Use of microbes to fight microbes

Microbes face a constant challenge to survive in any given ecological niche. Each microbial cell must compete with friends and foes alike for usually scarce resources (macro- and micronutrients), be able to react quickly to changing external conditions, and be able to rapidly respond to threats from existing and newly arrived competitors. This competition is particularly acute on body surfaces and reaches its apex in the gut, which contains what is probably the densest, most highly populated ecological niche on Earth. It is no wonder then that most microbes which occupy these niches have developed sophisticated strategies to promote their survival under these challenging conditions. These include strategies aimed at the destruction of rivals (and very often the most closely related strains or species can represent the greatest competitive challenge for any given niche), the expression of structures and strategies designed to assist a microbe to occupy selected sites within a given niche, tactics such as motility, and strategies aimed at outcompeting other microbes for scarce resources. It is tempting to consider that we could take advantage of these strategies to design therapies which would use harmless commensal bacteria to combat pathogenic bacteria. There are many instances where probiotic bacteria have been shown to play a role in limiting or even preventing infection. This chapter will consider some of the mechanisms involved in this phenomenon, and how we can exploit microbial competition to improve health status in humans.