Susceptibilitytesting using the MGIT960 system with EpiCenter TB eXiST software. The MGIT960 system was used for primary isolation and standard susceptibilitytesting of first-line drugs (10) as recommended by the manufac- turer. MGIT tubes supplemented with 0.8 ml of supplement (MGIT960 SIRE supplement; Becton Dickinson) were inoculated with 0.1 ml of the drug solution and 0.5 ml of the test strain suspension. For preparation of the drug-free growth control tube, the organism suspension was diluted 1:100 with sterile saline, and then 0.5 ml was inoculated into the tube (proportion testing). For quantitative DST using the MGIT960instrumentation, we requested Becton Dickinson to develop custom-designed software with the following characteristics: automated recording of the readings, additional incubation time beyond the time to posi- tivity of the drug-free control, minimization of the number of drug-free control tubes required, graphical representation of the growth unit (GU) value increase and storage of data, and easy handling and documentation. The software is EpiCenter, version 5.53, equipped with the TB eXiST module and available from Becton Dickinson. Table 1 lists the drug concentrations used for comparative analysis. The susceptibilitytesting sets were placed in the MGIT960 instrument and continuously monitored using EpiCenter (version 5.53) TB eXiST software. Results were interpreted as follows. At the time when the GU of the drug-free control tube was ⬎400, if the GU of the drug-containing tube to be compared was ⱖ 100, the strain was R. If the GU of the drug-containing tube was ⬍ 100, it was incubated for a further 7 days. If it was still ⬍100, the strain was S. If the GU of the drug-containing tube was ⱖ 100 during this further 7 days of incubation after the GU of the drug-free control tube reached ⬎400, the strain was intermediate (I).
As a routine practice, only primary drugs are tested initially. When strains test resistant to rifampin or any two of the pri- mary drugs, second-line drugs are then tested. When resistance to primary drugs is detected, the growth on LJ slants is usually adequate for the testing of second-line drugs. In this study, we used the growth from LJ slants as the main source for the inoculum preparation. With advances in molecular technolo- gies, isoniazid and/or rifampin resistance can be detected by rapid methods, such as those that use molecular beacons, line probes, or microchips (2, 3, 6, 7, 11, 13). Once rifampin resis- tance or MDR TB is detected, testing for susceptibility to second-line drugs should be performed along with testing for susceptibility to first-line drugs as early as possible, and testing should not wait for growth from LJ slants. Therefore, we in- cluded a subset of 23 specimens for validation of this assay using inocula prepared from positive MGIT tubes. Our proto- col prepares inocula from MGIT tubes by standardization of the growth to a 0.5 McFarland standard, as is done for LJ slants. DST can be performed from a primary MGIT tube and not subject to the manufacturer’s time schedule, as stated in the package insert for SIRE DST. This has two advantages: the inoculum does not contain coarse clumps, which helps to pro- duce more consistent results, and it does not require seeding of a MGIT tube and waiting for its growth for DST, thus improv- ing the TAT and reducing the cost for medium.
The reliability of the BACTEC Mycobacteria Growth Indicator Tube (MGIT) 960 system for testing of Mycobacteriumtuberculosissusceptibility to the three front-line drugs (isoniazid [INH], rifampin [RIF], and ethambutol [EMB]) plus streptomycin (STR) was compared to that of the BACTEC 460 TB system. The proportion method was used to resolve discrepant results by an independent arbiter. One hundred and ten strains were tested with an overall agreement of 93.5%. Discrepant results were obtained for seven strains (6.4%) with INH (resistant by BACTEC MGIT960; susceptible by BACTEC 460 TB), for one strain (0.9%) with RIF (resistant by BACTEC MGIT960; susceptible by BACTEC 460 TB), for seven strains (6.4%) with EMB (six resistant by BACTEC MGIT960 and susceptible by BACTEC 460 TB; one susceptible by BACTEC MGIT960 and resistant by BACTEC 460 TB), and for 19 strains (17.3%) with STR (resistant by BACTEC MGIT960 and susceptible by BACTEC 460 TB). After resolution of discrepant results, the sensitivity of the BACTEC MGIT960 system was 100% for all four drugs and specificity ranged from 89.8% for STR to 100% for RIF. Turnaround times were 4.6 to 11.7 days (median, 6.5 days) for BACTEC MGIT960 and 4.0 to 10.0 days (median, 7.0 days) for BACTEC 460 TB. These data demonstrate that the fully automated and nonradiometric BACTEC MGIT960 system is an accurate method for rapid susceptibilitytesting of M. tuberculosis.
Our findings carry obvious implications for the laboratory, clinician, and patient. First, DST remains expensive and techni- cally challenging, and so a laboratory usually will only have one validated method available for use. Thus, a high discrepancy rate of one’s chosen method will have direct clinical consequences. If our Bangladesh laboratory in this high MDR-TB setting were to use only the MGIT960 SIRE method, up to 49% of all EMB- susceptible findings might be false. The possibility of false suscep- tibility to EMB with the MGIT960 method has been reported since the scale-up of the platform (9) and has been postulated to be a function of isolates with MICs at or just above the critical concentration, or to be due to heteroresistance, where growth rates of resistant mutants are slowed in liquid medium but can be improved by late growth on solid agar. The latter explanation is possible, but the former seems insufficient, as we did not note any statistical difference in the MICs (among the MGIT “false” sus- FIG 3 Drugsusceptibilitytesting discrepancies by drug and methodology. Discrepancies versus the consensus gold standard were enumerated for INH, RIF, EMB, and STR by methodology. Discrepancy rates for EMB were higher for MGIT960 than with other methods (29%, versus 4 to 5% for other methods; P ⬍ 0.05). Discrepancies were further delineated as susceptible (boxed outline indicating the majority were susceptible) or resistant (dashed outline indicating the majority were resistant). There were 0 discrepancies for INH by L-J. nd, not done (GeneXpert only evaluates RIF and the Hain MTBDRplus method only evaluates INH and RIF).
tocol was used instead of the standard 4- to 13-day protocol for the INH and RIF indirect tests, to allow more time for the growth control tube to reach the required 400 growth units for a valid test. The indirect PZA test protocol is 4 to 21 days long; the extended incubation time allows more time for the M. tuberculosis bacteria to grow if the growth rate in the slightly acidified MGIT PZA medium is lower. The same protocol was used for the direct PZA test since it was not possible to adjust the instrument protocol, i.e., extend it beyond 21 days, using the Bactec MGITEpicenter sys- tem, which was not available in these laboratories. Slow growth of some M. tuberculosis strains in PZA medium may have been a cause for growth failures. It is more likely that the reason for in- sufficient growth in the control was that the inoculum density was too low. Although the inoculum for the control tube is a 1/10 dilution of the sputum pellet, instead of the 1/100 dilution used in the indirect test, the concentration of viable M. tuberculosis bacte- ria may have been very low in some sputum specimens despite these specimens being smear positive. Furthermore, it is possible that some strains had a delayed lag time before the beginning of replication and did not reach the threshold of detection before the end of the protocol.
Mycobacterial purity checks of suspensions used for SIRE and PZA susceptibility tests are important to ensure that the test is performed with a pure culture of a single mycobacterial species. When the test is performed in liquid medium, if the strain is resistant to any of the drugs tested, a mycobacterial purity check from at least one of the vials showing an unex- pected drug resistance is recommended (6). This approach permits checking for any possible contamination of the broths during the inoculation procedure, avoiding possible false-re- sistant results (ME). We have observed that bacterial contam- ination could occur in only one tube during the inoculation procedure, particularly when working with MGIT tubes, which use screw cups instead of rubber septa as with the B460. When more than one tube in the set showed resistant result, the different turbidities of the broths (contaminated broth and broth with growth of M. tuberculosis) could help to identify a possible contaminated tube, but for safety’s sake, in this study we preferred to check all broths showing drug resistance.
Conventional indirect drugsusceptibilitytesting of Mycobacteriumtuberculosis with liquid medium is well established and of- fers time-saving and reliable results. This multicenter study was carried out to evaluate if drugsusceptibilitytesting (DST) can be successfully carried out directly from processed smear-positive specimens (direct DST) and if this approach could offer substan- tial time savings. Sputum specimens were digested, decontaminated, and concentrated by the laboratory routine procedure and were inoculated in Bactec MGIT960 as well as Lowenstein-Jensen (LJ) medium for primary isolation. All the processed speci- mens which were acid-fast bacterium (AFB) smear positive were used for setting up direct DST for isoniazid (INH) and rifampin (RIF). After the antimicrobial mixture of polymyxin B, amphotericin B, nalidixic acid, trimethoprim, and azlocillin (PANTA) was added, the tubes were entered in the MGIT960 instrument using the 21-day protocol (Bactec 960 pyrazinamide [PZA] pro- tocol). Results obtained by direct DST were compared with those obtained by indirect DST to establish accuracy and time sav- ings by this approach. Of a total of 360 AFB smear-positive sputum specimens set up for direct DST at four sites in three differ- ent countries, 307 (85%) specimens yielded reportable results. Average reporting time for direct DST was 11 days (range, 10 to 12 days). The average time savings by direct DST compared to indirect DST, which included time to isolate a culture and perform DST, was 8 days (range, 6 to 9 days). When results of direct DST were compared with those of indirect DST, there was 95.1% con- cordance with INH and 96.1% with rifampin. These findings indicate that direct DST with the Bactec MGIT960 system offers further time savings and is a quick method to reliably detect multidrug resistance (MDR) cases.
Drugsusceptibilitytesting (DST) for both primary and sec- ondary antituberculosis drugs with the broth-based radiomet- ric BACTEC 460 TB system (Becton Dickinson Diagnostic Systems, Sparks, MD) is well established and is considered the “gold standard” (15). However, due to increasing concern about the use and disposal of radioactive material, there is a rapid trend toward using commercially available nonradiomet- ric broth-based culture and susceptibilitytesting methods. BACTEC MGIT960 (Becton Dickinson Diagnostic Systems) is a new nonradiometric system which is considered equivalent to the BACTEC 460 in performance. Recovery of mycobacte- ria from clinical specimens as well as DST for first-line drugs has been thoroughly studied for the MGIT960 system (3, 4, 5, 7, 8, 10, 11, 12). However, no thorough multicenter study has been carried out establishing DST for second-line and newer drugs currently being used in the treatment of tuberculosis. According to the WHO reports, global drug resistance is an increasing concern (18). Some countries are reporting high resistance even against second-line drugs (1, 17). Therefore, it is important that nonradiometric broth-based systems should also offer DST procedures for drugs other than those consid- ered first-line.
The emergence of multidrug-resistant tuberculosis (MDR- TB) caused by Mycobacteriumtuberculosis and, recently, exten- sively drug-resistant tuberculosis caused by an MDR strain that is also resistant to any fluoroquinolone and at least one of the three injectable second-line drugs (kanamycin [KAN], amika- cin [AK], and/or capreomycin [CM]) is a real threat for TB control programs (28). It is obvious that there is a great ne- cessity for rapid, reliable, and economical methods for testing the susceptibility of M. tuberculosis not only to first-line drugs but also to second-line drugs. Access to drugsusceptibilitytesting (DST) is a priority, and TB culture is an essential component of TB management. Using the standardized con- ventional DST methods, it takes a minimum of 3 to 8 weeks to identify resistant or susceptible strains on solid media (6, 7). The introduction of liquid culture media such as the manual mycobacterium growth indicator tube (MGIT) reduces the turnaround time compared to that of solid media, taking an average of 15 days to get results (1, 5, 19, 23, 25). In June 2007, the World Health Organization issued a recommendation for the use of liquid media for culture and DST in middle- and low-income countries to address challenges due to the epi- demic of human immunodeficiency virus-associated TB and drug-resistant TB, especially in resource-limited settings (29). Fully automated commercial systems such as the BACTEC MGIT960 (Becton Dickinson) have shown their usefulness for the rapid detection of resistance to second-line drugs (12, 24);
P yrazinamide (PZA) is a first-line antituberculosis (anti-TB) drug. It is often used in combination with isoniazid, ethambutol, and rifampin for the treatment of TB (1). PZA can shorten TB treatment time from the 9 to 12 months required prior to its introduction to the current standard treatment time of 6 months. This is often referred to as short-course chemotherapy (2). PZA is more effective against non- replicating, persistent Mycobacteriumtuberculosis than any other conventional anti-TB drug (3). However, reliable testing of the sus- ceptibility of M. tuberculosis to PZA in vitro is challenging since PZA has no apparent effect on actively growing TB bacilli under normal culture conditions at neutral pH. PZA is effective against M. tubercu- losis only at an acidic pH, and acidic media inhibit the growth of M. tuberculosis (4, 5). The Bactec 460 and MGIT960 are the only two methods recommended by the World Health Organization (WHO) for susceptibilitytesting ofM. tuberculosis to PZA, due to the difficulty in standardizing its susceptibility to PZA on solid media (6). Drugsusceptibilitytesting in liquid media is costly, especially in some re- gions that do not have enough economic capabilities (6). Several other drugsusceptibility-testing methods have been developed, in- cluding the molecular drugsusceptibility test (mDST) based on the detection of a pncA mutation, the pyrazinamidase (PZase) activity assay, and colorimetric methods based on a minimal inhibitory con- centration (MIC) or redox indicator (7, 8, 9, 10, 11, 12). In this study, we used 432 clinical M. tuberculosis isolates to compare five methods for determination of the susceptibility of M. tuberculosis to PZA: the MGIT960 system, the mDST, the PZase activity assay, and two colorimetric methods [the resazurin microtiter assay (REMA) and the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction test].
Emergence of multidrug-resistant tuberculosis (MDR-TB), defined as tuberculosis caused by a strain resistant to isoniazid (INH) and rifampin (RMP), is complicating tuberculosis con- trol efforts (11). Consequently, laboratories are challenged to provide rapid antimicrobial susceptibilitytesting (AST) to en- sure effective treatment of tuberculosis, and to prevent further development of drug resistance in the causative strain due to inadequate drug combinations for extended periods of time (15). In the current clinical routine, AST of Mycobacteriumtuberculosis is performed by either methods using solid media or the radiometric BACTEC 460 method (Becton Dickinson Diagnostic Instrument Systems, Sparks, Md.) (5) (see also the Becton Dickinson product and procedure manual MA-0029). The BACTEC 460 provides the best validated rapid approach for AST, but it has some disadvantages, e.g., use of radioactive medium, requirement of needles and syringes in transfer of inocula, and expensive instrumentation (8, 16).
values were associated with the use of MGIT960 (Tables 1 and 2) and similar results indicating a good discriminatory perfor- mance were obtained when DOR was used as a single indicator of accuracy (range, 5.00 to 9.84). The summary ROC curve, symmetric and very close to the upper left-hand corner of the graph, confirmed the results highlighted by DORs (Fig. 2). False- susceptibility results (VME) represent a serious drawback, as they can result in the failure of anti-TB chemotherapy. When compared with the B460 system, MGIT960 did not show any evidence of increased VME occurrence, indicating that this sys- tem is reliable for detecting true resistance (Tables 3 and 4). False-resistance results (ME), which label as ineffective a drug that can be successfully used, represent a less serious problem. In our analysis, the MGIT960 system showed an increased risk of ME (with a statistically significant difference in comparison with B460) for SM and INH (P values were 0.017 and 0.025, respectively), while no statistically significant difference was found for EMB (P ⫽ 0.065) despite a higher number of ME exhibited by this drug, whose results are well known to be less reproducible regardless of the system used (22, 24). When testing high concentrations (SM, INH, and EMB), the MGIT960 system did not show any evidence of increased ME risk (Tables 3 and 4). Four issues need to be carefully taken into consideration, as they can lead to inaccurate results: (i) purity of the culture, (ii) homogeneity of mycobacterial suspension, (iii) inoculum size, and (iv) environmental contamination of media during inoculation procedures. Purity check of suspen- sions (especially those prepared from resistant strains) guar- antees that a pure culture of a single mycobacterial species is used for DST inoculation, thereby preventing ME results. Bac- terial contamination from the surrounding environment may occur during DST hands-on procedures and sometimes in one tube only. In this context, a high contamination rate in com- parison with B460 has been reported by different authors (16, 33, 38), who addressed (i) richness of liquid medium and (ii) replacement of rubber septa with screw caps as being mainly responsible.
Drug-resistant Mycobacteriumtuberculosis strains represent a serious public health problem. Resistance to the four primary drugs, streptomycin (SM), isoniazid (INH), rifampin (RMP), and ethambutol (EMB) (a combination known as SIRE), makes tuberculosis difficult to treat (19). Multidrug-resistant strains have emerged within the last decade, and the rapid detection of these isolates is critical for the effective treatment of patients (28). As recommended by the National MDR TB Task Force, to combat multidrug-resistant tuberculosis (7), antimicrobial susceptibilitytesting (AST) must be performed on all initial and follow-up M. tuberculosis isolates from each patient. Among the methods used for drugsusceptibility test- ing, the agar proportion method (MOP) is universally accepted as the “gold standard” (18, 33). However, it requires a long time to report (generally 21 days after the test is set up). Since 1980 the BACTEC 460 TB radiometric system (Becton Dick- inson Diagnostic Instruments, Sparks, Md.), which is based on the modified version of the proportion method (26), has been introduced to perform AST. The BACTEC 460 TB method provides results within 5 to 6 days, with a significant time savings. Several studies (25) have demonstrated that AST re- sults obtained by BACTEC 460 TB were comparable with those of MOP, thus suggesting that the former method could be adopted for routine laboratory purposes. In 1995, the My- cobacteria Growth Indicator Tube (MGIT, 4 ml) (Becton
Tuberculosis (TB) is one of the most prevalent infectious diseases in the world. According to a recent report of the World Health Organization, in 2003 there were 8.8 million new TB cases and around 1.7 million deaths attributable to the disease (25). In addition, multidrug-resistant TB is becoming increasingly common and is a major health concern in many regions of the world, particularly in developing nations (24). Rapid, accurate diagnosis and the determination of drug sus- ceptibility are crucial to optimize treatment and prevent trans- mission. The most widely used method for Mycobacterium tu- berculosis drugsusceptibilitytesting is the proportion method, either on solid medium or on liquid broth. The BACTEC 460TB system (Becton Dickinson Biosciences, Sparks, MD) has been widely validated for approximately 20 years for the reliable and rapid testing of the susceptibilities of M. tubercu- losis isolates (9, 16, 21). The radiometric BACTEC 460TB test requires fewer than 14 days of incubation before results are available, but it is semiautomated and entails the disposal of a radioactive substance (16).
The objectives of this study were to (i) compare agreement of the MGIT960 system for first-line drugs with a methodology (the resistance ratio method [RRM]) that had been used in clinical trials, relating drugsusceptibility to clinical outcome; (ii) compare the performance of the MGIT960, RRM, and microtiter plate assay (MPA) methodologies for second-line drugtesting; and (iii) define critical concentrations for ciprofloxa- cin and moxifloxacin for liquid-culture-based testing. The large collection of clinical isolates of Mycobacteriumtuberculosis (n ⴝ 247) used included 176 (71%) multidrug-resistant isolates. The results for MGIT960 and the RRM for rifampin and isoniazid (n ⴝ 200) were in excellent (99 to 100%) agreement for all strains. For streptomycin, 97% of the results at the critical concentration and 92% at high concentration, and for pyrazi- namide 92% of results overall, were concordant, but for ethambutol, fewer than 85% (65% for the critical concentration and 84% for the high concentration) of the MGIT-based results were concordant with those for the RRM. The MGIT960, RRM, and MPA assays (n ⴝ 133) correlated well for most second-line drugs tested. For susceptibility to ofloxacin, the MGIT960 and MPA results were in full agreement. The amikacin and rifabutin results obtained by MGIT960 agreed with the RRM results in 131 (99%) cases, and for capreomycin, they agreed for 129 of 133 isolates tested (97%). For prothionamide testing, only a limited number of drug-resistant isolates were available for testing and drawing definitive conclusions. We propose critical concentrations of 1.0 g/ml and 0.125 g/ml for ciprofloxacin and moxifloxacin, respectively, for liquid- culture-based testing.
GenoType ® MTBDRplus line probe assay (HAIN Life Sciences, Germany) based on DNA strip technology is approved by World Health Organization for direct detection of multi drug resistant (MDR) M. tuberculosis from smear positive/negative pulmonary specimens and M. tuberculosis cultures after evaluation by the Foundation for Innovative new diagnostics (FIND) and has become available in many laboratories across the developing countries . This assay permits the molecular identification of the M. tuberculosis complex and resistance to Rifampicin (RIF) and Isoniazid (INH) and is based on multiplex PCR followed by reverse hybridization for detection of deletion in wild type gene loci and mutation in rpoB (RNA polymerase B subunit), katG (catalase peroxidase) and inhA (inoyl coenzyme A reductase) loci. This test is affordable and has become available in many high burden diagnostic laboratories for pulmonary tuberculosis and is user friendly with minimum staff training [8,9]. However this test has not been sufficiently evaluated for direct detection of M. tuberculosis and drug resistance from extra pulmonary patients .
Anti-TNF therapies for rheumatism have been very effective and have led to the development of several artificial monoclonal antibodies such as infliximab and rituximab that act as TNF ago- nists. However, TNF is pivotal in the immune response to M. tuberculosis and helps contain the infection. 72 Therefore, the use of these agonists may increase the risk of patients developing TB or other complications from infections by non-tuberculous mycobacteria. Keane et al. showed that there were 70 reported cases of TB, usually extra-pulmonary, from the 147,000 patients administered with infliximab worldwide between 1998–2001. 73 Almost all of the TB cases were reported in countries with low incidence of TB making this association a highly alarming one. Etarnecept, another TNF antagonist was found to fare marginally better than infliximab and adalimumab, but the TB incidences still remain high. 74,75 Professor O.M. Kon, Imperial College Healthcare NHS Trust (UK), highlighted the probability of most of these cases being LTBI that are activated as the host immune surveillance drops and recommend testing for it before initiating treatment for rheumatism. Therefore diagnosing LTBI in these patients is a high priority. Patients are usually risk- assessed, taking into account the risk of prophylaxis versus devel- oping TB and then put forward for diagnosis. However, risk stratification misses 2-thirds of the cases thereby lowering the sensitivity manifold. For diagnosis, chest X-rays, TST and IGRA are the methods of choice. The TST can be used for diagnosis but patients with rheumatism usually have an attenuated response to the test. 76 IGRAs in this case are highly specific for the detection of LTBI unless steroids are administered to the patient. In the absence of guidelines for practitioners, Professor Kon suggested a thorough follow-up and surveillance of patients by taking into account their contact and travel histories as treat- ment with biologics has been noted to be associated with reacti- vation of TB many years post cessation of treatment.
Comparison of M. paratuberculosis quantification by the BACTEC 460 and MGIT960 methods. Using plate counts as the reference method, CFU predic- tions for the same M. paratuberculosis cell suspensions were obtained by both the BACTEC 460 and MGIT960 methods. The BACTEC 460 method was per- formed as previously described (21), with the following slight modifications. Briefly, 100 l of 10-fold dilutions from each strain was inoculated into three BACTEC 12B vials containing 4 ml Middlebrook 7H12B medium supplemented with 2 g/ml of mycobactin J, 1.0 ml of egg yolk (Becton Dickinson, Sparks, MD), and the same final concentrations of antibiotics as those used for MGIT tubes. All vials were incubated at 37°C without shaking. The growth index for each vial was measured every 24 h for 8 weeks by the BACTEC 460 instrument using a normal atmosphere for gas exchange. The upper limit for the designation of no growth for inoculated vials as well as negative controls was 30 growth units after 8 weeks. The instrument was calibrated weekly according to the manufac- turer’s instructions. When cumulative growth index readings exceeded 2,000 units, a previously described algorithm was used to estimate CFU of M. paratu- berculosis inoculated.
et al. demonstrated that MGIT system could recover more mycobacteria from clinical specimen than solid culture media (19). Our findings are more or less the same as those of other investigators (13,18,19) who have stated that the recovery rate was highest in BACTEC followed by MGIT and L.J culture medium. However, if we combine results of all the four L.J tubes inoculated per specimen routinely in our laboratory, the performance of L.J medium improves significantly and the recovery rate would become as good as the other two media. Hence, we may increase the chance of recovery by using more L.J culture media in comparison with one culture tube used by MGIT/BACTEC system. The greatest advantage is the rapid reporting results by liquid system. The mean time of mycobacteriumtuberculosis detection by MGIT was 6.8 days versus 11.7 and 27 days by BACTEC-12B and L.J media, respectively (p<0.05). Therefore, the rapidity and non-radiometric component of the system can be the most obvious advantages of MGIT against BACTEC and L.J media.