Form and functions of the regular surface array (S-layer) of Aeromonas salmonicida.

The principal virulence factor of Aeromonas salmonicida is its S-layer (A-layer) which is comprised of tetragonally arrayed ∼50000 Mr protein subunits tethered to the cell surface via LPS. The detailed composition of its LPS is known, as is the primary sequence, and three-dimensional disposition of the A protein subunits. The A-layer physically protects the cell against bacteriophage, proteases, as well as immune and non-immune complement. The A-layer appears to be uniquely adapted towards binding biologically important molecules such as heme, and to various basement membrane proteins. In addition, the A-layer is required for macrophage infiltration and resistance. Specific mutants containing a disorganized A-layer are virulent and provide significant protection to salmonids when applied by immersion