Abstract
Competition between species plays a central role in the activity and structure of communities. Stable co-existence of diverse organisms in communities is thought to be fostered by individual tradeoffs and optimization of competitive strategies along resource gradients. Outside the laboratory, microbes exist as multispecies consortia, continuously interacting with one another and the environment. Survival and proliferation of a particular species is governed by its competitive fitness. Therefore, bacteria must be able to continuously sense their immediate environs for presence of competitors and prevailing conditions. Here we present results of our investigations on a novel competition sensing mechanism in the rhizosphere-inhabiting Pseudomonas putida KT2440, harbouring gfpmut3b-modified KanR TOL plasmid. We monitored benzyl alcohol (BA) degradation rate, along with GFP expression profiling in mono species and dual species cultures. Interestingly, enhanced plasmid expression (monitored using GFP expression) and consequent BA degradation were observed in dual species consortia, irrespective of whether the competitor was a BA degrader (Pseudomonas aeruginosa) or a non-degrader (E. coli). Attempts at elucidation of the mechanistic aspects of induction indicated the role of physical interaction, but not of any diffusible compounds emanating from the competitors. This contention is supported by the observation that greater induction took place in presence of increasing number of competitors. Inert microspheres mimicking competitor cell size and concentration did not elicit any significant induction, further suggesting the role of physical cell-cell interaction. Furthermore, it was also established that cell wall compromised competitor had minimal induction capability. We conclude that P. putida harbouring pWW0 experience a competitive stress when grown as dual-species consortium, irrespective of the counterpart being BA degrader or not. The immediate effect of this stress is a marked increase in expression of TOL, leading to rapid utilization of the available carbon source and massive increase in its population density. The plausible mechanisms behind the phenomenon are hypothesised and practical implications are indicated and discussed.
Highlights
Competition between species plays a central role in the activity and structure of communities [1]
The specific questions we raised were: (a) how would P. putida respond to the presence of another species, which might be a potential competitor for the resources? (b) How does it sense the presence of competitors? (c) Once in the competition race, does it modulate its plasmid expression for its own advantage? In this paper we examine these issues and demonstrate using microcosm experiments that planktonically growing P. putida senses presence of competitors by physical cell-cell interactions and not by means of diffusible substances and that such sensing is followed by enhanced expression of genes on plasmid pWWO, aiding rapid utilisation of the available carbon sources and massive increase in its population density
Samples were drawn at different intervals and analyzed for residual benzyl alcohol (BA)
Summary
Competition between species plays a central role in the activity and structure of communities [1]. Outside laboratories microorganisms usually coexist in multicellular communities and compete with one another for limited natural resources. This kind of competition between microbes plays a major role in framing the community structure and, in turn, helps proliferation of certain species in a given niche, where appropriate strategies help it to outcompete others [2]. It is widely demonstrated that plasmid replication and expression are dependent on various environmental stresses, including amino acid starvation, heat and cold shock responses, salt concentration, pH, etc. Extensive research has been carried out to study the biochemistry of replication, copy number maintenance and partitioning of plasmids, there are few reports focusing on the mechanisms underlying modulation of plasmid expression in response to environmental stimuli [6]. The role of plasmids in competitive survival strategies is less researched, except for a few reports where models have been developed for competitive behaviour of plasmid-bearing and plasmid-free organisms in fermentors and bioreactors [7,8,9]
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