Bacterial motility and chemotaxis in geometrically restrictive environments
Description
Motile bacteria swim by rotating their helical flagella. In the absence of any chemical concentration gradients in their surrounding medium, the swimming behavior of motile bacteria is referred to as random motility. At population level, bacterial swimming can be characterized by the random motility coefficient, whereas at individual cell level, swimming speed, tumbling frequency and the index of directional persistence are used to describe bacterial swimming. In the presence of chemical concentration gradients, bacteria move toward favorable substances termed chemoattractants and away from repellents. This phenomenon is called chemotaxis and it can be described through the 'chemotaxis coefficient' at population level To study random motility, experiments were conducted in a series of fine capillaries with inside diameters from 3 $\mu$m to 50 $\mu$m to study the geometrical restriction effects on bacterial random motility and the interactions between cells and the capillary wall, and among cells. These studies demonstrate, for the first time, that Escherichia coli cells were observed to swim unidirectionally in capillaries with inside diameters less than the length of the flagella. In 3$\mu$m-diameter capillaries, due to the extreme restriction, E. coli cells were found to form aggregates and move as clusters, a phenomenon which was not observed at larger-size capillaries. This aggregation was reversible. In the 3$\mu$m-diameter-capillaries, bacterial clusters could reverse their swimming directions even though single cells were unable to do so. Single-cell motility parameters-swimming speed, run length time and turn angle - were measured for E coli K 12 in the 6$\mu$m-diameter capillary tube For the study of chemotaxis, a new method was developed which enables chemotaxis parameters to be measured at a single-cell level inside a capillary. The chemotaxis chamber consists of two reservoirs communicating through a 50-$\mu$m diameter capillary tube. Chemotaxis parameters were measured inside the capillary using image analysis, after a near-linear attractant concentration gradient had been generated along the capillary by diffusion. Compared to previously published techniques, this method provides a well-characterized chemoattractant concentration profile in addition to allowing single-cell parameters to be measured inside a fine capillary. This procedure was used to measure the single-cell chemotaxis parameters for E. coli K12, and the results were in agreement to those of published studies on single E. coli cells chemotaxing in bulk