Antibiotics

Gram negative bacteria e.g. E. coli have a thin layer of peptidoglycan (murein) between two layers of inner and outer phospholipid membrane. Gram positive bacteria have a thick layer of peptidoglycan containing chemicals such as teichoic acid. e.g. MRSA.Beta-lactams such as penicillin inhibit the formation of peptidoglycan so are effective against gram positive bacteria. Penicillin inhibits the enzyme required for the synthesis and assembly of the peptide cross-linkages in bacterial cell walls. The cell walls are weakened. As water enters naturally by osmosis the cell wall is unable to withstand the pressure. The cell bursts and the bacterium dies.Glycopeptide antibiotics e.g. vancomycin are large polar molecules that cannot get through the outer membrane of gram negative bacteria but is effective against gram positive bacteria. Polypeptide antibiotics e.g. polymixins are effective against gram negative bacteria as they interact with the phospholipids in the outer membrane.Antibiotics are ineffective against viruses. Viruses do not have cell walls and they do not have their own metabolic pathways to disrupt. They do not have sites on the protein coat where antibiotics could work. Viruses mainly hide in the host cell making it difficult for antibiotics to reach them.