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|Drug: The Double Edged Knife (Part 9)|
Mohamed H. Dahir (Chairman Pharmaceutical Association of Somaliland)
Antimicrobial resistance (Cont’d)
Are antimicrobial drugs to blame?
No. Antimicrobial drugs do not cause resistance. But the process is accelerated when antimicrobials are misused. What happens is that natural selection - a natural biological process - favours the survival of microbes that develop resistance genes by chance when exposed to antimicrobials. All uses of antimicrobials - both appropriate and inappropriate - apply a selective pressure on microbial populations. However, the more antimicrobials are used, the greater this pressure will be. Thus it is critical to gain maximum benefit from the curative effects of antimicrobials - especially in developing countries, where they are not only misused, but often under-used due to financial constraints. At the same time, it is also essential to minimise the opportunities for resistance to emerge. In practice this means using antimicrobials both widely and wisely - neither too little, nor too much, and never inappropriately. Inappropriate prescribing practices - including the wrong choice of drug and incorrect dosage or length of treatment - poor compliance with treatment, and the use of low quality (sometimes counterfeit) drugs all contribute to the emergence of drug-resistannt microbes.
How does resistance develop?
If a person develops an acute infection such as pneumonea with a drug susceptible strain of streptococcus pneumoneae, for example, and is treated promptly with penicillin, the bacteria will be killed and the infection resolved before resistance has time to emerge. However, in the treatment of chronic infections such as TB and HIV/AIDS - especially if treatment compliance is poor - drug-resistant mutants have time to emerge and multiply and replace the drug-susceptible population of microbes. Under these circumstances, it is likely that the treatment outcome will be poor.
So why is it that the microbes involved in acute infections have also become resistant to many of the first-line drugs available? The problem is that antimicrobial drugs not only kill the microbe being targeted, they also "treat" other normally harmless microbes ("normal flora") in the body as well. For example, streptococcus pneumoniae as well as causing otitis, pneumonia and meningitis, is also carried by many people, especially children, as part of their normal throat flora, without causing any symptoms. So every time they take an antimicrobial - for whatever reason - their streptococci are exposed. If a mutant emerges, it will have a selective advantage and can spread to other people. A similar process occurs when salmonella bacteria are exposed to antimicrobials incorporated into animal feed while these bacteria may not cause the animal any harm, they can be spread to humans through the food chain.
What is multidrug-resistance?
There are many different classes of antimicrobials, and microbes have devised way s to resist the action of each and every one. In addition, a single microbial cell can carry resistance genes to a whole series of totally unrelated antimicrobial drugs. Over time, the dysentery-causing bacterium shigella, for example, has become resistant to each successive class of antimicrobials used in treatment. As a result, it has a string of genes each coding for resistance to a different antimicrobial. To make matters worse, this string of genes can be transmitted from one bacterial cell to another. Thus a previously susceptible shigella can, in one fell swoop, acquire five or six resistance genes.
Why is antimicrobial resistance spreading so fast?
Although mutations are rare events (about one in a million bacteria may show mutation which might lead to resistance), microbes multiply very rapidly - thereby enabling a single mutation to rapidly become dominant. Microbes also spread rapidly from person to person. Thus one patient infected with a resistant stain may be an important source of spread, not only of infection, but of a resistant infection. This is demonstrated in hospitals, where one patient infected with MRSA, for example, is often the source from which many others become infected or colonized.
Thus sin taking action to contain resistance, both emergence of resistance and the spread of resistant stains need to be considered.
Can Antimicrobial resistance be halted?
No. But it can be contained. Antimicrobial resistance is natural biological phenomenon - the respone of microbes subjected to the selective pressure of antimicrobial drug use. The main priority should be to prevent infection in the first place. After that, containment of the problem is best we can aim for. And since it is antimicrobila use that drives resistance, the focus of any containment strategy should be on minimizing any unnecessary, inappropriate or irrational use of antimicrobial drugs. Many groups of people play a role in determining how and where antimicrobials are used:
All of these groups need to be engaged in developing and implementing a resistance containment action plan.