Gram positive cell wall Dr Olga Perovic, CMID/NHLS/WITS 2007

Bacterial structure Capsule Extracellular polysaccharides outside of cell walls. Can’t be stained. Formed biofilm Protects from phagocytosis and death. Bacterial structure

Classification of bacteria with Gram positive cell wall

The bacterial cell wall definition and functions The bacterial cell wall provides structural integrity to the cell. Protects the cell from internal pressure caused by the much higher concentrations of proteins and other molecules inside the cell compared to outside the cell. The bacterial cell wall differs from all other organisms by the presence of peptidoglycan (poly-N-acetylglucosamine and N-acetylmuramic acid). Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and does not prevent for small substrates to pass trough.

The Gram positive cell wall structural components Teichoic acids-deep-seated in the Gram positive cell wall, and are polyalcohols. They are strongly antigenic, but are generally absent in Gram-negative bacteria. Lipoteichoic acid – teichoic acid linked with lipid, goes to the cytoplasmic membrane and links the peptidoglycan to the cytoplasmic membrane. They are antigenic, cytotoxic and adhesins

Why they are named Gram positive bacteria? Named by Danish doctor in 1884, Hans Christian Gram. Gram developed a staining procedure which divided almost all bacteria into two large groups. Depending on their retention of specific basic dyes there are: Gram positive and Gram negative.

The Characteristics of Gram positive cell wall organisms The Gram positive cell wall is characterized by the presence of a very thick peptidoglycan layer, which is responsible for the retention of the crystal violet dyes during the Gram staining procedure, opposite to Gram negative cell wall which doesn’t.

Application of Gram stain Routine use for primary microscopic examination of clinically significant specimens. To characterize bacteria growth from culture.

Comparison of the thick cell wall of Gram-positive bacteria with the comparatively thin cell wall of Gram-negative bacteria

The Gram stain and bacterial cell walls Property  Gram-positive Gram-negative Thickness of wall  thick (20-80 nm) thin (10 nm) Number of layers  1  2 Peptidoglycan (murein) content  >50% 10-20% Teichoic acids in wall present absent Lipid and lipoprotein content  0-3% 58% Protein content  9% Lipopolysaccharide content  0  13% Sensitivity to Penicillin G  yes no (1) Sensitivity to lysozyme no (2)

Importance of cell wall structure for selective activity of antibiotics The formation of the peptide bond between chains of peptidoglycan is blocked by a group of antibiotics of the  beta lactam class, Penicillin, cephalosporin, and carbapenems. The beta lactam antibiotics prevent the assembly of the bacterial cell wall. The wall becomes progressively weaker and weaker until the cell lyses or ruptures.

Assembly of Gram-positive peptidoglycan and activity of antibiotics A bridge of amino acids links the peptide side chains to one another-Gram-positive -interpeptide bridge, and is blocked by the beta lactam antibiotics.

Gram positive cocci Catalase - Enterococci Coagulase haemolysis Staphylococci Streptococci + - S.aureus Coagulase negative   non - staphylococci S.epidermidis haemolytic optochin Lancefield S.saprophyticus groups S R A,B,C,D S.pneumoniae F,G viridans e.g. group S.pyogenes (A) S.agalactiae (B)

Gram Positive Bacilli Corynebacterium diphteriae

Reference: Greenwood David, Medical Microbiology, 2002, 6th Edition