rpoB基因在结核分枝杆菌利福平/利福布丁交叉耐药株中突变情况的差异分析

目的 了解结核分枝杆菌利福平/利福布丁交叉耐药株与利福平耐药/利福布丁敏感株rpoB全基因突变特征的差异.方法 测定rpoB全基因序列,比较278株利福平/利福布丁交叉耐药(rifampicin/rifabutin cross-resistant,RIF/Rfb-R)株和40株利福平耐药/利福布丁敏感(rifampicinresistant/rifabutin-susceptible,RIF-R/Rfb-S)株,30株利福平/利福布丁敏感(rifampicin-susceptible/rifabutin-susceptible,RIF-S/Rfb-S)株及标准株H37 Rv之间的rpoB全基因序列突变差异.结果 利福平/利福布丁敏感株和H37Rv rpoB全基因未发现突变;利福平/利福布丁交叉耐药株的常见突变位点是531(70.5％)和526( 20.9％).223( 80.2％)株单点突变株分别含有S531L、S531W、H526D、H526Y、H526R、Q513K、Q513P、Q510H、V176F、P287R、Y395C及H404Y单点突变,55( 19.8％)株多重突变株分别以S531L、H526R、H526Y、H526D、D516G和Q513K位点与其他位点联合突变为主.利福平耐药/利福布丁敏感株的常见突变位点分别是516(65.0％)、526( 17.5％)和533( 10.0％).21(52.5％)株单点突变株分别含有L533P、H526L、H526S、S522L、D516V、D516Y和D516F单点突变,19(47.5％)株多重突变株分别以D516V和L533P位点与其他位点联合突变为主.结论 本批标本中,利福霉素耐药株rpoB基因突变率为100％,利福平/利福布丁交叉耐药株与利福平耐药/利福布丁敏感株rpoB全基因单点突变株的突变位置或氨基酸替换类型、多重突变株的突变位置或组合类型以及常见突变位点或其氨基酸替换类型完全不同,rpoB全基因DNA序列分析对临床科学应用利福平和利福布丁有指导意义.     

Direct detection in clinical samples of multiple gene mutations causing resistance of Mycobacterium tuberculosis to isoniazid and rifampicin using fluorogenic probes

BACKGROUND: This study evaluates a method based on real-time PCR for direct detection in clinical samples of the common mutations responsible for isoniazid and rifampicin resistance of Mycobacterium tuberculosis. METHODS: Six pairs of fluorogenic 5' exonuclease probes (Taqman), mutated and wild-type, were designed for six targets: codon 315 of katG, substitution C209T in the regulatory region of inhA, and codons 513, 516, 526 and 531 of rpoB. RESULTS: A total of 98 clinical samples harbouring resistant bacilli from 55 patients and 126 samples harbouring susceptible bacilli from 126 patients were processed. The isolates from samples were tested for drug susceptibility with the radiometric method and sequenced for the same genetic targets. Among the samples, 93 harboured isoniazid-resistant bacilli. According to the sequencing results, 30 had mutations in katG, 30 in inhA and 33 (35.4%) had no mutations in these targets. All 27 clinical specimens harbouring rifampicin-resistant bacilli showed mutations in rpoB. The detection threshold of this method in detecting target genes in serial dilutions of artificial samples was 1.5 x 10(3) cfu/mL. In clinical samples, the sensitivity ranged from 30.4 to 35.3% for smear-negative samples and from 95.1 to 99.2% for smear-positive samples, with a specificity of 100%. In this study, the overall sensitivity in detecting patients having the target mutations was 74.3%. CONCLUSIONS: The main advantage of the described method is the possibility of detecting rifampicin and isoniazid resistance within 48-72 h after sample collection, with a sensitivity of nearly 100% in smear-positive samples if the chosen target is responsible for the resistance.     

耐多药结核分枝杆菌耐药相关基因突变特征分析

目的了解中国耐多药结核分枝杆菌耐药相关基因的分子特征。方法对138株耐多药结核分枝杆菌和45株敏感菌的耐药相关基因inhA、katG和oxyR-ahpC间隔区(异烟肼)、rpob(利福平)、gyrA(氧氟沙星)和rrs(卡那霉素)进行序列测定,分析其基因突变特点。结果 138株耐多药结核分枝杆菌中,14.4%的菌株inhA基因发生突变,72.5%菌株的katG基因发生突变,15.9%菌株的oxyR-ahpC基因发生突变,同时考虑这3种基因,异烟肼耐药相关基因突变检出率可达90.6%;94.2%菌株的rpoB基因发生突变,74.5%菌株的gyrA基因发生突变,61.1%菌株的rrs基因发生突变,主要的突变位点为katG315(66.7%),inhA-15(9.4%),oxyR-ahpC-10(5.1%),rpoB516(13.8%),526(26.1%)和531(49.3%),gyrA90(21.6%)和94(51%),rrs1401(61.1%)。结论我国耐多药结核菌异烟肼、利福平、氧氟沙星和卡那霉素耐药相关基因最常见突变为katG315、inhA-15,rpoB531、526和516,gyrA94和90,rrs1401。     

Molecular characterization of isoniazid-resistant Mycobacterium tuberculosis isolates collected in Australia

Elucidation of the molecular basis of isoniazid (INH) resistance in Mycobacterium tuberculosis has led to the development of different genotypic approaches for the rapid detection of INH resistance in clinical isolates. Mutations in katG, in particular the S315T substitution, are responsible for INH resistance in a large proportion of tuberculosis cases. However, the frequency of the katG S315T substitution varies with population samples. In this study, 52 epidemiologically unrelated clinical INH-resistant M. tuberculosis isolates collected in Australia were screened for mutations at katG codon 315 and the fabG1-inhA regulatory region. Importantly, 52 INH-sensitive isolates, selected to reflect the geographic and genotypic diversity of the isolates, were also included for comparison. The katG S315T substitution and fabG1-inhA -15 C-to-T mutation were identified in 34 and 13 of the 52 INH-resistant isolates, respectively, and none of the INH-sensitive isolates. Three novel katG mutations, D117A, M257I, and G491C, were identified in three INH-resistant strains with a wild-type katG codon 315, fabG1-inhA regulatory region, and inhA structural gene. When analyzed for possible associations between resistance mechanisms, resistance phenotype, and genotypic groups, it was found that neither the katG S315T nor fabG1-inhA -15 C-to-T mutation clustered with any one genotypic group, but that the -15 C-to-T substitution was associated with isolates with intermediate INH resistance and isolates coresistant to ethionamide. In total, 90.4% of unrelated INH-resistant isolates could be identified by analysis of just two loci: katG315 and the fabG1-inhA regulatory region.     

Evaluation of the CombiChip Mycobacteria Drug-Resistance detection DNA chip for identifying mutations associated with resistance to isoniazid and rifampin in Mycobacterium tuberculosis

The CombiChip Mycobacteriatrade mark Drug-Resistance Detection DNA chip, recently developed by GeneIn (Pusan, South Korea), is an oligonucleotide microchip coupled with polymerase chain reaction for the detection of mutations associated with resistance to isoniazid (INH) and rifampin (RIF). This oligonucleotide chip was compared with DNA sequencing and phenotypic drug susceptibility testing with 69 INH- and/or RIF-resistant and 27 all tested drug-susceptible Mycobacterium tuberculosis isolates. Two selected codons (the katG codon 315 and inhA15) allowed identification of 84.1% of INH-resistant isolates and 100% of RIF resistance were detected by screening for 7 codons: rpoB511, rpoB513, rpoB516, rpoB522, rpoB526, rpoB531, and rpoB533. The overall specificity of this oligonucleotide chip for detecting INH and RIF resistance were 100 and 95.3%, respectively. This level of sensitivity and specificity is concordant with that from the determination of M. tuberculosis drug resistance by DNA sequencing. This oligonucleotide chip is a rapid and reliable genotypic method capable of detecting multiple mutations associated with INH and RIF resistance simultaneously in a single microchip slide.     

Molecular characterization of multidrug-resistant Mycobacterium tuberculosis isolated in Nepal

Despite the fact that Nepal is one of the first countries globally to introduce multidrug-resistant tuberculosis (MDR-TB) case management, the number of MDR-TB cases is continuing to rise in Nepal. Rapid molecular tests applicable in this setting to identify resistant organisms would be an effective tool in reversing this trend. To develop such tools, information about the frequency and distribution of mutations that are associated with phenotypic drug resistance in Mycobacterium tuberculosis is required. In the present study, we investigated the prevalence of mutations in rpoB and katG genes and the inhA promoter region in 158 M. tuberculosis isolates (109 phenotypically MDR and 49 non-MDR isolates collected in Nepal) by DNA sequencing. Mutations affecting the 81-bp rifampin (RIF) resistance-determining region (RRDR) of rpoB were identified in 106 of 109 (97.3%) RIF-resistant isolates. Codons 531, 526, and 516 were the most commonly affected, at percentages of 58.7, 15.6, and 15.6%, respectively. Of 113 isoniazid (INH)-resistant isolates, 99 (87.6%) had mutations in the katG gene, with Ser315Thr being the most prevalent (81.4%) substitution. Mutations in the inhA promoter region were detected in 14 (12.4%) INH-resistant isolates. The results from this study provide an overview of the current situation of RIF and INH resistance in M. tuberculosis in Nepal and can serve as a basis for developing or improving rapid molecular tests to monitor drug-resistant strains in this country.     

结核分枝杆菌katG、inhA、ahpC等突变与异烟肼耐药关系的研究

目的：了解结核分枝杆菌katG、inhA、ahpC、fabG1、sodA及sodC基因突变的特征及其与耐异烟肼的关系。方法对127例活动性肺结核患者痰标本进行菌型鉴定及结核分枝杆菌药敏试验,提取结核分枝杆菌菌株DNA,应用PCR扩增katG、inhA及ahpC、fabG1、sodA及sodC基因片段,并进行DNA序列分析。结果结核分枝杆菌药物敏感试验显示127株结核分枝杆菌中,其中47株耐异烟肼,80株对异烟肼敏感,耐异烟肼率为37.01%。47株耐异烟肼中,29株存在katG和（或）inhA基因突变,其中22株（46.81%,22/47）存在katG基因单位点突变,3株（6.38%,3/47）存在inhA基因单位点突变,4株（8.51%,4/47）存在katG及inhA基因联合位点突变。22株katG基因单位点突变中,20株为AGC315ACC、AGC315AAC （42.55%,20/47）突变,2株（2.13%,1/47）分别为CTG378CCG（Leu378Pro）、ACG394ATG（Thr394Met）突变,该突变位点及突变形式尚未见文献报道。18株katG及inhA未突变结核分枝杆菌均未检测到ahpC、fabG1、sodA及sodC基因突变。结论结核分枝杆菌对异烟肼耐药主要与katG和inhA基因突变有关。耐异烟肼结核分枝杆菌临床分离株378和394新突变位点的发现为进一步研究耐药机制以及耐药结核病的快速检测提供了依据。     

Molecular basis of rifampicin resistance in Haemophilus influenzae

OBJECTIVES: To determine the molecular basis of rifampicin resistance in Haemophilus influenzae. METHODS: Mutations in the rifampicin-resistance determining region of the rpoB gene of H. influenzae were analysed by gene amplification and sequencing in 12 rifampicin resistant, one intermediate and four susceptible isolates. RESULTS: All clinical resistant isolates except one had at least one amino acid substitution in the beta-subunit of RNA polymerase. Eleven resistant isolates had amino acid changes at codons 513, 516, 518, 526 and 533 of cluster I, with the most common amino acid substitution being Asp-516-->Val. Only one resistant isolate also had a second mutation Asn-518-->Asp in cluster I; transformants obtained with DNA of this isolate also had both mutations. All the amino acid changes in cluster I were detected in isolates with a high level of rifampicin resistance (MIC > or =32 mg/L), except the Asp-516-->Ala mutation in a low-level resistant isolate (MIC 4 mg/L). Only one serotype f isolate with an MIC of 2 mg/L had a mutation in cluster II. Cluster III presented no amino acid changes. In in vitro-generated high-level rifampicin-resistant mutants, only amino acid changes at codons 516 and 526 were seen, with new amino acid changes appearing at codon 526 of cluster I, while His-526-->Asn was associated with low-level resistance. CONCLUSIONS: Rifampicin resistance in H. influenzae is due to point mutations in the rpoB gene, and the resistance levels are dependent on both the location and the nature of amino acid substitution.     

耐药结核分枝杆菌基因突变分析

目的 探讨结核分枝杆菌耐药表型与基因突变位点之间的相互关系.方法 采用序列特异性引物分别扩增92株结核分枝杆菌利福平耐药基因rpoB,异烟肼耐药基因katG、inhA、ahpC,链霉素耐药基因rrs、rpsL,乙胺丁醇耐药基因embB及喹诺酮耐药基因gyrA,SSCP筛选出突变序列,DNA测序分析突变性质.结果 59株利福平耐药株rpoB基因突变检出率94.9%(56/59),以Ser450Trp突变最多;90株异烟肼耐药株中,katG基因突变检出率38.9%(35/90),以Ser315Thr最多,3株检出inhA基因突变,ahpC基因无突变检出;34株喹诺酮耐药株中gyrA基因突变检出率82.4%(28/34),主要为Asp94Gly,其次为Ala90Val;31株链霉素耐药株中,15株检出rrs突变,最常见为A514C和A1041G,10株发生rpsL Lys88Arg突变,总的链霉素基因突变检出率为77.4%(24/31);31株乙胺丁醇耐药株中embB 基因突变检出率19.4%(6/31),主要为Met306Val.结论 耐药结核分枝杆菌耐药情况较为严重,以DNA测序为基础的基因突变分析能快速有效地检测结核分枝杆菌的rpoB、katG、gyrA、rrs、rpsL、embB 等耐药分子标识,显示了西安地区耐药性结核分枝杆菌的突变特点,为结核病的临床诊断和合理用药提供了实验依据.     

Peptide nucleic acid-mediated competitive PCR clamping for detection of rifampin-resistant Mycobacterium tuberculosis

Peptide nucleic acid-mediated competitive PCR clamping, which can selectively amplify mutant alleles, was developed to detect mutations in four codons (513, 516, 526, and 531) of the rpoB gene in Mycobacterium tuberculosis strains. This simple method successfully identified the mutations in all 40 of the M. tuberculosis strains tested.     

首次复治肺结核病结核分枝杆菌对利福平耐药性与rpoB基因RRDR突变的研究

目的：研究分离自首次复治肺结核病患者的结核分枝杆菌（Mycobacterium tuberculosis, Mtb）临床菌株对利福平的耐药性, 及其与rpoB基因利福霉素耐药决定区（rifampicin resistance determining regions, RRDR）位点突变间的相关性。方法：收集2013年2月至2013年12月国内19家医院的首次复治肺结核患者的Mtb, 进行利福平药敏检测;使用PCR技术检测Mtb的rpoB基因RRDR位点突变。结果：来自全国12个省19家医院共141例患者的141株临床菌株获得药敏结果, 而RRDR位点测序分析显示首次复治结核患者中有53%（75/141）对利福平耐药。RRDR DNA测序共检测到72株Mtb存在rpoB耐药突变区突变, 其中71株的突变发生在RRDR, 主要突变位点分别为531位（42株）、526位（11株）和516位（10株）。有75株Mtb对利福平耐药, 其中70株（93.3%）检测到rpoB突变, 69株的突变发生在RRDR;除了2株L511P、H526N突变的Mtb对利福平敏感, 70株rpoB突变株均对利福平耐药。对于复治结核患者, 检测RRDR突变判断利福平耐药有较高的特异度。结论：首次复治肺结核病患者对利福平耐药率较高, Mtb耐利福平与rpoB基因RRDR突变密切相关, RRDR突变检测可以用于快速鉴定复治肺结核病患者的Mtb利福平耐药。     

四川德阳地区结核分枝杆菌耐药基因突变位点的分布特点及基因型耐药的分析

目的探讨四川德阳地区结核分枝杆菌耐药基因突变位点的分布特点及基因型耐药的分析。方法收集在2010年2月-2013年3月检出的257例肺结核病患者结核分枝杆菌DNA阳性的痰液标本,采用聚合酶链式反应（PCR）和反向点杂交（RDB）相结合的基因芯片技术对结核分枝杆菌耐药基因突变位点进行检测和分析。结果 257例肺结核患者中检测出49例存在结核分枝杆菌耐药基因位点突变,其中发生katG 315、rpsL43、embB 306和rpoB 531丝氨酸（S531L）位点突变的分别为30例（11.67%）、18例（7.00%）、11例（4.28%）和10例（3.89%）。234例初治肺结核患者对异烟肼、利福平、乙胺丁醇及链霉素的基因型耐药率分别为9.83%、4.27%、3.42%和5.13%,耐多药率为2.99%;23例复治肺结核患者对上述药物的基因型耐药率分别为52.17%、26.09%、13.04%和43.48%,耐多药率为13.04%。初治患者的基因型耐药率及耐多药率均明显低于复治患者。结论四川德阳地区结核分枝杆菌常见耐药基因突变位点分别为katG 315、rpsL 43、embB 306和rpoB 531丝氨酸（S531L）突变位点;肺结核初治患者的基因型耐药率及耐多药率均明显低于复治患者;该地区肺结核的耐多药率相对较低。     

Phenotypic and genotypic characterization of drug-resistant Mycobacterium tuberculosis strains

Of 142 pulmonary tuberculosis patients, 76 were considered high risk for the development of resistance, and 24 were confirmed as resistant strain carriers. Resistant isoniazid strains presented a high frequency of katG and ahpC mutations (90%) correlated with an MIC >4 microg/mL (94%). inhA mutations were not seen. rpoB mutations were identified in 78.6% of rifampicin-resistant strains, usually in codon 531 (72.7%), and 75% had an MIC >16 microg/mL. katG and rpoB mutations recognized 88.2% of multidrug-resistant strains and proved more efficient than the katG and rpoB mutations alone. Seventy percent of resistant pyrazinamide strains had pncA mutations between genes 136 and 188, 62.5% of them with an MIC >900 microg/mL. Pyrazinamidase inactivity was not an efficient resistance marker because 60% of pncA-mutated strains maintained enzymatic activity despite displaying good correlation with high resistance levels. Resistant ethambutol strains had embB mutations in codon 306, with MIC >16 microg/mL.     

基因芯片技术在结核分支杆菌耐药突变基因检测中的应用

目的通过基因芯片检测系统，快速检测临床样品中结核分支杆菌耐药突变情况。方法根据结核分支杆菌标准株H37Rv序列，设计了覆盖rpoB、katG，inhA基因突变区的系列寡核苷酸探针，制作膜芯片，检测临床样品中结核分支杆菌基因突变情况，以此判断耐药结果。结果在305例临床病例中，共检出阳性病例125例，其中阳性敏感病例64例，阳性突变病例61例，阳性率为40．98％，在125例阳性样品中，共发现有8种突变类型，其中10例531L，占7．94％，19例315M，占阳性样品中总数的15．08％。结论PCR与膜芯片杂交技术可临床检测结核分支杆菌对利福平和异烟肼的耐药性，并具有快速、简便、敏感的特点。     

Relationship between rifampin MICs for and rpoB mutations of Mycobacterium tuberculosis strains isolated in Japan.

We analyzed the relationship between rifampin MICs and rpoB mutations of 40 clinical isolates of Mycobacterium tuberculosis. A point mutation in either codon 516, 526, or 531 was found in 13 strains requiring MICs of > or = 64 micrograms/ml, while 21 strains requiring MICs of < or = 1 microgram/ml showed no alteration in these codons. However, 3 of these 21 strains contained a point mutation in either codon 515 or 533. Of the other six strains requiring MICs between 2 and 32 micrograms/ml, three contained a point mutation in codon 516 or 526, while no alteration was detected in the other three. Our results indicate that the sequencing analysis of a 69-bp fragment in the rpoB gene is useful in predicting rifampin-resistant phenotypes.     

Allele-specific rpoB PCR assays for detection of rifampin-resistant Mycobacterium tuberculosis in sputum smears

We describe an allele-specific PCR assay to detect mutations in three codons of the rpoB gene (516, 526, and 531) in Mycobacterium tuberculosis strains; mutations in these codons are reported to account for majority of M. tuberculosis clinical isolates resistant to rifampin (RIF), a marker of multidrug-resistant tuberculosis (MDR-TB). Three different allele-specific PCRs are carried out either directly with purified DNA (single-step multiplex allele-specific PCR), or with preamplified rpoB fragment (nested allele-specific PCR [NAS-PCR]). The method was optimized and validated following analysis of 36 strains with known rpoB sequence. A retrospective analysis of the 287 DNA preparations from epidemiologically unlinked RIF-resistant clinical strains from Russia, collected from 1996 to 2002, revealed that 247 (86.1%) of them harbored a mutation in one of the targeted rpoB codons. A prospective study of microscopy-positive consecutive sputum samples from new and chronic TB patients validated the method for direct analysis of DNA extracted from sputum smears. The potential of the NAS-PCR to control for false-negative results due to lack of amplification was proven especially useful in the study of these samples. The developed rpoB-PCR assay can be used in clinical laboratories to detect RIF-resistant and hence MDR M. tuberculosis in the regions with high burdens of the MDR-TB.     

Molecular Characterization of Multidrug-Resistant Isolates of Mycobacterium tuberculosis from Patients in North India

The World Health Organization has identified India as a major hot-spot region for Mycobacterium tuberculosis infection. We have characterized the sequences of the loci associated with multidrug resistance in 126 clinical isolates of M. tuberculosis from India to identify the respective mutations. The loci selected were rpoB (rifampin), katG and the ribosomal binding site of inhA (isoniazid), gyrA and gyrB (ofloxacin), and rpsL and rrs (streptomycin). We found known as well as novel mutations at these loci. Few of the mutations at the rpoB locus could be correlated with the drug resistance levels exhibited by the M. tuberculosis isolates and occurred with frequencies different from those reported earlier. Missense mutations at codons 526 to 531 seemed to be crucial in conferring a high degree of resistance to rifampin. We identified a common Arg463Leu substitution in the katG locus and certain novel insertions and deletions. Mutations were also mapped in the ribosomal binding site of the inhA gene. A Ser95Thr substitution in the gyrA locus was the most common mutation observed in ofloxacin-resistant isolates. A few isolates showed other mutations in this locus. Seven streptomycin-resistant isolates had a silent mutation at the lysine residue at position 121. While certain mutations are widely present, pointing to the magnitude of the polymorphisms at these loci, others are not common, suggesting diversity in the multidrug-resistant M. tuberculosis strains prevalent in this region. Our results additionally have implications for the development of methods for multidrug resistance detection and are also relevant in the shaping of future clinical treatment regimens and drug design strategies.     

广西壮族自治区结核分枝杆菌异烟肼与利福平耐药基因联合突变特征分析

  目的   了解广西壮族自治区（广西）结核分枝杆菌异烟肼与利福平耐药基因联合突变特征，为耐多药结核病的分子诊断和治疗提供依据。  方法   2017—2018年从广西30个结核病防治定点机构收集的结核分枝杆菌中选取49株耐多药菌株和459株全敏感菌株进行全基因组测序。  结果   耐多药表型与基因联合突变的符合率为71.43%。 单基因突变率和基因联合突变率在耐多药菌株中均高于全敏感菌株（χ2=5.753，P=0.016; χ2=284.034，P<0.001）。 katG和rpoB的单基因突变率在耐多药菌株中高于全敏感菌株（χ2=7.524，P=0.006; χ2=4.353，P=0.037）。 katG＋rpoB基因联合突变在耐多药菌株中高于全敏感菌株（χ2=279.956，P<0.001）。 在基因联合突变的位点分布中，以katG315＋rpoB450和katG315＋rpoB445位点突变为主，占40.82%（20/49），2种形式的基因位点联合突变率在耐多药菌株中均高于全敏感菌株（χ2=144.232，P<0.001; χ2=19.014，P<0.001）。  结论   对异烟肼和利福平耐药基因联合突变的检测可作为广西耐多药筛查的重要指标。 广西结核分枝杆菌异烟肼和利福平耐药基因突变以katG315、rpoB450和rpoB445位点突变为主，基因联合突变以katG＋rpoB形式为主。 katG315＋rpoB450和katG315＋rpoB445位点突变是广西地区耐多药产生的主要分子机制。     

Molecular characterization of isoniazid resistance in Mycobacterium tuberculosis: identification of a novel mutation in inhA

Multiplex allele-specific PCRs detecting katG codon 315 and mabA (bp -15) mutations could specifically identify 77.5% of isoniazid-resistant Mycobacterium tuberculosis strains in the South China region. One clinical isolate harboring InhA Ile194Thr was characterized to show strong association with isoniazid resistance in Mycobacterium tuberculosis.     

Characterization of spontaneous, In vitro-selected, rifampin-resistant mutants of Mycobacterium tuberculosis strain H37Rv

Resistance to rifampin in Mycobacterium tuberculosis results from mutations in the gene coding for the beta subunit of RNA polymerase (rpoB). At least 95% of rifampin-resistant isolates have mutations in rpoB, and the mutations are clustered in a small region. About 40 distinct point mutations and in-frame insertions and deletions in rpoB have been identified, but point mutations in two codons, those coding for Ser(531) and His(526), are seen in about 70% of rifampin-resistant clinical isolates, with Ser(531)-to-Leu (TCG-to-TGG) mutations being by far the most common. To explore this phenomenon, we isolated independent, spontaneous, rifampin-resistant mutant versions of well-characterized M. tuberculosis laboratory strain H37Rv by plating 100 separate cultures, derived from a single low-density inoculum, onto rifampin-containing medium. Rifampin-resistant mutants were obtained from 64 of these cultures. Although we anticipated that the various point mutations would occur with approximately equal frequencies, sequencing the rpoB gene from one colony per plate revealed that 39 (60.9%) were Ser(531) to Leu. We conclude that, for unknown reasons, the associated rpoB mutation occurs at a substantially higher rate than other rpoB mutations. This higher mutation rate may contribute to the high percentage of this mutation seen in clinical isolates.