Combined use of Amplified Fragment Length Polymorphism and IS6110 -RFLP in fingerprinting clinical isolates of Mycobacterium tuberculosis from Kerala, South India
© Krishnan et al; licensee BioMed Central Ltd. 2007
Received: 01 December 2006
Accepted: 28 July 2007
Published: 28 July 2007
DNA fingerprinting by IS6110-RFLP has shown a high incidence of Mycobacterium tuberculosis isolates having no and low copies of the insertion sequence in Kerala, South India. Amplified Fragment Length Polymorphism (AFLP) would scan the entire genome rather than a few repetitive elements, we thought that this technique would help us in differentiating the large reservoir of isolates from an endemic region. Here we evaluate the ability of Amplified Fragment Length Polymorphism (AFLP) to type clinical isolates.
Fifty clinical isolates of M. tuberculosis were analysed by conventional radioactive AFLP and IS6110- RFLP. M. bovis, M. bovis BCG and two non tuberculous mycobacteria were also analysed to see species specific differences generated by AFLP. Cluster analysis was performed using the AFLP profile that showed the maximum polymorphism within M. tuberculosis and this was compared to the number of copies of IS6110 insertions.
For AFLP, out of ten primer pairs tested, the EO/MC pair generated maximum polymorphism among the clinical isolates of M. tuberculosis. The similarity between the isolates ranged between 88 and 99.5%. Majority (nearly 85%) of the 'low copy' IS6110 isolates clustered together, while the rest clustered irrespective of the copy numbers. AFLP could show rare differences between isolates of M. tuberculosis, M. bovis and M. bovis BCG. The AFLP profiles for non-tuberculous mycobacteria were highly different from those of M. tuberculosis.
Polymorphism generated by AFLP within the M. tuberculosis species is limited and hence AFLP alone seems to have limited use in fingerprinting the isolates in Kerala. The combined use of AFLP and IS6110-RFLP showed relatively better differentiation of 'high copy' IS6110 isolates, but failed to differentiate the 'low copy' isolates. However, the technique may be efficient in inter-species differentiation, and hence potentially useful in identifying and developing species- specific markers.
Tuberculosis claims about 500, 000 deaths annually in India and the disease is endemic in Kerala, a small state in the southwest . DNA fingerprinting by RFLP-hybridization based on the insertion sequence IS6110  showed a high prevalence of 'no copy' and single copy isolates of Mycobacterium tuberculosis in Kerala . This, and other studies point out that IS6110-based methods have limited use in regions where there is a high incidence of strains carrying few copies of IS6110 [4, 5]. Spoligotyping, the next widely used fingerprinting method for M. tuberculosis complex organisms can efficiently differentiate 'low copy' (5 or fewer copies of IS6110) strains but again is not as efficient as RFLP for 'high copy' (6 or more copies of IS6110) strains [6, 7]. Fingerprinting techniques such as Random Amplification of Polymorphic DNA (RAPD) and Amplified Fragment Length Polymorphism (AFLP) would scan the entire genome rather than a few repetitive elements, and since AFLP can be applied to any genome irrespective of its complexity , we thought that this technique would help us in differentiating the large reservoir of isolates from an endemic region. AFLP has been used for strain typing within different bacterial genera  and has also been evaluated in inter- and intra-species typing of mycobacteria [10–13]. In the present study, we analyzed clinical isolates of M. tuberculosis from Kerala using AFLP and looked at the ability of the technique to further differentiate isolates typed by IS6110-RFLP. The ability of AFLP to provide inter-species differentiation was used to prove the validity of the methodology adopted by us.
Clinical isolates and laboratory strains
Fifty clinical isolates of Mycobacterium tuberculosis, isolated from TB patients and identified based on their growth characteristics, colony morphology and biochemical tests  were used in this study. The Laboratory strains of mycobacteria were M. tuberculosis H37Rv, H37Ra, M. bovis and M.bovis BCG. Two atypical mycobacteria obtained from clinical samples were also included in the study. Using INNO-LiPA Mycobacteria (LiPA; Innogenetics, Zwijnaarde, Belgium), one was identified as M. chelonae, while the other could be identified to genus level only.
The isolates used in this investigation were collected by our laboratory as a part of a study to screen for anti-TB drug sensitivity and became part of our local repository. Since this was a retrospective study on the isolates already available with us, it was not put up for any clearance to an ethical committee.
Genomic DNA was isolated from mycobacteria using CTAB method as reported earlier .
Sequences of AFLP adapters and primers(Vos et al., 1995).
AFLP adapter (EcoR I) oligo 1 : 5'-CTC GTA GAC TGC GTA CC-3'
AFLP adapter (EcoR I) oligo 2 : 5'-AAT TGG TAC GCA GTC TAC-3'
AFLP adapter (Mse I) oligo 1 : 5'-GAC GAT GAG TCC TGA G-3'
AFLP adapter (Mse I) oligo 2 : 5'-TAC TCA GGA CTC AT-3'
Primer, EO (non selective)a : 5'-GAC TGC GTA CCA ATT C-3'
Primer, MO (non selective)a : 5'-GAT GAG TCC TGA GTA A-3'
The PCR products were analyzed by urea-PAGE. Gel images were visualized in a Phosphor imager (Personal Molecular Imager FX, Bio-Rad, Hercules, CA).
This was performed according to the standard protocol .
Cluster analysis was performed using similarity coefficient based on bands that showed differences among isolates. Bionumerics software v 3 (Applied Maths, Belgium) was used to create UPGMA dendrogram.
Comparison of AFLP with IS6110- RFLP
In the present study we have used conventional radioactive AFLP in order to evaluate its potential to differentiate the M. tuberculosis isolates of Kerala. This is an endemic region where we have reported a high incidence of M. tuberculosis strains with low numbers of IS6110 which makes it difficult to be type them . We used this method instead of multiplex Fluorescent AFLP, as we were also interested in developing AFLP-derived markers for the characterization of the Kerala isolates. However, the data presented in this paper is restricted to AFLP profiles and their comparison to IS6110 copy number data for a set of 50 isolates, of which 33 were 'low copy' isolates. Initially, our results on twenty-five clinical isolates showed 0–3 polymorphic bands per primer pair while using ten primer combinations. Using EcoR I/Mse I enzyme combination, approximately 4% of the total number of bands/primer combination was polymorphic for M. tuberculosis. There have been earlier reports on AFLP analysis of Mycobacterium tuberculosis. A study using multiplex FAFLP employing EcoR I and Mse I enzymes and four primer combinations had shown that 28% of the total number of fragments obtained were polymorphic . In contrast, another study using radioactive AFLP and a different enzyme combination (Apa I/Taq I), demonstrated poor discrimination within the M. tuberculosis complex . The discrepancy between these studies could be due to the choice of restriction enzymes and the methodology. FAFLP involves multiplexing which is not feasible with the radioactive methodology. Therefore, we have used a combination of the above methodologies using EcoR I/Mse I enzyme combination and conventional radioactive AFLP. Even though one may screen other enzyme combinations to increase polymorphism, our results suggest that radioactive AFLP is not an easy method for strain differentiation in endemic regions. However, AFLP could clearly differentiate M. tuberculosis from isolates of non-tuberculous mycobacteria. In our study, even though the close relationship between members of M. tuberculosis complex is reflected in AFLP analysis, the rare differences could also be detected. We characterized a few bands that differentiated M. tuberculosis and M. bovis/BCG by sequencing and identified three fragments belonging to RD1, RD4 and RD5 (results not shown). It has been reported that RD1 is absent only in BCG, while RD4-RD10 are deleted in both M. bovis and BCG . Our results therefore correlate well with this data.
Comparing AFLP profile with IS6110 typing data showed that in AFLP analysis isolates clustered irrespective of the IS6110 copy number. The IS6110 'low copy' isolates were not well differentiated as nearly 85% of the low copy isolates clustered together (Fig 3, Cluster 1b). Therefore, it could be speculated that the low copy strains of Kerala are not very diverse. But all strains with a single copy of IS may not be identical, as they fell into different clusters (both 1 and 3) in the AFLP analysis. The high copy isolates were relatively better separated than the low copy isolates.
We conclude that conventional radioactive AFLP alone does not seem to be an efficient fingerprinting method for M. tuberculosis. The combined use of IS6110-RFLP and AFLP again can differentiate the 'high copy' isolates to some extent, but does not differentiate the 'low copy' isolates efficiently. However the technique provides inter-species differentiation within and outside the M. tuberculosis complex and therefore is useful in the development of species-specific markers. The latest addition to the fingerprinting methods for M. tuberculosis, MIRU-VNTR [16, 17], is efficient and is being adopted by several labs and we are currently analyzing our isolates using MIRU fingerprinting. But different fingerprinting techniques cluster the strains differently, and therefore generally appear to work independently of each other. Therefore, a unified fingerprinting system that can neatly pigeonhole all different strains is yet to be developed.
BVJ and ML are recipients of Research Fellowships from Council for Scientific and Industrial Research (CSIR), Government of India. This study received financial assistance under Program Support from the Department of Biotechnology, Government of India. The strains used in this study were collected under an SRF award of the Kerala State Council for Science, Technology and Environment to SM and AK. The type strains were kind gifts of Dr V M Katoch, JALMA, Agra and Dr S Vijaya, Indian Institute of Science, Bangalore and Tuberculosis Research Centre, Chennai.
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