Tuberculosis Strains are Mutating to Resist Drugs and Increase Fitness
by Kathleen Jia
For most people, coughing and sneezing may rarely be acknowledged beyond a perfunctory Òbless youÓ, but these symptoms are very serious for patients suffering from tuberculosis. Often thought of as a disease of the past, tuberculosis is actually a major health problem in the world today. A Stanford research project recently made progress toward a better understanding of tuberculosis (TB) and multi-drug resistant tuberculosis (MDR-TB). Led by Stanford epidemiologists Gary K. Schoolnik and Peter M. Small (now at the Bill and Melinda Gates Foundation), Associate Professor of Biological Sciences Brendan J. M. Bohannan, former postdoctoral fellow Sebastien Gagneux and graduate student Clara Davis Long, the study examined a long-held belief that bacteria with drug-resistant mutations are at a competitive disadvantage compared to drug-susceptible bacteria. Results of the study, however, revealed that some types of mutations carry no competitive cost.
Tuberculosis Today According to the World Health Organization, one-third of the world's population is currently infected with the TB bacillus, although most cases are dormant. Approximately 1.7 million people die from TB every year, making it the second leading cause of death from infectious disease after AIDS. In fact, HIV/TB co-infection is dismayingly common in areas with high HIV infection. Drug-resistant TB arises mainly in areas lacking proper control programs, where health care workers may administer improper treatment or fail to ensure that patients complete the whole course of treatment.
Although tuberculosis is a curable disease with a high survival rate, MDR-TB is much more difficult and expensive to treat. Drug-susceptible TB is treated with a combination of two to four powerful first-line drugs over a period of 6-9 months. If TB develops resistance to the two most powerful drugs rifampin and isoniazid, then it is considered MDR-TB. MDR-TB treatment requires expensive and toxic second-line drugs for a period of two years. Despite the long treatment, the cure rate of MDR-TB is only about 50%. Strains that are resistant to a single drug have been documented in every country, and strains of TB resistant to all major anti-TB drugs have been found. For example, XDR-TB, or Extensive Drug Resistant TB (also referred to as Extreme Drug Resistance) is MDR-TB that is also resistant to at least three of the six classes of second-line drugs. XDR-TB may be virtually untreatable with the TB drugs that are available today.
Clinical TB vs Lab-Grown TB
Dr. Gagneux describes the source of clinical TB used for the recent study comparing the fitness of drug-sensitive TB and drug resistant TB. ÒWe took advantage of a long-term study of active tuberculosis in San Francisco which started in 1991. At the time, an outbreak of disease swept through the City and Drs. Schoolnik and Small began collecting samples from every TB case in San Francisco both before and after treatment. Based on this database, we were able to identify ten patients who had developed drug resistance during their treatment and used their stored TB isolates in our study.Ó
Previous data using lab-grown Mycobacterium tuberculosis, the bacteria responsible for TB, supports the belief that antibiotic resistance often causes reduced competitiveness as compared to drug-susceptible strains. However, current research looking at TB from patients draws a different conclusion. This research measured the competitive fitness of clinical bacterial strains resistant to the powerful first-line drug rifampin against their drug-sensitive ancestors.
Clinical MDR-TB with Greater Fitness than TB
DNA fingerprinting methods were used to show that resistant (MDR-TB) and non-resistant (TB) strains from the same patient were of common ancestry. Rifampin resistance is caused by mutations in rpoB, the gene encoding a subunit of RNA polymerase necessary for gene transcription. Each of the mutated strains was distinct, but five had an rpoB S531L mutation, replacing the 531st amino acid serine with leucine. The remaining five had other rpoB mutations. Results of the study showed that all mutants without the S531L mutation had relative fitness less than 1.0, meaning they were less competitively fit than wild-type, drug sensitive TB ancestors. In contrast, four of the five clinical strains with the rpoB S531L mutation had a relative fitness greater than or equal to 1.0, meaning they were more or equally fit than their drug sensitive TB ancestors. These four strains were the only rifampin-resistant mutants found to have no fitness cost.
ÒA major point in the study is that this low cost mutation exists independent of the strainÕs genetic background, whereas the fitness effects of other rpoB mutations depends on the strain background,Ó said Dr. Gagneux. The results of the study could be explained in two ways. There may be no fitness cost associated with the S531L mutation. Alternatively, enough time had passed after this mutation for compensatory mutations to arise that ameliorate the high fitness cost. The second explanation is supported by Gagneux's data, because clinical mutants taken from patients had a competitive fitness of 1.04, which is greater than the laboratory-derived S531L mutants that had an average competitive fitness of 0.93.
The Future of TB
The results of this research have implications for predictions about the future of TB worldwide. When given time, drug-resistant TB will mutate further to gain increased fitness. As Dr. Gagneux describes it, ÒOur research also shows we have to do everything possible to prevent the emergence of drug resistance in the first place. Some people think that to stop drug resistance, we only need to stop drug treatment and wait until the drug susceptible bacteria overpowers the less fit drug-resistant bacteria. This idea is wrong.Ó Essentially, powerful new TB drugs requiring a shorter treatment period are needed to effectively treat and control TB, and to keep MDR-TB from gaining increased fitness.
The project to track TB transmission in San Francisco is still an ongoing effort. The project provides information not only for the study of drug susceptible TB, but also for drug resistant MDR-TB. Dr. Gagneux explains that Òtracking the genotypes of ongoing transmission helps TB control by narrowing the focus to target stains that are spreading and causing outbreaks.Ó
Confronting TB is a major worldwide health issue that must continue to be addressed in the coming years and decades. Though the task is difficult and daunting, Gagneux's study and others can point TB control in the right direction and help make TB truly a disease of the past.