3.1 | Twitching motility influences R. solanacearum micro-colony morphology in a density-dependent manner
We studied twitching motility in R. solanacearum at different dilutions. Microcolonies of R. solanacearum were observed at different dilutions after 16 h. Interestingly we observed that the shape of microcolonies was more like linear shape at higher concentrations (~105 CFU ml-1) whereas the shape of the microcolonies seems to be circular in shape when the bacterial concentration was low (~104 CFU ml-1). No such distinct colony morphology variation across different concentrations was observed in the case of E. coli where twitching motility is absent (Fig. 1). This suggested that magnitude of twitching motility influences colony morphology in this bacterium. While observing the twitching motility it is obvious that cells at the peripheral region of the spotting area exhibited stronger twitching motility than the cells towards the centre. It might be that bacteria in the peripheral region have dual experiences such as more moisture towards the centre of the spotted area and dryness towards the outward area. It indicated that a more fluidal environment results in lowering the twitching motility.
It is pertinent to note that bacterial cell concentration determines the shape of microcolonies. At higher cell concentrations such as 105 CFU ml-1, the microcolonies take a slender shape, unlike the lower cell concentrations such as 104 CFU ml-1 where the microcolonies take on a more circular shape (Fig. 1). It seems that at higher cell numbers, individual microcolonies tend to recognize the presence of its adjacent counterparts by quorum sensing and propel to a direction unexplored by other microcolonies attributing it an asymmetric, elongated shape. Besides controlling several other phenotypes across bacterial populations ; this is a distinct demonstration of quorum sensing affecting the shape of bacterial micro-colonies by piloting asymmetry in the magnitude of twitching motility in all directions. However, the microcolony margins of E. coli appear smooth irrespective of their cell concentrations. Therefore, it can be said with conviction that while twitching motility influences microcolony morphology, which in turn is influenced by cell density within a spotted area.