Purpose of review: A diverse array of microbes colonizes the human intestine. In this review, we seek to outline the current state of knowledge on what characterizes a 'healthy' or 'normal' intestinal microbiome, what factors modify the intestinal microbiome in the healthy state and how the intestinal microbiome affects normal host physiology.
Recent findings: What constitutes a 'normal' or 'healthy' intestinal microbiome is an area of active research, but key characteristics may include diversity, richness and a microbial community's resilience and ability to resist change. A number of factors, including age, the host immune system, host genetics, diet and antibiotic use, appear to modify the intestinal microbiome in the normal state. New research shows that the microbiome likely plays a critical role in the healthy human immune system and metabolism.
Summary: It is clear that there is a complicated bidirectional relationship between the intestinal microbiota and host which is vital to health. An enhanced understanding of this relationship will be critical not only to maximize and maintain human health but also to shape our understanding of disease and to foster new therapeutic approaches.