Modes of action of antibacterial agents

Peptidoglycan is a vital component of virtually all bacterial cell walls (see Chapter 4). It accounts for approximately 50% of the weight of gram-positive bacterial walls, around 30% of mycobacterial cell walls and 10%–20% of the gram-negative envelope. The peptidoglycan polymer is responsible for the shape, mechanical strength and integrity of bacterial cells. If the synthesis of peptidoglycan is selectively blocked by antibiotic action, bacteria undergo a number of changes in shape, the cell wall is greatly weakened and they ultimately die as the result of cell lysis. Mammalian cells do not possess a cell wall or macromolecular structures that resemble peptidoglycan. Consequently, antibiotics that interfere with peptidoglycan synthesis have excellent selective toxicity. A few organisms of medical importance produce cell walls without peptidoglycan, for example, Chlamydia trachomatis and Chlamydophila pneumoniae, causative agents of urogenital tract and respiratory tract infections respectively. Some other organisms are capable of surviving in certain environments without their cell wall. For example, the L-forms of Escherichia coli and Proteus spp. are said to be involved in certain kinds of urinary tract infections. Finally, certain organisms, such as Mycoplasma pneumoniae, which causes respiratory tract infections, do not naturally produce a protective cell wall. Bacterial L-forms and mycoplasmas can survive only in environments where the osmotic pressure is not extreme.