Since Anton van Leeuwenhoek first observed a surface-associated multicellular structure of bacterial cells in the 17th century, it has been shown to exhibit an ability to form a biofilm by numerous bacterial species. The biofilm formation is composed of distinct developmental stages, which include an attachment/adhesion of a single cell, a proliferation toward monolayered coverage, a propagation to aggregated microcolony, a maturation to 3-dimensional structure, and subsequently a local degradation. Investigation to identify the essential factors for bacterial biofilm formation has been performed via classical genetic approaches as well as recently developed technologies. The initial stage requires bacterial motility provided by a flagellum, and outermembrane components for surface signal interaction. Type IV-pilus and autoaggregation factors, e.g., type I-fimbriae or Ag43, are necessary to reach the stages of monolayer and micro colony. The mature biofilm is equipped with extracellular polymeric matrix and internal water-filled channels. This complex architecture can be achieved by differential expressions of several hundred genes, among which the most studied are the genes encoding exopolysaccharide biosyntheses and quorum-sensing regulatory components. The status of our knowledge for the biofilms found in humans and natural ecosystems is discussed in this minireview.
