The concept of attacking invading microorganisms without harming the host was first introduced by Paul Ehrlich. In 1910 he discovered “salvarsan”, which he announced as a magic bullet for the treatment of syphilis. Penicillin, produced by the fungus, Penicillium notatum, was first discovered by Alexander Fleming in 1928, purified by Florey and Chain in 1940, and shown to have wide applicability in the treatment of infection caused by a variety of bacteria. With the help of colleagues in the USA, it was produced in sufficient quantity to be a miracle cure for wound infections during the Second World War. But within several years, resistance had developed in bacteria that were formerly thought to be uniformly susceptible, and it became increasingly recognised that for optimal treatment and cure, it was important to test the infecting bacterial culture for susceptibility to antibiotics, and to treat only with antibiotics that were active in vitro against the infecting organism.

The disc technique was used by various workers for this assay because it was relatively easy to set up, and the result could be ascertained the next day from interpretation of the zone size that was obtained. The test was performed by instillation of a standard amount …