1 | INTRODUCTION
Bacteria have developed a variety of motility mechanisms, which can be
categorized into two types: swimming motility in an aqueous medium and
twitching motility in a solid medium (Wadhwa et. al 2022).
Swimming motility is the individual cell movement in an aqueous medium
or over a semi-solid surface governed by rotating flagella. Twitching
motility is the flagella-independent movement of bacteria in a group
over solid surfaces driven by type IV pilus appendages (Liu et.
al 2001; Corral et. al 2020). Lautrop was the first to use the
term ”twitching motility” in 1961 to refer to the surface movement ofAcinetobacter calcoaceticus without the use of flagella (Lautrop
1961). The term originates from the discovery that cells moving in this
mode of motility appeared as a jerky movement when viewed under the
microscope, resembling twitching. Bacterial communities generally follow
this mode of motility for rapid colonization on new surfaces under high
nutrient availability as well as for the successful formation of biofilm
(Ward et. al 1997; Ward et. al 1999). Several bacteria
have been found to exhibit twitching motility among whichPseudomonas aeruginosa, Neisseria gonorrhoeae, and Myxococcus
xanthus have been studied extensively (Mattick et. al 2002).
Ralstonia solanacearum is a Gram-negative, β -
proteobacterium, that causes a lethal wilt disease in many host plants
(Genin and Boucher, 2002; Phukan et al., 2019; Naik et al., 2023). The
bacterium is a soil-borne, systemic phytopathogen but is also known to
infect several hosts without causing disease (Phukan et al., 2019;
Genin, 2010). It is a species complex armed with a wide array of
virulence determinants that allow it to infect over 200 crop species
belonging to 53 families (Genin, 2010). It has been observed that
twitching motility is one of the important phenomena in this bacterium
to colonize inside the host plant as well as disease progression to
cause systemic infection (Tans-karsten et. al 2001, Corralet. al 2020). Out of the several virulence determinants, the TFP
besides aiding in twitching motility also mediates diverse processes
such as biofilm formation, adhesion, aggregation, horizontal gene
transfer and virulence. The TFP are surface-exposed slender appendages
that are made of repeating PilA pilin subunits . They are polarly
localized in rod-shaped bacteria such as Pseudomonas aeruginosa(Talà et al ., 2019) and R. solanacearum but tend to be
peritrichously piliated in cocci-shaped Neisseria gonorrhoeae .
Albeit executing scores of biological processes, the fundamental
mechanism of TFP operations remains unchanged i.e., extension,
attachment and retraction. Recent studies on the ‘grouped’ behaviour of
twitching motility on rod-shaped bacteria throw some light on how PilG
and PilH aid in bacterial navigation by inducing polarization of the
adenosine triphosphatase PilB favouring forward migration and also its
reversal upon collision respectively to direct twitching in the response
to spatially resolved signals from the TFPs by exercising mechanotaxis .
Although various studies reported about the twitching motility inR. solanacearum and the factors associated with it (Kai et al.,
2015; Kang et al., 2002; Ray et al., 2015; Singh et al., 2018), its
impact on the microcolony shape and size (Bhuyan et al 2023) in this
bacterium in a density-dependent manner has not been reported yet. More
importantly, the dynamics as well as the directionality of twitching
motility exhibited by peripheral cells in a colony are yet to be studied
adequately. In this manuscript, we have tried to address these questions
by observing the twitching motility in the microcolonies through a
time-lapse video.