AbstractThe mechanisms underlying adaptive radiation or evolution have been extensively investigated using experimental bacterial populations in liquid cultures, referred to as microcosms. Evolving populations of Pseudomonas fluorescens SBW25 in static microcosms reproducibly lead to the emergence of the Wrinkly Spreader (WS) genotype. These produce a cellulose-matrix-based biofilm to colonise the airliquid (A-L) interface with significant fitness advantage over non-biofilm-forming competitors.
In this work, the first SBW25 colonists in static microcosms were shown to establish O2 gradients, thereby modifying the original environment to establish two new niches corresponding to a high-O2 region at the A-L interface and a low-O2 region lower down the liquid column. Greater O2 availability at the A-L interface was found to provide the selective pressure driving the emergence of the WS and underlay the fitness advantage WS have over non-biofilm-forming strains in this environment.
A second biofilm-forming mutant of SBW25, known as the CBFS (Complementary Biofilm Forming Strain), had been previously characterised. Here, wild-type SBW25 was shown to produce a third biofilm-type induced non-specifically by exogenous Fe, and referred to as the viscous mass (VM)-class biofilm. The CBFS, VM and WS biofilm-types could be differentiated by in situ measurements of biofilm attachment and strength, rheometry of biofilm samples, and strain characteristics. However, despite these differences, each provided similar levels of fitness in static microcosms subjected to increasing levels of physical disturbance. This suggests that each of the biofilm-types might arise independently in evolving SBW25 populations as the result of convergent evolution, in which the key ecological constraints are O2 availability and physical disturbance. However, invasion-from-rare experiments indicated that the WS was ecologically more successful, perhaps explaining why only WS-like genotypes have been isolated from evolving SBW25 populations in static microcosms.
Other pseudomonads had previously been shown to produce cellulose-based A-L interface biofilms in static microcosms. Here, a survey of New Zealand brown blotch-causing pseudomonads (BCP) recovered from white mushrooms (Agaricus bisporus) identified similar biofilm-producing, cellulose-expressing isolates. The ability to express cellulose by key BCP isolates was shown to be a fitness advantage in static microcosms, as well as on A. bisporus mushroom caps themselves. Cellulose-expression was also found to be of fitness advantage under water-limiting conditions, suggesting that cellulose may provide some resistance to water-stress in the natural environment.
The research undertaken for this Thesis comprise several significant advances in our understanding of the adaptive radiation of P. fluorescens SBW25 and the emergence of A-L interface biofilms in static microcosms. This work has identified O2 gradients and physical disturbance as the dominant environmental factors that guide the convergent evolution of biofilms in this environment. A consequence of convergent evolution in static microcosms is a variety of physically-different biofilms, all of which provide a fitness advantage over non-biofilm-forming competitors. It is also enlightening to realise that a common structural component of biofilms, cellulose, may also be used in another role to provide a fitness advantage in an ecologically more relevant and natural environment.
|Date of Award||Feb 2011|
|Supervisor||Andrew Spiers (Supervisor) & Wilfred Otten (Supervisor)|