Study uncovers costs of drug resistance for bacteria

Developing resistance to certain antibiotics may come at a price for the bacteria, new Swedish research suggests. Sara Thulin Hedberg of Örebro University studies the bacteria Neisseria meningitidis, one of the main causes of meningitis. She found that bacteria that are resistant to the drug rifampicin do not reproduce as fast as non-resistant bacteria and are not as good as infecting people. She hopes that her findings, which form part of her doctoral dissertation, will lead to the development of new, more effective antibiotics.

"By enhancing our knowledge of how bacteria change and are affected by developing resistance it may be possible to design antibiotics that bacteria find it more difficult to adapt to without excessive cost to themselves," she explained.

Meningococcal bacteria are usually harmless; between 10% and 25% of the population carries the bacteria in their throats without even knowing about it. However, for reasons which are not fully understood, the bacteria sometimes get into the bloodstream and make their way through the blood-brain barrier causing infection in the meninges, which line the brain and spinal cord. When this happens, intensive medical care is needed, but even with prompt medical treatment, around 10% of patients die.

In recent years, there have been reports from a number of countries of meningococcal bacteria with resistance to some of the main antibiotics used to fight meningitis, namely penicillin, ciprofloxacin and rifampicin.

Sara Thulin Hedberg investigated over 700 N. meningitidis samples taken from patients in Sweden between 1995 and 2008, testing them for susceptibility to 7 antibiotics.

Although some samples were resistant to individual antibiotics, overall levels of resistance remain low and these bacteria do not seem to be spreading in the population.

"We expected a more negative tendency, considering the dramatic increase in resistant bacteria in society, so these findings are both a surprise and a great relief," she commented.

In effect, the results mean that, in Sweden at least, meningococcal bacteria remain vulnerable to the frontline drugs traditionally used to treat and prevent meningitis, namely penicillin and ciprofloxacin. Nevertheless, she warns that doctors should still test samples for antibiotic susceptibility to ensure that the right drugs are being used and to pick up on any emerging resistance.

Further analysis of the bacteria resistant to rifampicin suggests that one factor holding back the spread of drug-resistant bugs may be the cost of becoming drug resistant. Experiments revealed that the drug-resistant bacteria are weaker than their susceptible counterparts. This means that in an antibiotic-free environment, the drug-resistant bugs simply cannot compete with susceptible bacteria.

Symptoms of meningitis include a stiff neck, fever, sensitivity to light, confusion, headaches and vomiting. Patients require immediate hospitalisation and treatment, but even if drugs are administered promptly, between 5% and 10% of cases prove fatal. Furthermore, up to a fifth of survivors may experience brain damage, hearing loss or learning disability.

Small clusters of cases occur sporadically throughout the world, but the highest burden of the disease is found in the so-called 'meningitis belt', which stretches across sub-Saharan Africa from Senegal to Ethiopia. During the dry season, cold nights and dust winds render the population more susceptible to respiratory infections. At the same time, overcrowded housing and large movements of people on pilgrimages and travelling to regional markets provide ideal conditions for the spread of the disease.

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