Bacterial evolution in the oral microbiome: the role of conjugative elements and horizontal gene transfer

As one of the most diverse bacterial populations within the human body, the oral microbiome encodes a wealth of genetic information. Horizontal gene transfer, driven by mobile genetic elements, takes advantage of this information to influence bacterial evolution and the spread of phenotypes (antibiotic resistances, virulence attributes, and metabolic capabilities) among oral microbes. Although widespread within microbial communities, fundamental aspects of the mobile elements that drive horizontal gene transfer within the oral cavity remain poorly understood. In this review, we explore what is known about the role of horizontal gene transfer in bacterial evolution within the oral microbiome and the elements that facilitate this transfer, with a specific focus on conjugative DNA transfer. Conjugative elements are found in virtually all bacterial phylogenetic clades, and some can mediate genetic exchange between distantly related organisms. This is of particular interest in the diverse microcosm of the oral cavity, specifically how it drives the evolution and virulence of dental pathogens. Finally, we highlight advances in our understanding of the unique biology within dental plaque and how these might influence our understanding of bacterial gene transfer, and thus human health and disease.