The potential for microorganisms to affect host behaviour has been recognised for decades, particularly with respect to parasites, but research into the wider influence of microorganisms on host behaviour and communication is now also gaining more interest. Microbial communities and associated metabolites can affect satiation, cravings and dysphoria that can influence feeding behaviour and nutrient acquisition. Microbial communities can also synthesise hormones that are analogous in structure and function to those produced by the host, as well as degrade or metabolise host-derived hormones. This can affect host mood and stress, as well as reproductive behaviours and fecundity. The gut microbiome also plays a key role in normal cognitive development of mammals, including aspects such as learning and memory. Furthermore, microbial metabolites are critical for scent communication, such as pheromone production, across many different species, and may also play a role in pollinator attraction by plants. However, the literature is largely biased towards mammals, and in many cases the mechanisms underlying microorganism-induced host behavioural changes still need identifying.

A classic example of microbiome function is its role in nutrient assimilation in both plants and animals, but other less obvious roles are becoming more apparent, particularly in terms of driving infectious and non-infectious disease outcomes and influencing host behaviour. However, numerous biotic and abiotic factors influence the composition of these communities, and host microbiomes can be susceptible to environmental change. How microbial communities will be altered by, and mitigate, the rapid environmental change we can expect in the next few decades remain to be seen. That said, given the enormous range of functional diversity conferred by microbes, there is currently something of a revolution in microbial bioengineering and biotechnology in order to address real-world problems including human and wildlife disease and crop and biofuel production. All of these concepts are explored in further detail throughout the book.