Research on bacterial movement tends to focus on the rod-shaped bacteria. With the aid of small waving flagella, each bacterial cell can push itself in the direction it wishes to go. They can also move in groups, forming large swarms that ripple and slide their way across Petri dishes. Spherical cells had always been assumed to be relatively sessile, due to their non-streamlined shape, however recent research on the bacteria Serratia marcescens has show that it is capable of movement, even when the cells are spherical.

S. marcescens is normally a rod-shaped Gram negative bacteria while growing and developing. If grown in an overnight culture the cells become highly crowded and shrink slightly to form spheres (during the stationary phase). When placed in a single drop of culture these spherical cells were seen swimming upwards, to the top of the drop and away from the main bulk of bacterial cells. On the surface of the drop, the cells formed a monolayer and moved in dynamic whirls and jets as shown in the video below.

This movement was constant; not dependent on the size or shape of the droplet. As a control, some of the droplets were hung upside-down, and the same swimming behaviour was viewed. By using their flagella the bacteria were swimming around the droplet, forming little swirling vortexes as they all moved.

One of the factors that can affect bacterial movement is the amount of a chemical called surfactant that is secreted by the bacteria. The research compared the movement of three different strains of bacteria: one that produced no surfactant, one that produced an increased amount of surfactant, and one that produced surfactant but was unable to move. They found that the first two strains moved in the same way, whereas the non-mobile cells showed nothing other than a vague tendency to drift around. Introducing the non-mobile cells into the mobile population didn't encourage them to move either, while the other cells continued to swim up to the surface, the non-mobile ones stayed put.

A schematic side view, illustrating how cells swim to the upper surface of the drop, forming a dense monolayer. Image from reference 1.

As S. marcescens is also a rod shaped bacteria researchers were able to compare the movement of the spherical and rod-shaped cells in the same species. Modelling the movements of both they found that there were some differences in the type of movement and the rod-shaped cells moved slightly faster (there's a more complex mathematical analysis in the reference!). As the cells are rod-shaped during the early growth phase of the bacteria, it may be more important for them to be able to scuttle around faster while the later spherical shape occurs once the bacteria have settled down into a colony.


Reference 1: Rabani A, Ariel G, Be'er A (2013) Collective Motion of Spherical Bacteria. PLoS ONE 8(12): e83760. doi:10.1371/journal.pone.0083760

Image credit for featured image: Serratia marcescens on an agar plate.