Penicillin-resistant bacteria contain an enzyme that chemically modifies penicillin, making it ineffective.
Some types of bacteria have an additional layer outside the cell wall. This layer is made from lipopolysaccharide (LPS), a combination of lipids and sugars. There are several consequences to possessing this outer layer. Of least import to the bacteria but significant for researchers, this layer preventsthem from retaining a particular dye (called Gram stain) that is used to classify bacteria. Bacteria that have this LPS layer are called Gram-negative, in contrast to Gram-positive bacteria, which do not have an outer LPS layer and which do retain the stain. Of more importance to both the bacteria and the organisms they infect is that one portion of the LPS layer, called endotoxin, is particularly toxic to humans and other mammals. Endotoxin is partly to blame for the damage done by infection from Salmonella and other Gram-negative species.
Within the cell wall is the plasma membrane, which, like the eukaryotic plasma membrane, is a phospholipid bilayer studded with proteins. Embedded in the membrane and extending to the outside may be flagella, which are whiplike protein filaments. Powered by molecular motors at their base, these spin rapidly, propelling the bacterium through its environment.
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