Defence against bacterial drug resistance.

How does resistance arise in bacteria ? One process is simply selection; treatment with an antibiotic eliminates sensitive strains and enables an originally very small minority of resistant ones to survive and spread. This is undoubtedly what happened among staphylococci in the early days of penicillin. When exposure to a drug-antibiotic or synthetic-is followed by resistance in a previously sensitive strain this is usually attributed to mutation. This is undeniable and has been conclusively proved in some instances, but when resistance increases slowly over a period ofyears some form ofadaptation seems a more likely explanation than a long series of small genetic changes, and this hypothesis had a strong advocate in the late Sir Cyril Hinshelwood.1 There are three mechanisms whereby genetic elements determining drug resistance, often multiple, can be transferred from one bacterial cell to another. They are transformation, the uptake of genetic material already liberated from another cell; transduction by bacteriophage, which can occur in many species; and conjugation, which occurs only between Gram-negative bacilli, although of many different genera.