Bacterial competition: surviving and thriving in the microbial jungle

Abstract

Most natural environments harbour a stunningly diverse collection of microbial species. In these communities, bacteria compete with their neighbours for space and resources. Laboratory experiments with pure and mixed cultures have revealed many active mechanisms by which bacteria can impair or kill other microorganisms. In addition, a growing body of theoretical and experimental population studies indicates that the interactions within and between bacterial species can have a profound impact on the outcome of competition in nature. The next challenge is to integrate the findings of these laboratory and theoretical studies and to evaluate the predictions that they generate in more natural settings.

Figure 1. Examples of interference competition between bacterial species

Top panel:. Many bacterial species produce antimicrobial toxins which facilitate interference competition with other species; pictured is a zone of inhbition in a lawn of Bacillus subtilis surrounding a paper disk soaked with culture supernatant from Burkholderia thailandensis, an antimicrobial producer (picture courtesy B. Duerkop). Middle panel: In biofilm cocultures, Pseudomonas aeruginosa (red cells) blankets the surface of Agrobacterium tumefaciens (green cells- overlay of the two cells is yellow). Biomass of A. tumefaciens decreases in the biofilms over time (left panel represents 24 h growth, right is 164 h), in a mechanism at least partly dependent on quorum sensing by P. aeruginosa (See box 2). Figure reproduced with permission from . Bottom panel: Overproduction of EPS by mutant strains of Pseudomonas fluorescens (middle, compared to the parent at left) enables these organisms to position themselves in the favorable environment of the air-liquid interface of liquid cultures, where oxygen is more plentiful. However, EPS production is a phenotype vulnerable to social cheating. If sufficient cheaters that fail to produce EPS accumulate in the floating mat, it will collapse (right). Left and middle panels reproduced with modification from , right panel courtesy P. Rainey.

Figure 2. Non-transitive competition networks

A model Escherichia coli non-transitive competition network, first described in ref. . A strain producing a colicin toxin (red) outcompetes a sensitive strain (blue), which outcompetes a resistant strain (green), which in turn outcompetes the producing strain.

Figure 3. Simplified models of siderophore mediated bacterial competition

a | Three competitive scenarios in iron limiting conditions where the utilization of a siderophore is necessary for iron acquisition. Species 1* represents a cheater of Species 1 that has lost the ability to produce siderophores but maintains the ability to utilize siderophores produced by cooperating individuals. Species 1 produces a higher affinity siderophore than does Species 2, which allows Species 1 to monopolize the available iron.. Competition between Species 1 and Species 3 is analogous to that between Species 1 and 1*, however, Species 3 has evolved the ability to utilize the heterologously produced siderophore of Species 1 and never had the ability to produce this siderophore. b | The predicted outcome of competition between Species 1 and Species 1* in iron limiting conditions. The * along the x-axis of the graph indicates the mutational event that eliminates the ability of Species 1* to produce the siderophore resulting in the cheating phenotype. c | The predicted outcome of competition between Species 1 and Species 2 , and d | the predicted outcome of competition between Species 1 and 3.