Asymmetric Adhesion Affects Microcolony Morphology

Thank you to the Microbiology Online for this chart, this website is a great source for the basics of microbiology

Colony morphology is the shape of bacteria when they grow together. It is the natural result of many individuals competing over the same limited space with many different shapes emerging, a few of which are shown on the right. As a surface is colonised and bacteria begin to grow on a solid surface they rely on their general adhesion to remain in contact and not be whisked away. After a few fissions a single cell has become a microcolony of many bacteria all adhering to the surface. At the start these spread outwards forming a monolayer of microorganisms only really limited by their container. How quickly a 3D structure develops depends on the exact nature of the boundary. For solid-liquid boundaries the bacteria will begin growing up into the liquid after a few division cycles while for solid-agarose (agar jelly) there will be considerable spread across the interface before any bacteria try growing perpendicular to it.

This protrusion into the third dimension occurs due to the fact that bacterial cells will elongate in their attempt to increase adhesion to the surface they cling to. This stretch results in pressure building up within the cells and when combined with jostling from all their neighbours trying to take room some cells become detached and hop onto a second layer.

This paper has taken to exploring the dynamic interaction between adhesion forces and cell tension forces and how this produces some of the structured microcolonies observed. To do this Escherichia coli, a kind of rod shaped bacteria responsible for urinary tract infections, had individual members tracked within a developing microcolony. It was observed that all bacteria elongate at the same rate, that older bacteria were pushed to the periphery and that the interactions between bacteria made small groups align over time. They then switched up the medium the bacteria were growing in and observed various colonies growing with various degrees of reduced surface adhesion. More specifically they noticed an asymmetry when it came to the bacteria elongation with a movement of their centre. This can be explained by the adhesion being stronger at the poles of the bacteria. It is believed that this is a key reason why circular colonies come into existence as opposed to the rhizoid structure seen in the diagram above.

Paper links: Asymmetric adhesion of rod-shaped bacteria controls microcolony morphogenesis