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.