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.
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