Tuberculosis (TB) once belonged to a bygone era. However, this bacterial disease is now one of the three most deadly infectious diseases globally. The bacteria can hide for years in the human immune system, but research reveals that they are not dormant but are active while they wait for the opportunity to strike. This also presents opportunities to combat them.
We go about our daily lives without knowing that one of the world’s greatest killers is inside some of us: Mycobacterium tuberculosis. It hides latent in the macrophages of our immune system and waits for better times – such as the immune system weakening. However, Danish researchers have now shown that the bacteria – contrary to what was previously thought – are active while latent. The Journal of Medical Microbiology has published the results.
“Previously we thought that the TB bacteria shut down almost completely while hiding in the immune system, but our results show that the bacteria’s genes mutate at a rate that suggests that they are quite active. This may seem alarming because it shows that the bacteria are indeed alive and can cause active TB if the immune system weakens. Nevertheless, this enables us to combat the bacteria before they erupt,” explains Troels Lillebæk, a main author of the article and Head of the International Reference Laboratory of Mycobacteriology at Statens Serum Institut.
Some researchers suspected that the bacteria were active, but they could not confirm this because of difficulties in finding old genetic material from living bacteria in humans that could be directly compared with more recent genetic material from living bacteria and thereby enable researchers to prove that the bacteria had changed.
Developing active TB 33 years after being infected
“Researchers previously had to settle for dead old genetic material of poor quality, such as from the mummies of people who had died from TB. Since Denmark traditionally stores live biological samples such as bacteria and can link these with Danish social security numbers, we found a father and son who had developed active TB 33 years apart – almost certainly from the same bacterial strain. We were therefore able to compare their TB strains.”
The father probably infected his son when the father developed active TB in 1961. Then the bacteria were latent for 33 years in the son until he developed active TB in 1994. The researchers could therefore determine based on these and other test pairs how much the bacteria had mutated in the intervening period. This turned out to be more than expected.
“For many years, the bacteria were believed to be inactive and not grow in their latent phase. However, the new results showed that they are just as active genetically as when they infect people during active TB. This may be because the human immune system affects and harms the DNA of the bacteria,” explains another main author of the article, Lars Jelsbak, Professor with Special Responsibilities, Department of Biotechnology and Biomedicine, Technical University of Denmark.
Bacteria in waiting
According to Lars Jelsbak, the mutations in the bacteria’s DNA may well stem from the bacteria experiencing selective pressure that continually tunes their DNA to enable them to erupt again.
“It is still too early to determine whether the genetic mutations result from one cause or the other. However, the results do suggest that the bacteria’s DNA continually undergo small mutations that enable the bacteria to survive better in the human immune system. The process also presumably optimizes their chances of being reactivated and prepares them to erupt and cause active TB.”
The new results are the first concrete human evidence that these bacteria, which can remain dormant for decades without apparently being active, are actually active. Previous research on animals had indicated this, but the experiments often only lasted for a couple of weeks. In this context, Statens Serum Institut’s unique collection of freeze-dried samples from people who have had active TB is an invaluable resource.
“Now that we can be almost certain that the bacteria are active, the chances of combating them improve correspondingly. The latent phase involves many fewer bacteria, so the potential for combating them more effectively is much greater,” adds Lars Jelsbak.
An enormous problem
Experts believe that one third of the world’s population is infected with Mycobacterium tuberculosis today. Of the people infected, 10% will develop active TB, and 10% of these will die. Active TB is most prevalent in Asia and Africa, where up to 80% of the people are infected in some areas and many more develop active TB because they are also living with HIV and therefore have a weakened immune system. Globally, more than 1.5 million people die from active TB each year.
“In Denmark, about 300–400 people develop active TB annually, and up to 10% of them die, often from other causes. Although, overall, TB is relatively limited in Denmark, it is still a major problem for socially disadvantaged people, who have a very high prevalence of active TB, even higher than in neighbouring countries. In addition, this problem may become even more severe in high-income countries as the numbers of multidrug-resistant bacteria increase. Denmark has had several cases of multidrug-resistant TB in the past 2 years,” explains Troels Lillebæk.
Multidrug-resistant bacteria annually cause almost 500,000 cases of TB disease globally, and extensively drug-resistant bacteria cause almost 50,000 cases. This may impose an enormous financial burden on health systems globally because the costs of treating one person who has extensively drug-resistant TB can total almost DKK 4 million.
Erupting again and again
According to Troels Lillebaek, one of the main reasons why TB remains one of the three most deadly infectious diseases globally, along with HIV infection and malaria, is the bacteria’s ability to remain latent for decades. In Denmark, the prevalence of Mycobacterium tuberculosis bottomed out in the 1980s – such that eradication was believed to be possible, but this did not happen.
“The lesson we learned is that Mycobacterium tuberculosis needs to be combated continually, because it tends to erupt in a new form again and again. In the 1980s, the focus shifted to other diseases considered to be more threatening. TB was therefore able to regain a foothold.”
This is why the new results may prove to be invaluable. The researchers have not only discovered that the bacteria might be combated in their latent phase because of their activity; they have also identified genetic profiles in Mycobacterium tuberculosis bacteria that are especially successful through other samples from Statens Serum Institut.
“We can see that one strain of Mycobacterium tuberculosis called cluster 2 has become more prevalent since 1992. At that time, about five such cases were reported per year in Denmark. Today, that has increased to about 60 cases per year. This indicates that these changes enable the bacteria to more easily spread and make people ill. We hope that our research can better characterize these bacteria so that we can hopefully combat them effectively.”
“Substantial molecular evolution and mutation rates in prolonged latent Mycobacterium tuberculosis infection in humans” has been published in the Journal of Medical Microbiology. In 2013, the Novo Nordisk Foundation awarded a grant to Lars Jelsbak for the project Genomic Adaptation of Mycobacterium tuberculosis in Latent Infection and Active Disease. Read more about TB research at Statens Serum Institut.