface tension becomes. Since the Marangoni flow streams from places with lower surface tension to places with higher surface tension, the production of rhamnolipid makes swarming of Pseudomonas possible.

To test the importance of such mechanism on the swarming of Pseudomonas, the authors mutate the gene encoding the enzyme responsible for rhamnolipid production. They found that the swarming was greatly suppressed after the mutation. They also used fluorescent proteins to confirm that the amount of rhamnolipid production is correlated with the bacterial density. Finally, they added some rhamnolipid at somewhere the bacterial had not colonized yet, and the swarming was also greatly suppressed, which proved that it was the “gradient” rather than the absolute scale of surface tension that matters.

Many pathogens other than Pseudomonas, such as Mycobacterium spp., Bacillus subtilis, Candida spp., Penicillium spp., Aspergillus spp., produce substances that decrease the surface tension. So does our type II pneumocyte. How does these substance affect their pathogenesis? Are there any possible application utilizing this mechanism for medical treatment? That’s what we are going to look for.
*Suggested reading:
Fauvart, M., Phillips, P., Bachaspatimayum D., Verstraeten N., Fransaer J., Michiels J. and Vermant J.. (2012). Surface tension gradient control of bacterial swarming in colonies of Pseudomonas aeruginosa. Soft Matter 8: 70-76.
[1] Daniels, R., Vanderleyden, J., Michiels, J. (2004). Quorum sensing and swarming migration in bacteria. FEMS Microbiology Review 28(3): 261-289.
[2] Netotea, S., Bertani, I., Steindler, L., Kerényi, A., Venturi, V. & Pongor S. (2009). Biology Direct 4: 6.
*Video about Marangoni flow: