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.