Emergence and Spread of Antimicrobial Resistance
The origin of antimicrobial resistance
For billions of years, certain bacteria and fungi have produced chemical substances to protect them from attack from other microorganisms. Those used in clinical medicine today are referred to as "antibiotics" or "antimicrobial agents". As a survival mechanism, other microbes have developed mechanisms for resisting the toxic effect of antimicrobials. "Antimicrobial resistance" is thus an ancient phenomenon encoded on resistance genes passed down through microbial lineages.
Susceptible strains can become resistant either through mutations in existing genes or by acquiring a resistance gene from another organism that is already resistant. This is the first step in the emergence of "new resistance". Fortunately for most organisms, susceptible organisms do not easily become resistant -- resistance that we observe today in clinical practice usually developed in another person, animal, or environmental reservoir in some other part of the world many years prior.
Though for most organisms the sudden appearance of new resistance is rare, this is not the case for all pathogens. For example, in patients with tuberculosis or HIV infection, new mutations in susceptible strains can occur within a patient, especially when therapy is suboptimal. The emergence of resistant strains during therapy greatly increases the risk of a poor clinical outcome, including death. Consequently effective treatment is absolutely critical to avoid the development of resistance during treatment.
Selection and spread
The main drivers of resistance rates are not new mutations but rather antimicrobial selection pressure and transmission.
- Antimicrobial selection pressure: A person's normal flora consists of millions of strains of both susceptible and resistant microbial strains and species. The use of an antimicrobial to treat an infection impacts not only the specific pathogen causing disease, but also decimates populations of susceptible organisms throughout the body. Resistant strains thrive and expand, putting the patient at higher risk of a resistant infection in the future.
- Transmission of resistance microbes: Because of the movement of microorganisms between patients, healthcare workers, and family contacts, antimicrobial use in a single patient poses risks locally and ultimately to the global community. Resistant organisms spread via direct contact, environmental surfaces, waterways, and food.