A Pseudomonas species, isolated from a feeder culture for Euglena viridis, shows an unusual form of tetragonal array on the external surface of the cell wall in negatively stained preparations. These regular surface [RS] layers have been demonstrated on a variety of gram positive and gram negative bacteria and form the primary interface between the cell wall and its environment. It is thought to function as a protective barrier layer against predators and other deleterious agents such as enzymes.Ammonium molybdate was the most effective negative stain at relatively high concentrations (4-8% w/v); uranyl acetate did not display the RS layer. The micrographs were investigated with the aid of optical diffraction and digital image enhancement. Areas of micrographs that were free from obvious irregularities such as stacking faults and uneven staining were selected visually, Fig. 1(a).
