Evolution of drug resistance in TB

Student post submitted by Pritha Singh

In the article “Evolution of Drug Resistance in Mycobacterium tuberculosis: Clinical and Molecular Perspective”, the author Stephen Gillespie describes the clinical circumstances and the molecular mechanisms that are involved in the emergence of drug resistance in tuberculosis (TB). Even after so many years of introduction of very effective drug therapy for TB, the number of people infected worldwide is still increasing due to the development of drug resistance. The basic tool that the medical community has used to control this deadly disease is the combination therapy that uses antibiotics like isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), ethambutol (EMB) and streptomycin (SM).

The author first describes the clinical circumstances for the resistance development in Mycobacterium tuberculosis. He states that the reason why drug therapy for tuberculosis is different than the most bacterial infections is due to the long generation time and ability of dormancy in M. tuberculosis. Apart from this, it has a very slow metabolic rate which makes it difficult to target. There are many different populations of the bacterium present within the host. M. tuberculosis may be found in the pulmonary cavities, empyema pus and solid caseous material. The location of the bacterium makes it very difficult for the antibiotics to penetrate or the low pH conditions interfere with the activity of the anti-TB drugs. Thus each of the anti-TB drugs has a unique role in dealing with these different populations of mycobacterium. For example, INH is a drug that is active only against aerobically growing organisms and so it plays a major role very early in the drug therapy against bacteria growing in pulmonary cavities. On the other hand, PZA is a drug only active in low pH conditions thus is used for killing bacilli residing inside the caseous necrotic foci. RIF is most effective in killing the mycobacterium that are metabolizing slowly. Hence, due to the specific roles of these drugs, poor adherence to the drug therapy can result in resistant strains in mycobacterium.

Gillespie goes on to explain the molecular mechanism for the emergence of drug resistance. The way the researchers have started to understand the molecular mechanism of resistance in M. tuberculosis is from the action of these anti-TB drugs. The resistance in this bacterium occurs through single step mutations at the chromosomal level. He states that rate at which resistance emerges is different for all the anti-TB drugs which could be calculated by using the mutation rate. Mutation rate rather than mutation frequency is used to calculate the rate of resistance because calculating frequency has a risk of recording mutation per cell division. However, mutation rate is more apt as it records the proportion of the mutant cells. The author provides an equation and hypothetical calculations for resistance rate in the mycobacterium which suggests that even a small deviation from the standard drug regimen may lead to the emergence of resistance in a TB patient.

To describe the development of drug resistance in M. tuberculosis, the author talks about two anti-TB drugs: streptomycin (SM) and rifampin (RIF). A point mutation in rpsL gene results in high level of resistance to SM which can be categorized into restrictive and nonrestrictive mutations. Restrictive mutations are associated with an attenuation of virulence, whereas nonrestrictive mutations are not. He lists various clinical studies that show that resistant strains were equally divided between restrictive and non-restrictive mutations. However, the author does not explain how SM works on the bacterium or how the mutation in a specific gene leads to resistant strains which could have helped in better understanding of this section. Moreover, he does not mention an additional mutation that takes place in rrs operon which could also lead to drug-resistance.

The mycobacterium strains that are resistant to RIF have a mutation on the beta subunit of rpoB gene encodes for DNA-dependent RNA polymerase. Experimental studies have shown that more than 70% of the RIF’s mutations are restricted to the rpoB gene. He also mentions the study conducted by using the model of guinea pigs that were infected with M. tuberculosis in which katG gene was inactivated. These studies showed that the virulence of the strains was far less than the parent strain. However, the virulence was restored when katG gene was reintegrated in the genome. The author does a fair job in stating the clinical studies, however does not mention anything about the drug mechanism. KatG gene encodes for enzymes that are involved in mycolic acid biosynthesis but there was no mention of how mutation in katG gene is what leads to the INH resistance.

Lastly, the author mentions about human studies of resistance emergence. He mentions a report in which a brother and a sister suffered multiple drug resistance due to non adherence to the drug therapy. This case provides the evidence that variation in biological fitness has an affect on outcome of the therapy. In this study, the bacterium in one case showed multiple-drug resistance (MDR) whereas in the other case, it was completely susceptible which suggested that fitness deficit was directly related to the difference in susceptibility. However, Gillespie also mentions that multidrug-resistant M. tuberculosis strains with identical susceptibilities have different in vitro fitnesses. He provides data of a study conducted on an infected human female that suggests that changes occur on passage in humans. Since initially resistant strains have a fitness deficit, transmission of the organism in a group of immunocomprised people may allow the bacterium to adapt and be transmitted while adapting. This also agrees with previous studies that show MDR numbers have increased in immunocomprised patients.

The author ends his articles by stating that these molecular and clinical studies have shown that resistant organisms over time can become fully virulent. Thus to prevent the multi drug resistance tuberculosis, steps must be taken that patients are effectively treated. Drug-resistance is a major threat to human population and to stop these numbers from rising, it is important to understand the mechanisms of resistance development in this organism. Treatment with internationally approved regimens is very effective in preventing the resistance development. This is due to the fact that combination of drugs involved in the therapy makes it very unlikely for spontaneous mutations to occur to all the antibiotics involved. Inadequate treatment of tuberculosis or treatment with only one type of drug can lead to resistant strains. When these strains that are resistant to single agents are exposed to the combination drug therapy, the effectiveness of the drug therapy is depleted which further leads to multiple-drug resistance.

The author of this article Stephen H. Gillespie is a Professor of Medical Microbiology at University College London. He has done a lot of research in the treatment and diagnosis of tuberculosis and other respiratory infections. The author provides plenty of reference articles and studies to refer for further clarification for the reader. He backs up his statements with evidence collected from various research studies. However, the author’s use and interpretation of the evidence was unclear to me in many sections. To wrap up, my opinion of this article is that it was very difficult to follow at times and the author should have included some basic information about the drug action on the mycobacterium before talking about the gene mutations that lead to the drug resistance.

Reference:

Gillespie, S.H. (2002). Evolution of Drug Resistance in Mycobacterium tuberculosis: Clinical and Molecular Perspective. Antimicrobial Agents and Chemotherapy, 46(2), 267-274.