Microbial resistance to antimicrobials is a matter of great importance; if sensitive strains are supplanted by resistant ones, then a valuable drug may become useless. Just as:

Some are born great, some achieve greatness, and some have greatness thrust upon them.9

so microorganisms may be naturally Cborn') resistant, 'achieve' resistance by mutation or have resistance 'thrust upon them' by transfer of plasmids and other mobile genetic elements.

Resistance may become more prevalent in a human population by spread of microorganisms containing resistance genes, and this may also occur by dissemination of the resistance genes among different microbial species. Because resistant strains are encouraged (selected) at the population level by use of antimicrobial agents, antibiotics are the only group of therapeutic agents which can alter the actual diseases suffered by untreated individuals.

Problems of antimicrobial resistance have burgeoned during the past decade in most countries of the world. Some resistant microbes are currently mainly restricted to patients in the hospital, e.g. MRSA, vancomycin-resistant enterococci (VRE), and coliforms that produce 'extended spectrum ß-lactamases'. Others more commonly infect patients in the community, e.g. penicillin-resistant Streptococcus pneumoniae and multiply-resistant Mycobacterium tuberculosis. Evidence is accruing that the outcomes of infections with antibiotic resistant bacteria are generally poorer than those with

9 Malvolio in Twelfth Night, Act 2 Scene 5, by William Shakespeare (1564-1616).

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