How is it that sulfanilamide is selectively toxic?
both microbes and the host cells require a substance called ‘folic acid’ for growth. Folic acidis available in the body and crosses into the host’s cells which are then able to use the folic acid. Folicacid is, however, unable to enter microbes. The microbes must, therefore, have some device for makingfolic acid if they are to continue to grow. They do this by taking up a compound called paraaminobenzoic acid (PABA) which is then used for the manufacture of folic acid. Sulfanilamide closely resembles PABA in structure. The microbe cannot distinguish between sulfanilamide and PABA and is fooled into trying to use sulfanilamide instead of PABA. An analogy to this state of affairs, is that PABAcan be considered the key for the lock. Sulfanilamide closely resembles the key, gets into the lock and blocks the lock so that PABA (the key) cannot get into and open the lock. For this reason, folic acid is not made by the microbes and they cannot continue to grow. The body’s defence mechanism, namely the immune system, is then able to overpower the bacteria. Since the host cell uses preformed folic acid, it does not require PABA and is therefore not affected by the PABA antagonist, sulfanilamide.