The Structure of Bacterial Surfaces and its Influence on Stainability

Abstract

Prokaryotes are extremely small cells possessing a number of unusual structures, many of which can be stained to aid taxonomic characterization. For example, the number and arrangement of flagella can be important, as can the ability of bacteria to form endospores. The most important stain for bacterial characterization is the Gram stain. Prokaryotes are currently divided into 2 domains, the Archaeal and the Bacteria, and each can be characterized by the manner in which they interact with the Gram stain. By far the best studied group is the Bacteria; once stained these cells can have 1 of 3 responses—they can be Gram positive (purple), Gram negative (red) or Gram variable (red or purple). All of these responses ultimately depend on the structure and chemistry of the bacterial wall. Archaea are not as well studied since the members of this domain are more recently discovered and frequently inhabit extreme environments that are difficult to sample (eg, the hyper- thermphiles, methanogens, and halobacteria).Recent advances in electron microscopy have further advanced our understanding of prokaryotic cell wall structure, and a new platinum-based mordant for the stain has allowed the Gram reaction to be followed by high-resolution electron microscopy and energy dispersive X-ray spectroscopy. Gram-positive cells stain purple because the crystal violet–modant complex cannot escape through their thick cell walls during the decolorization procedure. Gram-negative have thinner, more membranous cell walls that do not retain the crystal violet-mordant complex, thereby allowing the red secondary stain (carbol fuchsin or safranin) to pre-dominate after staining. Gram-variable cells have walls of intermediate complexity and, therefore, can stain either Gram positive or Gram negative depending on how cell growth stage affects their cell wall structure. Most bacteria can be accurately characterized by the Gram stain but archaea, because of their broad range of cell wall structures and chemistries, require more study before dependable characterizations can be made. (The J Histotechnol 25:55, 2002)Submitted and accepted: August 31, 2001