Variations in moisture retention of contact-printed soil bacteria surface colonies using confocal microscopy

The retention of soil moisture enables vegetation to remain viable between rainfall or irrigation events. Microbial production of extracellular polymeric substances (EPS) can influence soil moisture in two ways: first, by retaining moisture directly in the carbohydrate-rich hydrogel material itself; and second, by promoting an aggregated structure of soil grains. In whole soils, it can be difficult to decouple these two effects. The objective of this study was to measure the direct moisture-retaining properties of extracellular polymeric substances produced by a common soil isolate. Here, water retention in biofilm-like colonies of GFP-expressing Sinorhiobium meliloti were measured under controlled relative humidity as a volume change with time using confocal microscopy. S. meliloti strains with muccoid (EPS+) and non-muccoid (EPS-) traits were compared. Methods to collect and contact-print biofilm-like colonies on a glass substrate and to quantify volume changes with time were developed. The results show EPS+ biofilms are more resistant to drying than are EPS-biofilms. Under identical atmospheric conditions, similarly-sized EPS-biofilms decreased in volume to 17% of their original size, while EPS+ biofilms retained 50% of their original volume. This work may enable development of environmental biotechnologies to improve moisture retention and enhance agricultural productivity.