Bacteria form communities, termed biofilms, in which cells adhere to each other within a matrix, typically comprised of polysaccharides, proteins, and extracellular DNA. Biofilm formation by the marine bacterium Vibrio fischeri requires the Syp polysaccharide, but the involvement of matrix proteins is as yet unknown. Here we identified three genes, termed bmpA, -B, and -C (biofilm maturation protein), with overlapping functions in biofilm maturation. A triple bmpABC mutant, but not single or double mutants, was defective in producing wrinkled colonies, a form of biofilm. Surprisingly, the triple mutant was competent to form pellicles, another biofilm phenotype, but they generally lacked a three-dimensional architecture. Transmission electron microscopy revealed that the extracellular matrix of the bmp mutant contained electron-dense, thread-like structures that were also present in the wild type but lacking in syp mutant strains. We hypothesized that the bmp mutant produces the Syp polysaccharide but fails to produce/export a distinct matrix component. Indeed, a mixture of the bmp and syp mutants produced a wrinkled colony. Finally, BmpA could be detected in cell-free supernatants from disrupted pellicles. Thus, this work identifies a new matrix protein necessary for biofilm maturation by V. fischeri and, based on the conservation of bmp, potentially other microbes.
Bacteria can be found as planktonic cells or within sessile communities called biofilms, which are composed of bacterial cells encased in an extracellular matrix (1-3). The matrix is generally self-produced and comprised of polysaccharides, proteins, extracellular DNA, and other bacterial products, such as outer membrane vesicles (OMVs). These matrix components promote adherence of the bacteria to each other and to a variety of surfaces, including host tissues and medical devices, such as catheters (reviewed in reference 4). Furthermore, due at least in part to the protective nature of the matrix, resident cells exhibit increased resistance to predation (5), desiccation (6), host defenses (4), and antibiotics (7). While much is known about the process of biofilm formation and its regulation from the study of a variety of bacteria, we still know little about the constituents of the matrix, in particular the matrix proteins, and their contributions to biofilm formation.Matrix proteins can be anchored to or associated with the cell surface or released into the extracellular space, where they appear to promote cell-matrix, cell-cell, matrix-matrix, and cell-surface interactions. For example, in Pseudomonas aeruginosa, the matrix protein CdrA is both cell associated and secreted and likely promotes cell-matrix and matrix-matrix interactions by binding to the Psl polysaccharide (8), which itself is cell associated and secreted (9). Additionally, the lectins LecA, specific for D-galactose (10), and LecB, specific for L-fucose (11), are thought to promote cell-cell interactions (12) during P. aeruginosa biofilm formation.Perhaps the most is known about the r...