Vulnerable shields — the cell walls of bacteria and fungi

Abstract

In the search for differences between microbial pathogens and animal cells that could provide the basis for selective antimicrobial attack, one evident distinction lies in their general structure. The animal cell is relatively large and has a complex organization; its biochemical processes are compartmentalized and different functions are served by the nucleus with its surrounding membrane, by the mitochondria and by various other organelles. The cytoplasmic membrane is thin and lacks rigidity. The cell exists in an environment controlled in temperature in mammals and birds and also in osmolarity. It is constantly supplied with nutrients from the extracellular fluid. Bacteria and fungi live in variable and often hostile environments and they must be able to withstand considerable changes in external osmolarity. Some micro-organisms have relatively high concentrations of low molecular weight solutes in their cytoplasm. Such cells suspended in water or in dilute solutions develop a high internal osmotic pressure. This would inevitably disrupt the cytoplasmic membrane unless it were provided with a tough, elastic outer coat. This coat is the cell wall, a characteristic of bacteria and fungi which is entirely lacking in animal cells. It has a protective function but at the same time it is vulnerable to attack, and a number of antibacterial and antifungal drugs owe their action to their ability to disturb the processes by which the walls are synthesized. Since there is no parallel biosynthetic mechanism in animal cells, substances affecting this process may be highly selective in their antimicrobial action.