Bacterial evolution in simple microcosms

Microcosms have been used extensively to investigate ecological factors, evolutionary dynamics and the underlying molecular biology of adapting populations of bacteria in experimental evolution studies. One such experimental system has combined simple glass vials containing King’s B liquid growth medium with the model soil and plant-associated pseudomonad, Pseudomonas fluorescens SBW25. In these, evolution reproducibly generates the Wrinkly Spreader genotype, a novel class of adaptive mutations that enable Pf. SBW25 to produce a robust, physical cohesive–class biofilm at the air-liquid interface. The analysis of Wrinkly Spreader mini-Tn5 mutants led to the identification of cellulose as the main matrix component of the biofilm, and ultimately to the discovery of the mutation in a chemosensory-like system that activated the Wrinkly Spreader phenotype through the over-production of cyclic-di-GMP. Parallel ecological investigations have shown the importance of O2 availability for the evolution and success of the Wrinkly Spreader. The initial colonisers establish an O2 gradient, dividing the microcosm into a 100 – 200 µm high-O2 zone at the air-liquid interface, and second, deeper low-O2 zone in the rest of the liquid column. Growth is O2–limited in King’s B medium, and it is the domination of the air-liquid interface that guarantees access to O2 and underlies the fitness advantage the Wrinkly Spreader has over non-biofilm–forming competitors including the ancestral wild-type Pf. SBW25. However, recent experiments demonstrate that Wrinkly Spreader fitness is reduced in static microcosms containing lower levels of nutrients, or in those in which King’s B liquid viscosity has been increased. These findings demonstrate that the Wrinkly Spreader biofilm is a finely-balanced solution to the colonisation of the air-liquid interface in static microcosms, and changing environmental conditions will impact directly on Wrinkly Spreader fitness. This work highlights the importance of environmental factors, which along with competition, help guide the evolution of Pf. SBW25 in simple microcosms, invariably leading to the rise of the Wrinkly Spreaders.