In simple terms, antimicrobial resistance just means that germs have come up with a way of stopping a drug from killing or damaging them. The principle behind the development of antimicrobial resistance is 'Survival of the Fittest'. Most germs multiply very rapidly. By chance (due to a genetic mutation) a germ may become resistant to an antimicrobial. As susceptible germs are killed by antimicrobial drugs, resistant microbes survive. The next time infection occurs the antimicrobial drug may not work if the germs are resistant.
Because each germ is naturally susceptible to different antimicrobial drugs, different germs develop resistance to different drugs. Although all germs can potentially develop resistance, there is more evidence for the development of resistance in bacteria so in the following questions we will look at the case of the bacteria to illustrate the principles of resistance.