Insight into the relationship between type IV pilus function and biofilm formation

Type IV pili (T4P) are protein nanofibers that can be extended and retracted from the surfaces of many bacterial taxa. They are involved in many aspects of bacterial physiology that differ between bacterial species, including surface motility, DNA uptake, and host-cell adherence, but genetically and structurally distinct type IV pilus systems from distantly related bacterial species have also been found to promote the formation of bacterial biofilms. The molecular mechanisms underpinning the promotion of biofilm remain an area of active investigation and may be both manifold and variable between type IV pilus systems. Two areas of recent interest are interactions between T4P and extracellular DNA and the relationship between surface-adhered biofilms and suspended aggregates. In the present review, we critically discuss the current state of knowledge of type IV pilus function and how these structures may interact with other biomolecules to influence the formation of multicellular bacterial communities. We examine the evidence for how alterations in DNA-binding, pilus retraction, and pilus composition have downstream effects on the formation of bacterial biofilms.