Bacteria must be able to find food in order to survive. They have evolved various chemotactic strategies to efficiently locate and track nutrient gradients, several of which have been defined. The strategies usually consist of a series of run phases in which the bacteria swim in straight lines followed by phases of tumbles, arcs, stops and reversals. A tight control over these phases allows them to direct and adapt movements towards nutrient sources.
Their intricate pathways of signalling proteins allow them to detect chemical changes in the environment which in turn will affect their run phase. Bacteria must maintain straight trajectories to pick up vital environmental cues and react appropriately. However due to their size and shape, the lengths of their movements are restricted by Brownian motion.
The rotation friction (fr) co-efficient is the cell’s resistance to being rotated, which is dependent on both cell size and shape. Several models have been suggested for spherical and ellipsoidal cell types. However, despite theoretical modelling, there has been little empirical evidence.
August’s Paper of the Month, from the Universities of York and Lincoln, focussed on validating the theoretical understanding of how cell size and shape affect bacteria run phases.
To create bacteria with different aspect ratios they treated E.coli with cephalexin, which was found to elongate the cell. They compared this form to the normal wildtype E.coli. With increasing cell length, they found an increase in flagella along the cell body.
Flagella are primarily motile appendages, but also sensory, and are found commonly in the cell bodies of bacteria.
Using phase contrast microscopy, they then recorded and tracked both the control and elongated group. Samples were imaged while placed on a Linkam PE-100 ZAL system heated to 33°C.
Their results indicated the elongated cells had shorter runs but longer tumble phases. This finding agreed with the veto model which suggests an increase in flagella increases the average tumble time.
Although mechanistically different, they also found elongated E.coli performed a run and reverse strategy. This pattern has been described within natural populations of marine and soil bacteria and has been found to be advantageous within these particular niches.
When asked about the role of the stage, Dr Oscar Guadayol said, “For this kind of study, the ability to perfectly control the environment at the microscale is critical, and thus the PE100 has become an absolutely essential piece of equipment in our everyday exploration of the microbial behaviour.”
Their work experimentally demonstrated long-standing theoretical predictions about how cell elongation may affect the capability of bacteria to swim and to navigate their chemical landscape and how different morphologies can lead to vital changes in motility patterns.
They are now taking their results one step further and using microfluidic devices with the PE100 to characterize bacterial chemotaxis.
A microfluidic chip placed on a Linkam PE100 ZAL system.