Antibiotics have been one of the major advances in public health, and are still relied on to treat bacterial infections. However, bacteria are evolving, and so inevitably develop genetically-based resistance. Antimicrobial resistance (AMR) is now regarded as one of the most important threats to modern healthcare. Some bacterial species (e.g. Mycobacterium tuberculosis and Neisseria gonorrhoeae which cause TB and gonorrhoea respectively) are becoming untreatable – there are strains that are resistant to all known antibiotics.
Evolution of AMR is a complex process. At the population level, it depends on the frequency at which bacteria are exposed to antibiotics, i.e. how often an infection is recognised, diagnosed and treated, which itself depends on how often people are infected, which itself depends on how susceptible they are to infection. Consequently, a lot of AMR is associate with hospitals and other healthcare facilities, because the people in hospital are more susceptible and more likely to be treated if they become infected. Infections that are transmitted in hospitals are known as “nosocomial”.
Methicillin-resistant Staphylococcus aureus (MRSA) has become one of the most important nosocomial infections of the past decade. From 1995 until 2006 there was a dramatic, four-fold increase in the numbers of deaths associated with MRSA (see figure from ONS). This rise was due to two interacting processes. First, the control imposed on MRSA was not scalable, i.e. the resources available (such as isolation facilities) were fixed, so that when they became full they were ineffective at preventing transmission. This leads to catastrophic dynamics where hospitals suddenly experience very high prevalence. Second, patients who are discharged from hospital have a much higher risk of returning in the near future compared to the general population. Consequently, people who become infected during one short stay in hospital can transmit infection on subsequent visits. This leads to "stealth dynamics" where the prevalence of infection outside of hospitals gradually increases even though the prevalence within hospital appears to be relatively low. Eventually the pressure on infection control resources builds up, and they fail catastrophically. These two processes meant that MRSA was very hard to control and predict until widespread infection control ("clean hands") came into common practice.
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