Adhesins involved in attachment to abiotic surfaces by Gram-negative bacteria

The ability of bacterial cells to adhere to and interact with surfaces to eventually form a biofilm is a crucial trait for the survival of any microorganism in a complex environment. As a result, different strategies aimed at providing specific or nonspecific interactions between the bacterial cell and the surface have evolved. While adhesion to abiotic surfaces is usually mediated by nonspecific interactions, adhesion to biotic surfaces typically requires a specific receptor-ligand interaction ( 1 ). In both cases, these interactions usually originate from the same fundamental physicochemical forces: covalent bonds, Van der Waals forces, electrostatic forces, and acid-base interactions ( 2 ). Strong adhesion occurs if a bacterium and a surface are capable of forming either covalent, ionic, or metallic bonds, but weaker forces, such as polar, hydrogen bonding, or Van der Waals interactions, can also strengthen or achieve strong interactions when a high number of contacts are involved ( 2 , 3 ). Due the net negative charge of their cell envelopes, bacteria are subjected to repulsive electrostatic forces when approaching surfaces. Bacterial cells also encounter repulsive hydrodynamic forces near the surface in a liquid environment. To overcome these two repulsive barriers, bacteria typically use organelles, such as flagella or pili, which act either as an active propeller or a grappling hook ( 4 – 6 ). Once on the surface, the cell can enhance attachment to the surface via specific and/or nonspecific adhesins to eventually trigger irreversible attachment. This irreversible attachment is strongly influenced by environmental factors (i.e., pH, salinity, etc.) and the physicochemical properties of the surface (i.e., rugosity, hydrophobicity, charge, etc.) but also by the presence of the conditioning film, a layer of organic and inorganic contaminants adsorbed on the surface which changes its physicochemical properties ( 7 ). To achieve permanent adhesion under such variable conditions, bacterial cells have developed a series of adhesins able to facilitate adhesion under various environmental conditions ( 8 , 9 ). In this article, we will focus exclusively on nonspecific adhesins, which are primarily responsible for biofilm formation and bacterial adhesion to abiotic surfaces. We will review the current knowledge of fimbrial, nonfimbrial, and discrete polysaccharide adhesins involved in adhesion to abiotic surfaces and cell aggregation in Gram-negative bacteria.