Bacterial cells are surrounded by a cell wall that contains peptidoglycan, which is only found in prokaryotes. Peptidoglycan is a polymer held together by covalent bonds. A molecule made of four amino acids, a tetrapeptide, helps hold peptidoglycan together. Not all bacteria have the same tetrapeptide which accounts for the differences in shape. The cell wall of gram-positive bacteria is composed of 90% peptidoglycan in addition to teichoic acids. Teichoic acids are responsible for the net negative charge on gram positive cells. The cell walls of gram-negative bacteria are composed of a thin layer of peptidoglycan and an outer membrane that encloses the peptidoglycan. The outer membrane forms an additional permeability barrier in these organisms. Humans are born with non-immune factors that protect them from infection from bacteria. Lysozyme is one of these factors and is found in most bodily fluids like tears and saliva. Lysozyme can break down peptidoglycan which results in cell lysis and death.
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