Bacterial Cell Wall
Bacteria have cell walls. These cell walls help protect the bacteria, and are constantly being maintained by being broken down and rebuilt. A cell wall is made up of certain carbohydrates and proteins in a layer known as the peptidoglycan. In gram-positive bacteria, the molecules are cross-linked by a pentapeptide bridge. In gram-negative bacteria, the peptidoglycan molecules are covalently bound to one another. This causes gram-positive bacteria to be more susceptible to penicillin.
Mechanism of Action
Penicillin is known as a β-Lactam antibiotic, because it contains a β-Lactam ring. It is bactericidal. All β-Lactam antibiotics work by inhibiting the formation of cross-links in the peptidoglycan portion of bacterial cell walls. It does so by inhibiting the enzyme DD-transpeptidase. Penicillin's β-Lactam ring permanently binds to and inactivates DD-transpeptidase.
This prevents bacteria from creating more peptidoglycan. Osmotic pressure builds up, and the cell wall eventually bursts, killing the cell.
Also of note: the peptidoglycan precursors build up, but are unable to be placed in the peptidoglycan. This increase in the concentration of the precursors causes activation of the enzymes that cause cell wall hydrolysis. This causes the cell wall to break down even faster.
Donald J. Tipper and Jack L. Strominger. "Mechanism of Action of Penicillins: A Proposal Based on their Structural Similarity to Acyl-D-alanyl-D-alanine." Proceedings of the National Academy of Sciences of the United States of America. 1965. Pages 1133-1141.
Image 1 source: http://water.me.vccs.edu/courses/env108/clipart/cellwall.gif
Image 2 source: http://classes.midlandstech.edu/carterp/Courses/bio225/chap04/04-13b_BactCellWall_1.jpg
Image 3 source: http://www.chem.ucla.edu/harding/IGOC/B/beta_lactam01.jpgBanner Image Source: http://en.wikipedia.org/wiki/File:Penicillin_spheroplast_generation_horizontal.svg
- Caption: "Bacteria that attempt to divide in the presence of penicillin fail to do so, and instead end up shedding their cell walls."
Image 1 source: http://water.me.vccs.edu/courses/env108/clipart/cellwall.gif
Image 2 source: http://classes.midlandstech.edu/carterp/Courses/bio225/chap04/04-13b_BactCellWall_1.jpg
Image 3 source: http://www.chem.ucla.edu/harding/IGOC/B/beta_lactam01.jpgBanner Image Source: http://en.wikipedia.org/wiki/File:Penicillin_spheroplast_generation_horizontal.svg
- Caption: "Bacteria that attempt to divide in the presence of penicillin fail to do so, and instead end up shedding their cell walls."