Gram Positive Bacteria

GRAM-POSITIVE BACTERIA are characterised by having as part of their cell wall structure peptidoglycan as well as polysaccharides and/or teichoic acids. The peptidoglycans which are sometimes also called murein are heteropolymers of glycan strands, which are cross-linked through short peptides.

THE BASIS OF THE MUREIN are chains of alternating residues of N-acetylglucosamine and N-acetyl muramic acid which are Beta -1,4-linked. The muramic acid is a unique substance associated with bacterial cell walls. These chains are cross-linked by short polypetide chains consisting of both L- and D-aminoacids. While in Gram-negative bacteria the peptidoglycan is simple in structure and comparatively uniform throughout most genera, in Gram-positive bacteria there is a very big variation in structure and composition. In general the peptidoglycan is multilayered. There have also been recorded some minor variations in composition in some groups. Thus, in Mycobacterium and Nocardia the N-acetyl moiety of the muramic acid is replaced by the oxydised form N-glycolyl. The amino acid composition of the both the cross-linking as well the stem polypeptides can vary extensively with different groups. These differences form the basis for the taxonomy of these organisms.

THE LIPOPOLYSACCHARIDES of the Gram-positive bacteria have not been extensively studied apart from those of the streptococci, where they are the basis of their serological subdivision. They have also been used in investigating the serological relationships between streptococci, enterococci and lactobacilli. The specific polysaccharides associated with the acid-fast bacteria of the Corynebacterium-Mycobacterium-Nocardia group have also been extensively studied. They are arabinogalactans and arabinomannans linked to mycolic acids. The presence in the cell wall of these genera of arabinose, galactose and meso-diaminopimelic acid is characteristic of these genera.

THE TEICHOIC ACIDS , which are polyols consisting predominantly of glycerol, ribitol and mannitol, are covalently linked to the peptidoglycan through phosphodiester bonds and can be substituted by sugars, aminosugars or D-alanine residues. Some rare teichoic acids lack polyols. Teichoic acids are found in some actinomycetes, bacilli, lactobacilli, listeria and staphylococci.

IN ONLY A LIMITED NUMBER of Gram-positive genera including the Corynebacterium-Mycobacterium-Nocardia group are found the mycolic acids. They are 3-hydroxy acids of high molecular weight with a long alkyl branch in position 2.