 How Antibiotics Work:
Antibiotics selectively attack bacteria without harming cells belonging to the host organism. Any given antibiotic does this in one of two general ways:
- Bactericidal antibiotics (such as penicillin and cephalosporin) kill bacteria by inhibiting cell wall synthesis and allowing cell contents to leak out. Human and animal cells do not have cell walls, so these antibiotics do not damage them.
- Bacteriostatic antibiotics (including tetracycline and erythromycin) stop bacteria from reproducing by inhibiting nuclei acid formation (DNA and/or RNA), or by inhibiting protein synthesis by cell structures called ribosomes. Antibiotics that inhibit DNA and RNA affect bacteria cells more than human or animal cells. Antibiotics that inhibit protein synthesis can cause side effects because some human and other animal ribosomes are similar to those in bacteria.
Antibiotics can also be categorized by the types of bacteria they affect. The cell wall of some species of bacteria is made of a thick layer of peptidoglycan. Other bacteria have a thin layer of peptidoglycan combined with both an outer and inner membrane. These differences in structure can be seen when bacteria are dyed with gentian violet a procedure called Gram's stain.
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Bacterial cells stained with Gram's stain. |
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Bacteria with thick peptidoglycan can appear purple when stained and are referred to as gram-positive. Bacteria with thin peptidoglycan show up either colorless or red and are referred to as gram-negative. Antibiotics that work against primarily gram-positive bacteria are know as narrow-spectrum. Broad-spectrum antibiotics target both gram-positive and gram-negative bacteria.
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