Fig. 3

Schematic representation of bacterial richness and diversity concepts. In the figure, each color represents a different bacterial taxon. Richness and diversity can be considered at different levels of taxa, from phylum to species. Richness represents the variety of bacterial communities observed in a specific ecosystem and is greater with the number of different bacterial species found in it. Diversity is related to the preponderance of bacterial communities one over the others, in the ecosystem: when a bacterial taxon is overrepresented in the niche, microbial diversity decrease. Various indices are used to estimate the bacterial diversity. Shannon index estimates the bacterial diversity and increases with the number of different species in an ecosystem. Simpson index increases with the probability that two random species within an ecosystem are the same and is therefore higher when one or more bacterial species are preponderant in an ecosystem. The figure includes material available from Servier Medical Art (https://smart.servier.com) under a Creative Commons license