Drug resistance by mutation

Bacteria multiply at such a rapid rate that there is always a chance that a mutation will render a bacterial cell resistant to a particular agent. This feature has been known for a long time and is the reason why patients should fully complete a course of antibacterial treatment even though their symptoms may have disappeared well before the end of the course. 

If this rule is adhered to, the vast majority of the invading bacterial cells will be wiped out, leaving the body s own defence system to mop up any isolated survivors or resistant cells. If, however, the treatment is stopped too soon, then the body s defences struggle to cope with the survivors. Any isolated resistant cell is then given the chance to multiply, resulting in a new infection which will, of course, be completely resistant to the original drug. 

These mutations occur naturally and randomly and do not require the presence of the drug. Indeed, it is likely that a drug-resistant cell is present in a bacterial population even before the drug is encountered. This was demonstrated with the identification of streptomycin-resistant cells from old cultures of a bacterium called E. coli which had been freeze-dried to prevent multiplication before the introduction of streptomycin into medicine. 

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