No abstract

For enumerating viable bacteria, traditional dilution plating to count colony forming units (CFU) has always been the preferred method in microbiology owing to its simplicity, albeit laborious and time-consuming. Similar CFU counts can be obtained by quantifying growing microcolonies in conjunction with the perks of a microscope. Here, we employed a simple method of five microliter spotting of differently diluted bacterial culture multiples times on a single petri plate followed by finding out CFU by counting microcolonies using phase contrast microscope. In this method within four-hour period CFU of an Escherichia coli culture can be found out. Further, within ten-hour period, CFU in a culture of Ralstonia solanacearum, a bacterium with generation time around 3 h, can be estimated. The CFU number determined by microcolonies observed is comparable with that obtained by the dilution plating method. Microcolonies number observed in the early hours of growth (2 h in case of E. coli and 8 h in case of R. solanacearum) were found to remain consistent at later h, though there was noticeable increase in size of the colonies. It suggested that microcolonies observed in the early hours indeed represent the bacterial number in the culture. Though manual, the method is less labor intensive apart from being simple, easy, economical and rapid.

Ralstonia solanacearum is a rod-shaped phytopathogenic bacterium that causes lethal wilt disease in many plants. On solid agar growth medium, in the early hour of the growth of the bacterial colony, the type IV pili-mediated twitching motility, which is important for its virulence and biofilm formation, is prominently observed under the microscope. In this study, we have done a detailed observation of twitching motility in R. solanacearum colony. In the beginning, twitching motility in microcolonies was observed as a density-dependent phenomenon that influences the shape and sizes of the microcolonies. No such phenomenon was observed in Escherichia coli, where twitching motility is absent. In the early phase of colony growth, twitching motility exhibited by the cells at the peripheral region of the colony was more prominent than the cells towards the centre of the colony. Using a time scale photography and merging those into a video, twitching motility was observed as an intermittent phenomenon that progresses in layers in all directions as finger-like projections at the peripheral region of a bacterial colony. Each layer of bacteria twitches on top of the other and produces a multi-layered film-like appearance. We found that the duration between the emergence of each layer diminishes progressively as the colony becomes older. This study on twitching motility demonstrates distinctly heterogeneity among the cells within a colony regarding their dynamics and the influence of microcolonies on each other regarding colony shape and size.