|Year : 2014 | Volume
| Issue : 3 | Page : 281-284
Comparison of manual mycobacteria growth indicator tube and epsilometer test with agar proportion method for susceptibility testing of Mycobacterium tuberculosis
N Karabulut1, B Bayraktar2, Y Bulut3
1 Department of Medical Microbiology, Mental Health Hospital, 23000 Elazig, Turkey
2 Department of Medical Microbiology, Sisli Etfal Training and Research Hospital, 34371 Sisli, Istanbul, Turkey
3 Department of Medical Microbiology, Medical Faculty, Firat University, 23119 Elazig, Turkey
|Date of Submission||31-Oct-2012|
|Date of Acceptance||25-Nov-2013|
|Date of Web Publication||10-Jul-2014|
Department of Medical Microbiology, Mental Health Hospital, 23000 Elazig
Source of Support: None, Conflict of Interest: None
Background and Objectives: Antimycobacterial susceptibility tests take weeks, and delayed therapy can lead to spread of Mycobacterium tuberculosis. Therefore, rapid, accurate and cost-effective methods are required for proper therapy selection. In this study, the Mycobacteria growth indicator tube (MGIT) and epsilometer test (Etest) methods were compared to the agar proportion method for susceptibility testing of Mycobacterium tuberculosis. Materials and Methods: The susceptibility tests against isoniazid (INH), rifampin (RIF), streptomycin (STM) and ethambutol (ETM) of 51 M. tuberculosis complex isolates were analyzed by the MGIT, Etest and agar proportion methods. Results: The concordance between MGIT/Etest and agar proportion methods was 98% for INH and 100% for RIF, STM, ETM. There were not statistically significant differences in results of the susceptibility tests between MGIT/Etest and the reference agar proportion method. Conclusion: The results have shown that MGIT and Etest methods can be used instead of the agar proportion method, because these two methods are more rapid and easier than the agar proportion method.
Keywords: Agar proportion method, epsilometer test, mycobacteria growth indicator tube, Mycobacterium tuberculosis
|How to cite this article:|
Karabulut N, Bayraktar B, Bulut Y. Comparison of manual mycobacteria growth indicator tube and epsilometer test with agar proportion method for susceptibility testing of Mycobacterium tuberculosis. Indian J Med Microbiol 2014;32:281-4
|How to cite this URL:|
Karabulut N, Bayraktar B, Bulut Y. Comparison of manual mycobacteria growth indicator tube and epsilometer test with agar proportion method for susceptibility testing of Mycobacterium tuberculosis. Indian J Med Microbiol [serial online] 2014 [cited 2019 Jul 18];32:281-4. Available from: http://www.ijmm.org/text.asp?2014/32/3/281/136565
| ~ Introduction|| |
The incidence of tuberculosis has increased in developed countries in the last decade and in developing countries, tuberculosis still remains as a serious public health problem. Early diagnosis of tuberculosis and susceptibility testing for effective treatment has vital importance to control the spread of Mycobacterium tuberculosis. Therefore, rapid, accurate and cost-effective methods are needed to control this disease efficiently.  Furthermore, rapid identification and susceptibility testing of isolates contribute to prevention of spread of resistant organisms.  The clinical and laboratory standards institute (CLSI) subcommittee recommends that laboratories use a quick method of susceptibility testing to achieve the goal of reporting the results of M. tuberculosis complex susceptibility testing within 28 days of receipt of a specimen in the laboratory. 
The methods recommended by the CLSI are the agar proportion and BACTEC broth methods. Although the agar proportion method is inexpensive, this method has the disadvantage of requiring 3 weeks of incubation. The BACTEC system can determine mycobacteria isolates within less time, but this method has certain limitations such as requiring expensive instrumentation, being standardised for only a few of the first-line anti tuberculosis drugs. In addition, both methods use a single critical drug concentration for determination of drug resistance. ,,,
The mycobacteria growth indicator tube (MGIT) is a non-radiometric approach for the detection of mycobacteria. The MGIT contains modified Middlebrook 7H9 broth and oxygen-quenching fluorescence method to detect the amount of oxygen consumed by growing organisms. , The epsilometer test (Etest) method was developed for mycobacterial susceptibility testing by Wanger and Mills. The Etest strip contained an exponential gradient of antimicrobial agent determinates minimum inhibitory concentrations (MIC) for antimicrobial agents. ,, The potential advantages of these methods are relatively easy to perform and provide susceptibility results in about a week. ,,
In this study, MGIT and Etest which were suggested as alternative methods to test susceptibility against isoniazid (INH), rifampin (RIF), streptomycin (STM) and ethambutol (ETM) of M. tuberculosis clinical isolates were compared with agar proportion method.
| ~ Materials and Methods|| |
A total of 51 M. tuberculosis0 complex strains were isolated from 467 tuberculosis suspected patients at Medical Microbiology Laboratory of ͺiΊli Etfal Training and Research Hospital, Istanbul, Turkey. All clinical isolates were grown on Lowenstein-Jensen agar (L-J) slants and MGIT media; identified presumptively as mycobacteria by colony morphology and Erlich-Ziehl-Neelsen and Auromine-Rhodamine staining. Identification was confirmed by using MGIT p-nitro benzoic acid, cord factor, nitrate reductase and niacin accumulation tests.
We included the quality-control strains of M. tuberculosis H37RV, which was susceptible to all first-line drugs; ATCC 35838, which was resistant to rifampin; ATCC 35837, which was resistant to ethambutol; ATCC 35822, which was resistant to INH; ATCC 35820, which was resistant to streptomycin.
MGIT susceptibility testing
A volume of 0.5 ml of oleic acid-albumin-dextrose (OADC) and 0.1 ml of the drug stock solution were aseptically added to each MGIT (Becton Dickinson, USA). The growth control (GC) tube did not contain antibiotics. 0.5 ml of the 1:5 diluted suspension of the test culture was inoculated into each MGIT. The final concentration of drugs in MGIT was 0.8 μg/ml for STM, 0.1 μg/ml for INH, 1.0 μg/ml for RIF and 3.5 μg/ml for ETM. All tubes were incubated at 37°C. MGITs were read daily by placing them on a 365-nm ultraviolet transilluminator. An isolate is considered susceptible if the drug-containing tube is not emitting fluorescence within 2 days after GC tube showed positivity. It is considered resistant if the drug-containing tube showed growth on the day of GC positivity or within the following 2 days. ,
Etest susceptibility testing
Etest strips, containing gradients of STM (0.064-1024 μg/ml) and INH, RIF, ETM (0.016-256 μg/ml) were provided by AB BIODISK, Solna, Sweden. Freshly prepared Middlebrook 7H10 agar with OADC supplement was used. 3-4 week-old colonies of M. tuberculosis from L-J slants were suspended in 3 ml of Middlebrook 7H9 broth in tubes. The tubes were vortexed vigorously for 3-5 min to homogenise the suspension. The large particles were allowed to settle and the supernatant was adjusted to a turbidity equivalent to a McFarland 3.0. The inoculum was swabbed onto the entire surface of the agar plate. The Etest strips were placed on the agar surface after plates were preincubated at 35°C in 5-10% CO 2 for 24 h. The plates were then incubated under the same conditions until an inhibition ellipse was visible (7-10 days). The MIC was interpreted as the point at which the ellipse of inhibition crosses the Etest strip, as described in the Etest technical guide. ,
Agar proportion susceptibility testing
Commercially prepared drug-impregnated discs (Sensi-Discs, Becton Dickinson, USA) were used as the source of INH, RIF, ETM, STM drugs. Discs used in agar proportion susceptibility tests and drugs concentrations of discs were INH 0.2 and 1 μg/ml, RIF 1 μg/ml, ETM 5 μg/ml, STM 2 μg/ml. Antibacterial activity was determined by an agar dilution technique using Middlebrooks 7H10 agar with OADC enrichment. Colonies collected from a L-J tube were homogenized in Middlebrooks 7H9 broth for achieving turbidity equal to a McFarland 1.0 standard. The standardised suspensions were diluted 10− 2 and 10− 4 in sterile saline and 0.1 ml were inoculated onto the control quadrant and onto each of the drug-containing quadrants. Plates were incubated at 37°C in an atmosphere of 5% CO 2 for 3 weeks. The colonies on the plates were counted after 3 weeks of incubation. Susceptibility to tested drug was determined by comparing the number of colony forming units growing in the drug containing quadrants to the number in the drug-free control quadrant [Figure 1]. Quadrants showing 1% growth or more compared with growth in the control quadrant were considered resistant to that concentration of tested drug, as described in the NCCLS document. 
|Figure 1: (a) Control quadrant of agar proportion susceptibility test, (b) Isoniazid and streptomycin in agar proportion susceptibility test (c) Rifampin and ethambutol in agar proportion susceptibility test|
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Statistical analyses were conducted using the SPSS 16 (SPSS Inc, Chicago, IL). The differences in susceptibility results were evaluated by using the Chi-square test, with a P ≤ 0.05 considered to be significant.
| ~ Results|| |
A total of 51 M. tuberculosis complex isolates was tested for susceptibility to STM, INH, RIF and ETM with three different methods. The susceptibility patterns of the strains are listed in [Table 1]. Whereas 47 (92%) of the 51 isolates were sensitive to all four drugs by the agar proportion method, 48 (94%) of them were found sensitive by both MGIT and Etest methods.
|Table 1: The susceptibility patterns of M. tuberculosis complex strains by the agar proportion, MGIT and etest methods |
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While two (4%) isolates were resistant to INH by the agar proportion method, one of these resistant isolates was found susceptible to INH by both MGIT and Etest methods. MIC of this strain was established as 0,125 μg/ml (indicating susceptibility) for INH by the Etest. The other two (4%) isolates were established as resistant to STM by agar proportion, MGIT and Etest methods. The concordance between MGIT/Etest with agar proportion methods was 98% for INH and 100% for RIF, STM, ETM. The sensitivity of the MGIT and Etest was 50% for INH, 100% for RIF, STM and ETM. The specificity of these methods was 100% for all four drugs. While positive predictive value was 100% for four drugs, negative predictive value was 98% for INH, 100% for RIF, STM and ETM. On the basis of the Chi-square test, the difference in performance between MGIT/Etest methods with the agar proportion method turned out not to be statistically significant.
| ~ Discussion|| |
Rapid, accurate and cost-effective susceptibility testing methods are required for proper therapy selection for M. tuberculosis and the prevention of the spread of resistant organisms. MGIT and Etest methods may be considered as they are rapid, accurate and easy to perform.
In this study, an excellent agreement was demonstrated between MGIT/E test with the agar proportion method. The concordance between MGIT/Etest with agar proportion methods was 98% for INH and 100% for RIF, STM, ETM. While no false positive result was reported for all drugs, only one false negative result was reported against INH. There was not a significant difference between MGIT/Etest with agar proportion methods (P > 0.05 for all drugs). Similarly, the study of Jaloba et al. found strong correlations between Etest and agar proportion, which are 100% for STM, RIF, ETM and 98% for INH. In another study, between the Etest and the proportion method for drug susceptibility testing of M. tuberculosis was also found considerable agreement by Varma et al. The authors evaluated Etest to be a more rapid method (10 days) than agar proportion (3 weeks) for testing of M. tuberculosis with INH and RIF.
The correlation between proportion method with MGIT and Etest for the first line antituberculosis drugs was investigated by Birinci et al. The study reported that the Etest has the advantage of being a quick and simple method without the need for any additional equipment. It was also suggested that the MGIT and Etest methods could be routinely used instead of the proportion method. Fegou et al. compared the manual MGIT and Etest to the proportion method. Although they established that manual MGIT showed superior sensitivity at detecting resistance to INH, STM and ETM compared to the Etest, they concluded that the proportion method remains the method of choice. The Etest was compared with the standard proportion method using Lowenstein Jensen medium in testing INH, RIF and STM with M. tuberculosis by Freixo et al.  Although they achieved extremely good agreement between the methods, they reported Etest to be less useful for developing countries because of requiring expensive media, Etest strips and the specialised equipment (CO 2 incubator).
| ~ Conclusions|| |
The agar proportion method is more inexpensive than other methods, but this method has disadvantages such as requiring 3 weeks of incubation, consisting a single critical drug concentration for determination of drug resistance, requiring technical expertise for the interpretation of results. Although the Etest and MGIT methods compared with agar proportion is expensive, these methods are relatively simple and provide susceptibility results in less than a week may be effective for developing countries. These two methods can beroutinely used as they are rapid and easy to perform instead of the agar proportion method.
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