ABSTRACT

Bacteria belong to the kingdom Procaryotae. Bacteria are, like all cells, limited by a metabolically active and complex cell membrane. Many bacteria can be segregated on the basis of the Gram stain. Gram-positive organisms retain the dye crystal violet; in contrast, the dye is washed out of Gramnegative bacteria by acetone-alcohol. Cell walls of Gram-positive bacteria are thick, poorly defined structures, comprised mostly of peptidoglycan. The nonpeptidoglycan portion includes a wide variety of polysaccharides and teichoic acids, which are covalently bound to the peptidoglycan. The lipopolysaccharides are major somatic antigens of Gram-negative bacteria in addition to their important biological activities. Colonies of bacterial mutants having only core polysaccharide and lipid A in their cell walls may have a visibly rough surface that can be distinguished on the agar plate from the smooth colonies of parent bacteria. Anaerobic Gram-negative bacteria are involved, probably only synergistically, in suppurative infections. Bacteroides fragilis, a normal bowel organism, is characteristically associated with abdominal infections.