Bacterial survival in microscopic droplets

Abstract

Plant leaves constitute a huge microbial habitat of global importance. How microorganisms survive the dry daytime on leaves and avoid desiccation is not well-understood. There is evidence that microscopic wetness in the form of thin films and micrometer-sized droplets, invisible to the naked eye, persists on leaves during daytime due to deliquescence - the absorption of water until dissolution - of hygroscopic aerosols. Here we study how such microscopic wetness affects cell survival. We show that, on surfaces drying under moderate humidity, stable microdroplets form around bacterial aggregates due to deliquescence and capillary pinning. Notably, droplet-size increases with aggregate-size and the survival of cells is higher the larger the droplet. This phenomenon was observed for 13 different bacterial species, two of which - Pseudomonas fluorescens and P. putida - were studied in depth. Microdroplet formation around aggregates are likely key to bacterial survival in a variety of unsaturated microbial habitats, including leaf surfaces.