The arrival of drug-resistant tuberculosis has significantly complicated global efforts to decrease the scourge of the disease.
Each year more than nine million people are infected with TB and another 1.5 million die. But the latest figures show that at least 20% of people diagnosed with the disease have "multiple-drug-resistant" TB. And about 9.7% of these also have "extensively-drug-resistant TB".
TB is caused by bacteria that attacks the lungs. Most TB treatments target bacteria that actively grow in the body. But a very important subset of bacteria is able to survive treatment. These are known as persistent bacteria.
Though these persistent bacteria only represent a very small proportion of the bacteria that causes TB, failing to get rid of them can have devastating consequences. They are responsible for lengthy drug treatment, and could contribute to drug resistance. They therefore should also be the target of TB therapies.
The challenge with these persistent bacteria is that they are very difficult to isolate. This makes it difficult to study them and therefore difficult to develop drugs to kill them.
As a team of scientists at Stellenbosch University in South Africa, together with colleagues at Imperial College London, we found a new way to identify, isolate and target persistent bacteria. Our technique, which has never before been applied in TB research, will help scientists understand why some bacteria respond to treatment and others become resistant.
How this bacteria works
Persistent bacteria plays a particularly important role in latent TB - when bacteria that can cause TB hibernate in the body. Someone with latent TB will not have any clinical symptoms and will therefore not know that he or she has the disease. Latent TB can survive in the body for decades and only flare up when someone's immune system is compromised.
Latent TB can therefore progress to full-blown disease in people who have compromised immune systems. These are often people who have HIV/AIDS, suffer from malnutrition, are ageing or have a substance-abuse problem. About one-third of the world's population carries latent TB.
Conventional thinking has held that persistent bacteria are also present in people who have latent TB. These bacteria are thought to either stop growing or are slowly growing, although they still survive in the body.
But emerging research has started to question this assumption on two fronts:
Some research shows that proportions of the bacteria continue to grow while others die.
Other research argues that the bacteria do not grow.
Understanding the bacteria present in latent TB is important to choose the best TB treatments. This is especially important because of the difficulties associated with treating persistent bacteria that can survive treatment.
For this reason our research is focused on finding ways to study and target persistent bacteria. We used specific bacteria-associated labels and sophisticated laser-based methods to identify and isolate this bacteria.
A new method to study persistent bacteria
The technique, known as fluorescence dilution, uses two fluorescent proteins to label the bacteria. One protein tracks live bacteria and the other measures its growth. It is applied to identify and isolate individual bacteria to study it.
The technique can best be described as using "micro-tweezers" to physically pick out the slow-growing bacteria from the rest. This enables us to find the hard-to-identify persistent bacteria.
We were able to do this by applying the same approach that's been used to isolate the bacteria that causes food poisoning, Salmonella. This involves subjecting the bacteria to conditions that come closest to those found in the body as opposed to conditions in the laboratory.
Using this technique, we found that when bacteria entered a specific type of white blood cell, a population of non- or slowly-growing persistent bacteria appeared. White blood cells play a critical role in defending the body against invading bacteria. In the laboratory we use them to mimic the environment found in the body.
This finding is important because it shows that the numbers of persistent bacteria increase by being inside white blood cells. This means that the host's own defences can help the bacteria to survive TB treatment.
Hope for the future
These are only the first steps, but this technique offers unique opportunities to deepen scientists' understanding of why and how the body's response to TB treatment results in drug resistance.
We can now, for example, begin to study what drives bacteria into a latent state. Once we understand this better it will be possible to begin designing drugs that better manage latent TB. Importantly, this could help decrease the amount of time it takes to treat TB as well as minimise drug resistance.
Jomien Mouton, Postdoctoral research fellow in the MRC Centre for Tuberculosis Research, Stellenbosch University and Samantha Sampson, Associate Professor, SARChI Research Chair in Mycobactomics, Stellenbosch University