FORMATION OF MICROSCOPIC DROPS AROUND BACTERIAL AGGREGATES ON DRYING SURFACES

While water is essential to life, the largest terrestrial microbial habitats – soil, roots, and leaf surfaces – are not constantly saturated with water and are subjected to drying events on a daily basis. Bacterial cells in these habitats are observed as solitary cells and as aggregates of different size and shape. We ask how the type and size of an aggregate affect it propensity to adsorb water and how these features affect cells` survival. To this end, we have constructed an experimental system that provides saturated conditions for bacterial growth for a defined period of time and then gradually evaporates and becomes dry. Microscopy aided inspection of the dry surface revealed three distinct bacterial organization forms: (1) solitary cells (2) round dome-like aggregates (3) flat aggregates with irregular shape that originate from the water-air interface. Interestingly, while the surface itself appeared to be completely dry, the surface-attached cells where engulfed with micro-droplets of different sizes and shapes (figure 1). These micro-droplets maintained steady for long period of time (hours to days). We find a clear correlation between aggregates` characteristics and the surrounding drop size and shape. Similar results were observed with two distinct bacterial species (Pseudomonas Putida and P. Fluorescens – a soil and plant-surface model strains) – supporting the view that these phenomena is not strain specific. Currently, we are assessing the survival probability within hydrated aggregates over time and with respect to the drop size and shape. The formation and retention of such micro-droplets around aggregates, may explain how bacterial cells in terrestrial habitats keep their micro-environment hydrated, a feature that is likely key for their survival.