Susceptibility testing with the manual mycobacteria growth indicator tube (MGIT) and the MGIT 960 system provides rapid and reliable verification of multidrug-resistant tuberculosis

The objective of the study was to compare the manual Mycobacteria Growth Indicator Tube (MGIT) method and the BACTEC MGIT 960 system to the BACTEC 460 method for susceptibility testing of Mycobacterium tuberculosis. The evaluation was based on testing of 36 M. tuberculosis strains with various susceptibilities to isoniazid (INH), rifampin (RMP), ethambutol (EMB), and streptomycin (SM). In addition, five of the strains generating discrepant results in testing for EMB were analyzed for heteroresistance. For INH, the susceptibility test results obtained by the MGIT 960 and the manual MGIT systems agreed with the BACTEC 460 results in 94 and 97% of the cases, respectively. The results of susceptibility to RMP were all in agreement. For SM, 78 and 72% of the results obtained by the MGIT 960 and the manual MGIT systems, respectively, agreed with the BACTEC 460 results. In contrast, less than 80% of the results for susceptibility to EMB obtained by the two MGIT methods agreed with the BACTEC 460 results. All five strains analyzed for EMB heteroresistance were found to consist of resistant and susceptible subpopulations. The average turnaround times were 6.4 days for the MGIT 960 system, 6.5 for the manual MGIT system, and 8.7 days for the BACTEC 460 method. Both MGIT methods can be regarded as accurate and rapid alternatives to the BACTEC 460 method for detection of strains resistant to INH and RMP. However, more studies are needed for solving the problems associated with susceptibility testing to EMB and SM.

In vitro activities of rifamycin derivatives ABI-1648 (Rifalazil, KRM-1648), ABI-1657, and ABI-1131 against Chlamydia trachomatis and recent clinical isolates of Chlamydia pneumoniae

ABI-1648 (rifalazil) is a semisynthetic rifamycin with potent bactericidal activity against intracellular respiratory bacteria, including Mycobacterium tuberculosis, and a long half-life (approximately 60 h) and thus can be administered once weekly. We therefore tested the in vitro activities of ABI-1648, its derivatives ABI-1657 and ABI-1131, azithromycin, and levofloxacin against 10 strains of Chlamydia trachomatis and 10 recent clinical isolates of Chlamydia pneumoniae. The MICs at which 90% of the isolates were inhibited and the minimal bactericidal concentration at which 90% of the isolates were killed for ABI-1648, ABI-1657, and ABI-1131 were 0.0025 micro g/ml for C. trachomatis and 0.00125 to 0.0025 micro g/ml for C. pneumoniae. ABI-1648, ABI-1657, and ABI-1131 were 10- to 1,000-fold more active than azithromycin and levofloxacin.

Low-level resistance to rifampin in Streptococcus pneumoniae

Rifampin is recommended for combination therapy of meningitis due to beta-lactam-resistant Streptococcus pneumoniae. High-level rifampin resistance (MIC, > or =4 mg/liter) has been mapped to point mutations in clusters I and III of rpoB of the pneumococcus. The molecular basis of low-level resistance (MICs, > or =0.5 and <4 mg/liter) was analyzed. Spontaneous mutants of clinical pneumococcal isolates were selected on Columbia sheep blood agar plates containing rifampin at 0.5, 4, 10, or 50 mg/liter. Low-level resistance could be assigned to mutations in cluster II (I(545)N, I(545)L). Sensitive (MIC, <0.048 mg/liter) wild-type strains acquired low-level resistance at a rate approximately 10 times higher than that at which they acquired high-level resistance (average mutation frequencies, 2.4 x 10(-7) for low-level resistance versus 2.9 x 10(-8) for high-level resistance [P < 0.0001]). In second-step experiments, the frequencies of mutations from low- to high-level resistance were over 10 times higher than the frequencies of mutations from susceptibility to high-level resistance (average mutation frequencies, 7.2 x 10(-7) versus 5.0 x 10(-8) [P < 0.001]). Mutants with low-level resistance were stable upon passage. Sequencing of a clinical isolate with low-level resistance (MIC, 0.5 mg/liter) revealed a Q(150)R mutation upstream of cluster I. The frequencies of mutations to high-level resistance for this strain were even higher than the rates observed for the in vitro mutants. Therefore, a resistance-mediating mutation located outside clusters I, II, and III has been described for the first time in the pneumococcus. In vitro low-level rifampin resistance in S. pneumoniae could be mapped to cluster II of rpoB. Mutants of pneumococcus with low-level resistance may be selected in vivo during therapy in tissue compartments with low antibiotic concentrations and play a role in the development of resistance.

The role of KasA and KasB in the biosynthesis of meromycolic acids and isoniazid resistance in Mycobacterium tuberculosis

Mycobacterium tuberculosis has two discrete beta-ketoacyl synthases encoded by kasA and kasB that are located in tandem within a five-gene operon that has been implicated in isoniazid-sensitivity and mycolic acid synthesis. We have developed an in vitro meromycolic acid synthase assay to elucidate the anabolic role of these enzymes. Overproduction of KasA and KasB individually and together in M. smegmatis enabled cell-free incorporation of [(14)C]malonyl-CoA into lipids whose chain length was dependent upon the M. tuberculosis elongating enzyme used. KasA specifically elongated palmitoyl-CoA to monounsaturated fatty acids that averaged 40 carbons in length. KasB hyperproduction in the presence of KasA produced longer chain multiunsaturated hydrocarbons averaging 54 carbons in length. These products comigrated with a synthetic standard of meromycolic acid and their production was sensitive to isoniazid, thiolactomycin, and triclosan. KasA mutations associated with isoniazid resistance produced an enzyme that had a diminished overall catalytic activity but conferred enhanced resistance to isoniazid. In vivo analysis confirmed that overexpression of each of the four mutant KasAs enhanced isoniazid resistance when compared to overexpression of wild-type KasA. These results suggest discrete anabolic roles for both KasA and KasB in mycolic acid synthesis and substantiate the involvement of KasA mutations in isoniazid resistance.

Rapid detection of mutations in rpoB gene of rifampicin resistant Mycobacterium tuberculosis strains by line probe assay

BACKGROUND & OBJECTIVES

Multidrug resistant (MDR) tuberculosis (TB) is a problem of increasing importance in the world due to limited treatment options. Resistance to rifampicin results from nucleotide changes in the gene encoding the beta subunit of the RNA polymerase (rpoB) of Mycobacterium tuberculosis. Rifampicin resistance is considered as a marker for MDR TB. The nature and frequency of mutations in the rpoB gene of rifampicin resistant clinical isolates vary considerably according to the geographical location and very little information is available on specific mutational patterns in India. This study was undertaken to detect and characterize the rpoB gene mutation associated with rifampicin resistance in M. tuberculosis by line probe assay.

METHODS

A total of 36 strains of M. tuberculosis were analysed by INNO-LiPA Rif TB and compared with the results of conventional susceptibility testing method. After PCR amplification of the region of RNA polymerase involved in rifampicin resistance, the amplified product was hybridized with a set of 10 oligonucleotides immobilized onto a membrane strip. From the pattern obtained, the presence or absence of rifampicin resistance in the M. tuberculosis strains was assessed.

RESULTS

It was found that the M. tuberculosis probe was 100 per cent specific; the most frequently observed mutation was His-526-Tyr in the rpoB gene; and correlation between the results of the LiPA and those obtained by the classical susceptibility testing was excellent (100%).

INTERPRETATION & CONCLUSION

INNO LiPA was found to be a reliable, simple, rapid and informative tool for the early detection and characterization of rpoB mutation associated with rifampicin resistance in M. tuberculosis in the clinical laboratory setting and may constitute an important molecular method for the control of tuberculosis.

Antimycobacterial activity of diospyrin derivatives and a structural analogue of diospyrin against Mycobacterium tuberculosis in vitro

Three derivatives and one structural analogue of diospyrin were synthesized and investigated for their inhibitory activity against Mycobacterium tuberculosis employing the rapid radiometric method in vitro. A novel aminoacetate derivative was found to be more active than the parent compound, the MICs being 50 and 100 mg/L, respectively, for a drug-susceptible strain, H37Rv, of M. tuberculosis. This derivative also exhibited an MIC of 50 mg/L for a few multidrug-resistant strains of M. tuberculosis. The other two derivatives and the analogue did not show any significant antimycobacterial activity at the highest concentration (100 mg/L) tested.

Activities of sitafloxacin (DU-6859a), either singly or in combination with rifampin, against Mycobacterium ulcerans infection in mice

Efficacy of a new fluoroquinolone, sitafloxacin (DU-6859a), against Mycobacterium ulcerans was evaluated in vivo using the mouse footpad system. The growth of M. ulcerans in mouse footpads was completely inhibited when mice were fed with sitafloxacin at a dose of 25 mg/kg body weight per day; on the other hand similar effects were observed with ofloxacin at a dose of 100 mg/kg body weight per day. In the presence of rifampin, the above dose of sitafloxacin could be reduced by 75% to achieve total inhibition, while, under similar circumstances, the dose of ofloxacin could be reduced by only 50%. Either used singly or in combination with rifampin, the effects of sitafloxacin were bactericidal. The results suggest that sitafloxacin should be evaluated as a chemotherapeutic agent against M. ulcerans infection.

Rapid detection of rifampin resistance in Mycobacterium tuberculosis isolates by heteroduplex analysis and determination of rifamycin cross-resistance in rifampin-resistant isolates

Direct heteroduplex analysis and a universal heteroduplex generator assay were performed to detect rifampin resistance rapidly in Turkish Mycobacterium tuberculosis isolates. Cross-resistance to rifapentine, rifabutin, and rifalazil was investigated. A relationship between specific mutations and resistance patterns, which can guide the choice of an appropriate therapeutic regimen for tuberculosis patients, was identified.

N(2)-substituted D,L-cycloserine derivatives: synthesis and evaluation as alanine racemase inhibitors

A select series of N(2)-substituted D,L-cycloserine derivatives were prepared a ndevaluated for inhibitory activity against purified alanine racemases (alr gene product) from Escherichia coli, Staphylococcus aureus, and Mycobacterium tuberculosis, as well as in a growth inhibition assay. N(2)-Modification led to loss of enzymatic inhibitory activity in most cases consistent with a recent proposal for cycloserine function.

Sterilizing activities of fluoroquinolones against rifampin-tolerant populations of Mycobacterium tuberculosis

The bactericidal activities of ciprofloxacin, ofloxacin, levofloxacin, moxifloxacin, and gatifloxacin were tested in three models of rifampin-tolerant Mycobacterium tuberculosis persisters. Model 1 was a 100-day-old, unshaken, anaerobically adapted culture in which serial dilutions of the quinolones were incubated for 5 days and CFU counts were then done In models 2 and 3, 100 mg of rifampin/liter was added to the 100-day culture for 5 or 7 days to produce tolerant organisms that did not grow on plates; the rifampin was then washed off, fresh medium was added to allow recovery of growth on plates, and the culture was incubated for 7 days before CFU counts. In model 2, the quinolones were added after rifampin had been washed off, whereas in model 3 the quinolones were added to the cultures containing rifampin. In models 1 and 2, ciprofloxacin had the least bactericidal activity, ofloxacin and levofloxacin had greater activities, and moxifloxacin and gatifloxacin had the greatest activities. In model 3, ofloxacin had no detectable activity whereas moxifloxacin killed about log(10) 0.279 CFU of the persisters per ml at concentrations attainable in lesions; isoniazid had virtually no activity. These findings predict that ofloxacin will not be found to have effective sterilizing activity in clinical studies now planned whereas moxifloxacin will be able to shorten treatment.

Characterization of rpoB mutations in rifampicin-resistant Mycobacterium tuberculosis isolated in China

The objective of this study was for the elucidation of the characteristics of the rpoB gene mutation in rifampicin (RIF)-resistant Mycobacterium tuberculosis strains isolated in China. The rifampicin resistance determination regions (RRDR) of the rpoB genes of 242 M. tuberculosis strains were sequenced, including 193 RIF-resistant, 46 RIF-sensitive clinical isolates and three manually induced RIF-resistant tuberculosis strains. Mutations in the 81 bp RRDR of the rpoB gene were identified in 89.6% (173/193) of RIF-resistant clinical isolates. No mutation was observed in RIF-sensitive strains. Ser531Leu mutations accounted for 46.1% (89/193) of RIF-resistant strains; the mutation frequency of 526-His was 17.6% (34/193) in RIF-resistant strains. Furthermore, a combination of 2-3 single-point mutations was observed in 24 RIF-resistant strains (12.4%; 24/193). Mutations in three manually induced RIF-resistant tuberculosis strains were located at codons 531 and 526, respectively. The results indicate that about 90% of rifampicin-resistant M. tuberculosis strains isolated in China had rpoB mutations;531-Ser and 526-His were the most common positions substituted; high-level rifampicin-resistant strains had a higher frequency of 531-Ser mutations than low-level rifampicin-resistant strains. Sequencing analysis of the 81 bp fragment in the rpoB gene is useful in predicting the rifampicin-resistant phenotype.

[Determination of rifampin resistance in Mycobacterium tuberculosis isolates by the RNA/RNA mismatch method]

Rifampin is one of the most potent antituberculosis drugs and therefore, rifampin resistance leads to high clinical relapse rates. Detection of rifampin resistance could be an indication of multidrug resistance. In the recent years several molecular methods have been developed to evaluate the mutations in rpoB gene for the detection of rifampin resistance. The aim of the present study was to evaluate the performance of the RNA/RNA Mismatch Assay for detection of the mutations in the rpoB gene in 20 M. tuberculosis isolates which were determined as resistant to rifampin by agar proportion method. While RNA/RNA Mismatch Assay detected the mutations in the rpoB region in 16 of 20 (80%) M. tuberculosis isolates, the remaining four isolates yielded no band pattern indicating resistance. However, there may be situations where interpretation of the results is difficult in RNA/RNA Mismatch Assay which is already cheaper than DNA sequencing and other molecular methods. In conclusion, if the RNA/RNA Mismatch Assay can be optimized, it can be used for the rapid detection of rifampin resistance.

Molecular basis of rifampicin-induced inhibition of anti-CD95-induced apoptosis of peripheral blood T lymphocytes: the role of CD95 ligand and FLIPs

Rifampicin modulates immune response; however, mechanisms by which it exerts these effects are incompletely understood. Recently, rifampicin has been shown to bind to and activate glucocorticoid receptors. Because of the evidence for a role of glucocorticoids in lymphocyte apoptosis, we hypothesized that rifampicin may exert its influence on the immune system by regulating apoptosis. Therefore, we examined the effect of rifampicin on signaling pathway of anti-CD95-induced apoptosis in peripheral blood lymphocytes. Rifampicin, in a concentration-dependent manner, inhibited anti-CD95-induced apoptosis in both CD4+ and CD8+ T cells, which was associated with the inhibition of activation of both caspase-3 and caspase-8. In addition, rifampicin down-regulated the expression of CD95L and Bax. The inhibitory effects of rifampicin on apoptosis and caspase activation as well as its effect on the expression of CD95L and FLIPs were reversed by RU486, an antagonist of glucocorticoid receptor. These data suggest that rifampicin inhibits anti-CD95-mediated apoptosis in lymphocytes by modulating the expression of certain proteins that regulate apoptosis, at least in part, via glucocorticoid receptors.

Detection and drug-susceptibility testing of M. tuberculosis from sputum samples using luciferase reporter phage: comparison with the Mycobacteria Growth Indicator Tube (MGIT) system

Rapid diagnosis of drug-resistant M.tuberculosis (Mtb) is desirable worldwide. We (i) describe a new luciferase reporter phage (LRP), phAE142 for this purpose; (ii) compare it to the automated MGIT 960 for time-to-detection of Mtb in clinical specimens; and (iii) evaluate its use for species confirmation and antibiotic susceptibility testing(AST) of Mtb. Twenty sputum samples were inoculated for testing by LRP, or by MGIT 960. After "positives" were identified by either method, the LRP was used for confirmation of Mtb complex (TBC) and for AST. The LRP method proved comparably efficient to MGIT 960 at detecting Mtb. Using an antibiotic uniquely inhibiting TBC with LRP provided species assignment, concurrently with AST, in a median of 3 days, with a sensitivity of 97%. Overall agreement in susceptibility results was 96%. Reliable susceptibility results and identification of TBC can be completed in a median of 12 days (range 8 to 16d) with LRP applied to sputum samples.

Human organic anion transporting polypeptide-C (SLC21A6) is a major determinant of rifampin-mediated pregnane X receptor activation

Rifampin, a member of the rifamycin class of antibiotics, is well known for its ability to induce drug-metabolizing enzymes and transporters, through activation of the pregnane X receptor. Available data suggest rifampin entry into hepatocytes may be transporter-mediated. Accordingly, it is therefore plausible that modulation of the achievable intracellular concentration of rifampin by drug uptake transporters would influence the degree of induction. In this study, we expressed an array of known hepatic uptake transporters to show the key hepatic rifampin uptake transporters are liver-specific members of the organic anion transporting polypeptide family (OATP). Indeed, both OATP-C and OATP8 seemed capable of mediating rifampin uptake into HeLa cells. OATP-C, however, seemed to have far greater affinity and capacity for rifampin transport. In addition, several allelic variants of OATP-C known to be present among European and African Americans were found to have markedly decreased rifampin transport activity. In cell-based, transactivation assays, OATP-C expression was associated with increased cellular rifampin retention as well as potentiation of PXR reporter gene activity. This is the first demonstration of an uptake transporter such as OATP-C, in modulating PXR function, and sheds important new insight into our understanding of the molecular determinants of PXR-mediated inductive processes.

Molecular Cloning of a D-Cycloserine Resistance Gene from D-Cycloserine-producing Streptomyces garyphalus

A 3.5-kb DNA fragment that confers resistance to D-cycloserine (DCS) was cloned from the chromosomal DNA of a DCS-producing Streptomyces garyphalus into Streptomyces lividans by a shot-gun cloning technique. Nucleotide sequence analysis revealed the existence of four open reading frames (ORFs B, C, D, and E), together with two incomplete ORFs, A and F. By introduction of the cloned fragment into Escherichia coli, the host obtained resistance to DCS. We showed that ORF B, which consists of 903 bp, is a DCS resistance gene. The hydropathy plot analysis of a protein deduced from ORF B revealed that the protein carries membrane-integral domains spanning the membrane 10 times, which suggests that the DCS-resistance determinant may be a factor associated with DCS transport.

Use of receiver operating characteristic curves to assess the performance of a microdilution assay for determination of drug susceptibility of clinical isolates of Mycobacterium tuberculosis

The aim of this study was to apply receiver operating characteristic (ROC) analysis to the microplate Alamar blue assay, a recently developed alternative for drug susceptibility testing of mycobacteria. As this is a quantitative assay, its performance can be determined by ROC analysis, in which the area under the ROC curve represents a summary of test performance (the higher the area, the better the test's performance). Sixty isolates of Mycobacterium tuberculosis were tested by the microcolorimetric assay against six twofold dilutions of streptomycin, isoniazid, rifampin, and ethambutol. For each isolate, the susceptibility pattern was simultaneously established by the agar proportion method, the result of which represented the gold standard value for the ROC analysis. The critical concentration, area under the curve, and P value for each drug were determined by ROC curve analysis. The results of the assay were obtained in an average of 8 days of incubation. The performance of the assay was excellent for all four drugs: the area under the curves was >0.97, the P values were 0.000, and sensitivity was 94%, specificity 97%, predictive value for resistance >/=92%, predictive value for susceptibility 97%, and test efficiency 97%. According to ROC analysis, the microplate Alamar blue assay is a reliable method for determination of drug-susceptibility. Rapidity and cost efficiency are two additional qualities that make this test an excellent alternative for the drug susceptibility testing of Mycobacterium tuberculosis. The ROC curve analysis is a robust statistical approach for evaluating the performance of new quantitative methods for determination of drug sensitivity of Mycobacterium tuberculosis isolates.

Investigation of the in vitro activity of streptomycin against Mycobacterium tuberculosis

Streptomycin was the first antibiotic used against Mycobacterium tuberculosis. It was used for years in monotherapy regimens, leading to the emergence of resistance; therefore, its use gradually waned. Given the resistance rates detected with current antituberculosis drugs, the use of streptomycin has gained renewed interest. The mechanism of action of streptomycin is inhibition of protein synthesis of mycobacteria in the ribosome. Resistance emerges when mutations appear in genes encoding 16S rRNA and protein S12. The activity of streptomycin against 1,496 M. tuberculosis strains was investigated; 1,186 and 196 strains corresponded to pulmonary and extrapulmonary specimens, respectively. For 114 strains, the source was not indicated. Initially, the BACTEC 460 TB system was used for antibiotic susceptibility testing. Since 1996, the ESP II system was used. The strains ATCC27294 (sensitive to streptomycin, rifampin, ethambutol, and isoniazid) and ATCC35820 (resistant to streptomycin) were used as controls. An overall resistance rate of 2.2% was obtained. In all cases secondary resistance was observed. Multiresistance was observed in 23 strains.

[Effect of glutoxim in the combination with antitubercular agents of the second choice on the growth of drug resistant Mycobacteria tuberculosis in the cultured murine lung tissue]

It was demonstrated that glutoxim combination with second line drugs for tuberculosis treatment (cycloserine + rifabutine, cycloserine + protionamide) provided statistically significant decrease of intracellular mycobacteria growth in the murine lung tissue culture. The decrease rate when compared to the control group was 3-5 times. The investigated Mycobacterium tuberculosis strain was isolated from the patient and was multi-drug resistant (MDR). Glutoxim addition to the second line drugs combinations provided also decrease of the MDR bacteria microcolonies growth in the lung tissue culture. Glutoxim combination with second line antituberculosis drugs allowed to keep vitality and functional activity of lung tissue cells.

Characterization of rpoB mutations in rifampin-resistant clinical isolates of Mycobacterium tuberculosis from Turkey by DNA sequencing and line probe assay

Mutations in an 81-bp region of the rpoB gene associated with rifampin resistance were studied in 41 rifampin-resistant clinical strains of Mycobacterium tuberculosis isolated in Turkey. Fourteen different rpoB alleles, three of which had not been reported before, were found. A reverse hybridization-based line probe assay (the Inno-LiPA Rif.TB test) for rapid detection of the mutations was evaluated with these isolates. Rifampin resistance was correctly identified in 23 of 41 isolates (56.1%) with the kit's R probes specific for these mutations. Seventeen of 41 isolates (41.5%) yielded hybridization patterns, with at least one negative signal obtained with the S probes for the wild type. One isolate was identified as rifampin sensitive by the line probe assay. The rate of concordance of the results of the line probe assay with the results of the in vitro susceptibility test was high (97.6%). These results demonstrate that the line probe assay kit may be useful for the rapid diagnosis of rifampin-resistant tuberculosis.

Clinical strain of Pseudomonas aeruginosa carrying a bla(TEM-21) gene located on a chromosomal interrupted TnA type transposon

A clinical isolate of Pseudomonas aeruginosa was found to produce a clavulanic acid-inhibited extended-spectrum beta-lactamase with a pI of 6.4. PCR, cloning, and sequencing experiments showed that the corresponding bla(TEM-21) gene was part of a chromosomally located Tn801 transposon disrupted by an IS6100 element and adjacent to an aac(3)-II gene.

Novel Saccharomyces cerevisiae screen identifies WR99210 analogues that inhibit Mycobacterium tuberculosis dihydrofolate reductase

The ongoing selection of multidrug-resistant strains of Mycobacterium tuberculosis has markedly reduced the effectiveness of the standard treatment regimens. Thus, there is an urgent need for new drugs that are potent inhibitors of M. tuberculosis, that exhibit favorable resistance profiles, and that are well tolerated by patients. One promising drug target for treatment of mycobacterial infections is dihydrofolate reductase (DHFR; EC 1.5.1.3), a key enzyme in folate utilization. DHFR is an important drug target in many pathogens, but it has not been exploited in the search for drugs effective against M. tuberculosis. The triazine DHFR inhibitor WR99210 has been shown to be effective against other mycobacteria. We show here that WR99210 is also a potent inhibitor of M. tuberculosis and Mycobacterium bovis BCG growth in vitro and that resistance to WR99210 occurred less frequently than resistance to either rifampin or isoniazid. Screening of drugs with M. tuberculosis cultures is slow and requires biosafety level 3 facilities and procedures. We have developed an alternative strategy: initial screening in an engineered strain of the budding yeast Saccharomyces cerevisiae that is dependent on the M. tuberculosis DHFR for its growth. Using this system, we have screened 19 compounds related to WR99210 and found that 7 of these related compounds are also potent inhibitors of the M. tuberculosis DHFR. These studies suggest that compounds of this class are excellent potential leads for further development of drugs effective against M. tuberculosis.

An elevated mutation frequency favors development of vancomycin resistance in Staphylococcus aureus

The emergence of intermediate vancomycin resistance, mainly in methicillin-resistant Staphylococcus aureus strains, has become a great concern. Thorough characterization of clinical and laboratory vancomycin-intermediately resistant S. aureus (VISA) strains identified multiple, resistance-associated changes most probably due to stepwise mutations. We hypothesized that an elevated mutation frequency as found, e.g., in mutator strains defective in DNA mismatch repair could allow rapid acquisition of adaptive mutations in the presence of vancomycin. We therefore subjected S. aureus RN4220 and its isogenic mutator strain, the mutS-knockout mutant RN4220DeltamutS, to a stepwise vancomycin selection procedure. Vancomycin resistance evolved much more quickly in the mutator background than in the wild type (5 versus 19 passages, respectively). In addition, a higher resistance level could be reached (MIC, 32 versus 4 micro g/ml, respectively). The susceptibility to other antibiotics with the exception of teicoplanin remained unchanged. Concomitantly with increasing vancomycin resistance, a loss of phage typeability and differences in growth behavior as well as an improved ability to regrow at high vancomycin concentrations were observed. In conclusion, an elevated mutation rate in S. aureus led to the rapid development of vancomycin resistance, indicating that a high mutation frequency could be one of the factors that favor the emergence of vancomycin resistance in S. aureus.

Characterization of rpoB mutations in rifampin-resistant clinical Mycobacterium tuberculosis isolates from Kuwait and Dubai

Mutations conferring resistance to rifampin in rifampin-resistant clinical Mycobacterium tuberculosis isolates occur mostly in the 81 bp rifampin-resistance-determining region (RRDR) of the rpoB gene. In this study, 29 rifampin-resistant and 12 -susceptible clinical M. tuberculosis isolates were tested for characterization of mutations in the rpoB gene by line probe (INNO-LiPA Rif. TB) assay and the results were confirmed and extended by DNA sequencing of the PCR amplified target DNA. The line probe assay identified all 12 susceptible strains as rifampin-sensitive and the DNA sequence of RRDR in the amplified rpoB gene from two isolates matched perfectly with the wild-type sequence. The line probe assay identified 28 resistant isolates as rifampin-resistant with specific detection of mutation in 22 isolates including one isolate that exhibited hetro-resistance containing both the wild-type pattern as well as a specific mutation within RRDR while one of the rifampin-resistant strain was identified as rifampin-susceptible. DNA sequencing confirmed these results and, in addition, led to the specific detection of mutations in 5 rifampin-resistant isolates in which specific base changes within RRDR could not be determined by the line probe assay. These analyses identified 8 different mutations within RRDR of the rpoB gene including one novel mutation (S522W) that has not been reported so far. The genotyping performed on the isolates carrying similar mutations showed that majority of these isolates were unique as they exhibited varying DNA banding patterns. Correlating the ethnic origin of the infected TB patients with the occurrence of specific mutations at three main codon positions (516, 526 and 531) in the rpoB gene showed that most patients (11 of 15) from South Asian region contained mutations at codon 526 while majority of isolates from patients (6 of 11) of Middle Eastern origin contained mutations at codon 531.