Penicillin targets bacterial synthesis of the cell wall.
Bacteria are a large and diverse group of prokaryotes that have colonized many areas of the world, from oceans to swamps and even the digestive tracts of animals. A nearly universal feature of bacteria is the presence of a cell wall consisting of peptidoglycan. This cell wall lies outside the weak cell membrane that encompasses the bacterial cell. Variations in the cell wall between bacteria allow the cell wall to be used in identification, and the essential nature of the cell wall has provided a target in the treatment of many bacterial diseases.
Peptidoglycan
The weak bacterial membrane of most eubacteria is surrounded by a cell wall composed of peptidoglycan, also known as murein. Peptidoglycan consists of two amino sugars with a chain of five amino acids extending from one of the amino sugars. Amino sugars are sugars which have an amine substituted for one of the hydroxyls on a carbon atom. Individual units of peptidoglycan are linked together into chains by the removal of a water molecule from the amino sugars, leaving a bond called a glycosidic bond. These chains are then cross-linked by peptide bonds between the amino acids extending from the peptidoglycan chains.
Gram Staining
The bacterial cell wall is often one of the first characteristics used to identify and classify bacteria. The Gram stain uses a purple dye (crystal violet or methylene blue), followed by Gram's iodine solution to stain the bacteria. The bacteria are then rinsed with decolorizer (mixture of ethanol and acetone) and the addition of a pink dye (basic fuchsin or safranin). After Gram staining, Gram-positive bacteria such as Staphylococcus aureus appear purple as their thicker cell wall retains the large molecule formed by the crystal violet and iodine. Gram-negative bacteria such as Escherichia coli appear pink as their thin cell wall retains the pink dye but not the purple dye.
Acid-Fast Staining
Some medically important bacteria possess a waxy coating over their cell wall that produces a weak Gram-positive result during the Gram stain. These bacteria include members of the genus Mycobacterium which cause leprosy and tuberculosis. In the acid-fast stain, a red dye (carbolfuchsin) is used to stain the bacteria followed by a decolorizing rinse with acid-alcohol solution and counterstained with methylene blue. Acid-fast bacteria appear red on a purple background as they resist decolorization by the acid-alcohol solution.
Penicillin
Many antibiotics have been found and modified that attack the synthesis of the cell wall in bacteria. Among the most famous of these is penicillin. Bacteria use enzymes to unlink the peptidoglycan chains allowing growth following binary fission. These chains are then reconnected to newly added chains. Penicillin specifically targets the enzyme that links the chains back together, causing the bacteria to rupture from the strain placed on its weak cell membrane. Since humans and other eukaryotes lack a cell wall of peptidoglycan and the enzymes necessary for bacterial cell wall synthesis, penicillin targets the bacterium while ignoring the host. However, some individuals may develop an allergic response to the penicillin that is potentially life threatening.
Penicillinase
Penicillin is a natural compound produced by molds belonging to the genus Penicillium. It is then unsurprising that the natural presence of penicillin has resulted in the ability of some bacteria to counteract this compound. Penicillinase is an enzyme that breaks a specific feature in the structure of penicillin and closely related compounds. Penicillinase is coded for on plasmids, short pieces of DNA that replicate independently of the bacterial genome and can be transmitted between bacteria. Penicillinase producing strains of disease causing bacteria have become increasingly common with widespread and indiscriminate use of these antibiotics.
Bacteria Without Peptidoglycan
The genus Mycoplasma is unique among bacteria as they lack a cell wall entirely. These bacteria overcome the weakness of a bacterial cell membrane by incorporating compounds call sterols into their cell membrane. Interestingly, the bacteria are unable to manufacture the sterols, instead acquiring these compounds from a host organism. Members of the genus Chlamydia also deviate from the norm of peptidoglycan by using proteins in their place.
