Morphological and nanostructural surface changes in Escherichia coli over time, monitored by atomic force microscopy.

The present study aims at evaluating intrinsic changes in Escherichia coli (E. coli) surface over time, by Atomic Force Microscopy (AFM). For that purpose, bacteria were immobilized on mica or on mica previously functionalized by the deposition of a polyelectrolyte multilayer cushion. AFM images reveal that E. coli population goes through different stages. Firstly, after a week, the number of healthy bacteria decreases resulting in a release of cellular components which likely become, in turn, a nutrition source for increasing the healthy population after around two weeks. Finally, after one month, most of the bacteria is dead. Our study shows a transition of a healthy rod-shaped bacterium to a dead collapsed one. Most importantly, along with the morphological evolution of bacteria, are the structure changes and the mechanical properties of their outer membrane, emphasized by AFM phase images with very high resolution. Indeed, the surface of healthy bacteria is characterized by a phase separation pattern, thereafter mentioned as "ripples". Bacterial ageing goes along with the loss of this organized structure, turning into circular areas with irregular boundaries. These changes are likely caused by a re-organization, due to external stress, of mainly lipopolysaccharides (LPS) present in the outer membrane of E. coli.