Drug Resistance and Fitness in Mycobacterium tuberculosis Infection
Institut für Medizinische Mikrobiologie, Universitt Zürich, Zürich, Switzerland
Karolinska Institute and Swedish Institute for Infectious Diseases Control, Solna, Sweden
In a recent article, Burgos et al. [1] investigated the effect of drug resistance on the generation of secondary cases of tuberculosis, because previous studies have yielded contradictory data on the pathogenicity and transmission of drug-resistant strains of Mycobacterium tuberculosis [2]. On the basis of epidemiological data, those authors quantified the number of secondary cases generated by drug-resistant versus drug-susceptible strains, to calculate the relative secondary case-rate ratio (SR). They concluded that, in the context of an effective tuberculosis control program, strains that were resistant to isoniazid either alone or in combination with other drugs were less likely to result in secondary cases than were drug-susceptible strains. However, there were large differences in SRs for resistance to different drugs, such as a decrease in SR for isoniazid resistance (SR, 0.29), no effect on SR for streptomycin resistance (SR, 0.88), and an increase in SR for rifampicin resistance (SR, 2.33). Unless there are convincing reasons provided to explain these differences, these findings might indicate that the parameters of the study and the methods of analyses were not very robust. It is particularly difficult to explain why rifampicin resistance should increase the number of secondary cases. A priori, there is no reason why drug resistance should increase the SR, unless one assumes general factors, such as prolonged transmission due to ineffective drug treatment. This should, however, affect the drugs equally, if standard treatment procedures are used.
Experimental data and mathematical models have suggested that the reduction of bacterial fitness (i.e., reduced transmission between hosts and reduced persistence and growth within hosts) imposed by antimicrobial resistance could influence the frequency of drug-resistant microorganisms in a population [3, 4]. In this respect, we refer to a very eloquent article by G. Canetti [5]. Canetti addressed the question of resistance-related fitness costs by studying, at a phenotypic level, the resistance observed in primary drug-resistant strains versus those with acquired drug resistance (primary resistance is defined as infection with a resistant strain; acquired resistance is defined as drug resistance that emerges during chemotherapy). In contrast to acquired resistance, primary resistance reflects additional parameters, such as fitness and transmission. Canetti found that, for isoniazid but not for streptomycin, the proportion of high-level drug resistance was considerably lower for strains with primary resistance than for strains with acquired resistance. This seemed to indicate that, in contrast to streptomycin, high-level isoniazid resistance is associated with a defined fitness cost.
The results of epidemiological investigations are complemented by studies that have examined the mechanisms of drug resistance at a molecular level, particularly those that have addressed the question of fitness cost related to the acquisition of a resistance determinant. Although corresponding studies in mycobacteria have been scarce [610], some of them have provided interesting insights, such as those that relate to resistance to streptomycin and to isoniazid.
The fitness cost of various chromosomal mutations was experimentally determined and was found to be dependent on the chromosomal alteration that mediates resistance to streptomycin [9]. Drug-resistant mutants obtained by in vitro selection in the laboratory are characterized by a variety of different possible resistance mutations. In contrast, streptomycin resistance in clinical M. tuberculosis isolates nearly invariably is associated with the lysinearginine alteration at amino acid 42 of rpsL [7]. Interestingly, the lysinearginine alteration at amino acid 42 of rpsL is the only streptomycin resistance determinant among the mutational alterations studied that was found to not carry a fitness cost, as determined experimentally in vitro [9]. These basic molecular studies provide a mechanistic explanation for the pioneering epidemiological observations by Canetti.
Most of the many chromosomal alterations that result in resistance to isoniazid are associated with a significant fitness cost [6, 8], although the serinethreonine resistance mutation at amino acid 315 of KatG was found to not affect in vivo growth in an experimental animal model [8]. In accordance with the early findings of Canetti [5] and the results reported by Burgos et al. [1], it was found that isoniazid-resistant strains in general were significantly less clustered than were isoniazid-susceptible strains [11]. However, more-refined analysis of the epidemiological data revealed that aa-315 isoniazid-resistant mutants led to secondary cases of tuberculosis as often as drug-susceptible strains [12].
A picture emerges in which, among various resistance mutations that appear with similar rates, those associated with the least fitness cost are most likely to become selected in the population. To allow the implementation of rational strategies to combat the problem of drug-resistant tuberculosis, we need an integrated view that combines epidemiology, molecular mechanisms of resistance, and a relevant experimental determination of how drug resistance affects the entire life cycle of M. tuberculosis (the establishment of infection, progression to disease, and transmission).
References
1. Burgos M, DeRiemer K, Small PM, Hopewell PC, Daley CL. Effect of drug resistance on the generation of secondary cases of tuberculosis. J Infect Dis 2003; 188:187885. First citation in article
2. Cohen T, Sommers B, Murray M. The effect of drug resistance on the fitness of Mycobacterium tuberculosis. Lancet Infect Dis 2003; 3:1321. First citation in article
3. Andersson DI, Levin BR. The biological cost of antibiotic resistance. Curr Opin Microbiol 1999; 2:28993. First citation in article
4. Levin BR. Models for the spread of resistant pathogens. Neth J Med 2002; 60(Suppl 7):S5866. First citation in article
5. Canetti G. Present aspects of bacterial resistance in tuberculosis. Am Rev Respir Dis 1965; 92:687701. First citation in article
6. Cohn M, Kovitz C, Oda U. Studies on isoniazid and tubercle bacilli: the growth requirements, catalase activities, and pathogenic properties of isoniazid resistant mutants. Am Rev Tuberc 1954; 70:64164. First citation in article
7. Bttger EC, Springer B, Pletschette M, Sander P. Fitness of antibiotic-resistant microorganisms and compensatory mutations. Nat Med 1998; 4:13434. First citation in article
8. Pym AS, Saint-Joanis B, Cole ST. Effect of katG mutations on the virulence of Mycobacterium tuberculosis and the implication for transmission in humans. Infect Immun 2002; 70:495560. First citation in article
9. Sander P, Springer B, Prammananan T, et al. Fitness cost of chromosomal drug resistanceconferring mutations. Antimicrob Agents Chemother 2002; 46:120411. First citation in article
10. Mariam DH, Mengistu Y, Hoffner SE, Andersson DI. Effect of rpoB mutations conferring rifampicin resistance on fitness of Mycobacterium tuberculosis. Antimicrob Agents Chemother 2004; 48:128994. First citation in article
11. van Soolingen D, Borgdorff MW, de Haas PE, et al. Molecular epidemiology of tuberculosis in the Netherlands: a nationwide study from 1993 through 1997. J Infect Dis 1999; 180:72636. First citation in article
12. van Soolingen D, de Haas PE, van Doorn HR, Kuijper E, Rinder H, Borgdorff MW. Mutations at amino acid position 315 of the katG gene are associated with high-level resistance to isoniazid, other drug resistance, and successful transmission of Mycobacterium tuberculosis in The Netherlands. J Infect Dis 2000; 182:178890. First citation in article, 百拇医药(Erik C. Bttger, Michel Pl)
Karolinska Institute and Swedish Institute for Infectious Diseases Control, Solna, Sweden
In a recent article, Burgos et al. [1] investigated the effect of drug resistance on the generation of secondary cases of tuberculosis, because previous studies have yielded contradictory data on the pathogenicity and transmission of drug-resistant strains of Mycobacterium tuberculosis [2]. On the basis of epidemiological data, those authors quantified the number of secondary cases generated by drug-resistant versus drug-susceptible strains, to calculate the relative secondary case-rate ratio (SR). They concluded that, in the context of an effective tuberculosis control program, strains that were resistant to isoniazid either alone or in combination with other drugs were less likely to result in secondary cases than were drug-susceptible strains. However, there were large differences in SRs for resistance to different drugs, such as a decrease in SR for isoniazid resistance (SR, 0.29), no effect on SR for streptomycin resistance (SR, 0.88), and an increase in SR for rifampicin resistance (SR, 2.33). Unless there are convincing reasons provided to explain these differences, these findings might indicate that the parameters of the study and the methods of analyses were not very robust. It is particularly difficult to explain why rifampicin resistance should increase the number of secondary cases. A priori, there is no reason why drug resistance should increase the SR, unless one assumes general factors, such as prolonged transmission due to ineffective drug treatment. This should, however, affect the drugs equally, if standard treatment procedures are used.
Experimental data and mathematical models have suggested that the reduction of bacterial fitness (i.e., reduced transmission between hosts and reduced persistence and growth within hosts) imposed by antimicrobial resistance could influence the frequency of drug-resistant microorganisms in a population [3, 4]. In this respect, we refer to a very eloquent article by G. Canetti [5]. Canetti addressed the question of resistance-related fitness costs by studying, at a phenotypic level, the resistance observed in primary drug-resistant strains versus those with acquired drug resistance (primary resistance is defined as infection with a resistant strain; acquired resistance is defined as drug resistance that emerges during chemotherapy). In contrast to acquired resistance, primary resistance reflects additional parameters, such as fitness and transmission. Canetti found that, for isoniazid but not for streptomycin, the proportion of high-level drug resistance was considerably lower for strains with primary resistance than for strains with acquired resistance. This seemed to indicate that, in contrast to streptomycin, high-level isoniazid resistance is associated with a defined fitness cost.
The results of epidemiological investigations are complemented by studies that have examined the mechanisms of drug resistance at a molecular level, particularly those that have addressed the question of fitness cost related to the acquisition of a resistance determinant. Although corresponding studies in mycobacteria have been scarce [610], some of them have provided interesting insights, such as those that relate to resistance to streptomycin and to isoniazid.
The fitness cost of various chromosomal mutations was experimentally determined and was found to be dependent on the chromosomal alteration that mediates resistance to streptomycin [9]. Drug-resistant mutants obtained by in vitro selection in the laboratory are characterized by a variety of different possible resistance mutations. In contrast, streptomycin resistance in clinical M. tuberculosis isolates nearly invariably is associated with the lysinearginine alteration at amino acid 42 of rpsL [7]. Interestingly, the lysinearginine alteration at amino acid 42 of rpsL is the only streptomycin resistance determinant among the mutational alterations studied that was found to not carry a fitness cost, as determined experimentally in vitro [9]. These basic molecular studies provide a mechanistic explanation for the pioneering epidemiological observations by Canetti.
Most of the many chromosomal alterations that result in resistance to isoniazid are associated with a significant fitness cost [6, 8], although the serinethreonine resistance mutation at amino acid 315 of KatG was found to not affect in vivo growth in an experimental animal model [8]. In accordance with the early findings of Canetti [5] and the results reported by Burgos et al. [1], it was found that isoniazid-resistant strains in general were significantly less clustered than were isoniazid-susceptible strains [11]. However, more-refined analysis of the epidemiological data revealed that aa-315 isoniazid-resistant mutants led to secondary cases of tuberculosis as often as drug-susceptible strains [12].
A picture emerges in which, among various resistance mutations that appear with similar rates, those associated with the least fitness cost are most likely to become selected in the population. To allow the implementation of rational strategies to combat the problem of drug-resistant tuberculosis, we need an integrated view that combines epidemiology, molecular mechanisms of resistance, and a relevant experimental determination of how drug resistance affects the entire life cycle of M. tuberculosis (the establishment of infection, progression to disease, and transmission).
References
1. Burgos M, DeRiemer K, Small PM, Hopewell PC, Daley CL. Effect of drug resistance on the generation of secondary cases of tuberculosis. J Infect Dis 2003; 188:187885. First citation in article
2. Cohen T, Sommers B, Murray M. The effect of drug resistance on the fitness of Mycobacterium tuberculosis. Lancet Infect Dis 2003; 3:1321. First citation in article
3. Andersson DI, Levin BR. The biological cost of antibiotic resistance. Curr Opin Microbiol 1999; 2:28993. First citation in article
4. Levin BR. Models for the spread of resistant pathogens. Neth J Med 2002; 60(Suppl 7):S5866. First citation in article
5. Canetti G. Present aspects of bacterial resistance in tuberculosis. Am Rev Respir Dis 1965; 92:687701. First citation in article
6. Cohn M, Kovitz C, Oda U. Studies on isoniazid and tubercle bacilli: the growth requirements, catalase activities, and pathogenic properties of isoniazid resistant mutants. Am Rev Tuberc 1954; 70:64164. First citation in article
7. Bttger EC, Springer B, Pletschette M, Sander P. Fitness of antibiotic-resistant microorganisms and compensatory mutations. Nat Med 1998; 4:13434. First citation in article
8. Pym AS, Saint-Joanis B, Cole ST. Effect of katG mutations on the virulence of Mycobacterium tuberculosis and the implication for transmission in humans. Infect Immun 2002; 70:495560. First citation in article
9. Sander P, Springer B, Prammananan T, et al. Fitness cost of chromosomal drug resistanceconferring mutations. Antimicrob Agents Chemother 2002; 46:120411. First citation in article
10. Mariam DH, Mengistu Y, Hoffner SE, Andersson DI. Effect of rpoB mutations conferring rifampicin resistance on fitness of Mycobacterium tuberculosis. Antimicrob Agents Chemother 2004; 48:128994. First citation in article
11. van Soolingen D, Borgdorff MW, de Haas PE, et al. Molecular epidemiology of tuberculosis in the Netherlands: a nationwide study from 1993 through 1997. J Infect Dis 1999; 180:72636. First citation in article
12. van Soolingen D, de Haas PE, van Doorn HR, Kuijper E, Rinder H, Borgdorff MW. Mutations at amino acid position 315 of the katG gene are associated with high-level resistance to isoniazid, other drug resistance, and successful transmission of Mycobacterium tuberculosis in The Netherlands. J Infect Dis 2000; 182:178890. First citation in article, 百拇医药(Erik C. Bttger, Michel Pl)