Cell Wall of Bacteria

Bacterial cell wall is composed of saccharides called peptidoglycan. However, its structure varies for different types of bacteria. Depending on the looks of bacterial cell wall, these microorganisms can be divided into two major groups: Gram-positive bacteria (e.g. Bacillus subtilis) and Gram-negative bacteria (e.g., Escherichia coli). The difference in the structure of the cell wall of bacteria was discovered in 1884 by Christian Gram. Gram-positive bacteria have a cell wall composed mainly of thick layer of peptidoglycan, which binds the purple dye (eg. Crystal violet) very well – this bond is very strong. Gram-negative bacteria in comparison have a very thin layer of peptidoglycan on their surface, above which, however, have another layer of the membrane structure interwoven with pores. These structures bind the violet dye weakly and can be easily cleaned with alcohol and dyed with different color – typically with the red dye (e.g. safranin). So after the process of Gram staining , the Gram-positive bacteria appear purple and Gram-negative red. The difference in construction of the cell walls is not only in response to the staining, but it is essential e.g. in medicine. For instance, the type of cell wall is decisive in the choice of antibiotics suitable for treatment.