INT J TUBERC LUNG DIS 9(5):576–581 © 2005 The Union
Restriction fragment length polymorphism study of nationwide samples of Mycobacterium tuberculosis in Thailand, 1997–1998 D. Rienthong,* P. Ajawatanawong,† S. Rienthong,* S. Smithtikarn,* P. Akarasewi,* A. Chaiprasert,‡ P. Palittapongarnpim†§ * Tuberculosis Division, Department of Disease Control, Bangkok, † National Center for Genetic Engineering and Biotechnology, Pathumthani, ‡ Department of Microbiology, Faculty of Medicine, Siriraj Hospital, Bangkok, § Department of Microbiology, Faculty of Science, Mahidol University, Bangkok, Thailand SUMMARY S E T T I N G : During 1997–1998, a national anti-tuberculosis drug resistance survey was conducted in Thailand as a part of a global project. O B J E C T I V E : To evaluate the IS6110 hybridisation patterns and the level of clustering, which was expected to be low due to the short duration of the sample collection. D E S I G N : Eight hundred and twenty-eight bacterial isolates were available for fingerprinting by standard IS6110 hybridisation. R E S U L T S : The restriction fragment length polymorphism patterns varied with geographic locations, ages of the patients, and resistance to rifampicin and streptomycin. The Beijing strain was more common among younger patients, and their prevalence appeared to decrease with the distance from Bangkok, while the oppo-
site was true for the single-banded isolates. Excluding isolates containing five or less copies of IS6110, 26.4% were clustered. Clustering was more common among females. The clustered isolates were sometimes from different provinces and, if resistant to drugs, usually possessed different resistance profiles. C O N C L U S I O N S : The results question the validity of inferring recent transmission from the clustering of IS6110 hybridisation patterns in some settings in Thailand. The level of recent transmission in a nationwide study in a country with a high incidence of tuberculosis should be evaluated with caution. K E Y W O R D S : tuberculosis; molecular epidemiology; RFLP; Thailand; drug resistance
MYCOBACTERIUM TUBERCULOSIS (TB) is usually typed using the internationally recommended method of Southern hybridisation with IS6110.1 In industrialised countries, the restriction fragment length polymorphism (RFLP) pattern of each bacterial strain is usually sufficiently unique to be regarded as the DNA fingerprint. Although this method has been applied in Thailand on several occasions,2–4 the previously studied samples were not representative of the populations of M. tuberculosis patients in Thailand. It is assumed that if TB in studied patients results from endogenous reactivation of previously infecting bacteria, there is little chance that the isolates will have an identical RFLP pattern. Therefore, two isolates with an identical RFLP pattern are generally considered as epidemiologically linked and a result of recent bacterial transmission. This has become the basis of outbreak investigations, estimations of degree of transmission in communities and other epidemiological studies.5 However, it is commonly felt that
IS6110 clustering in large rural areas, especially in high-incidence countries, may not all represent recent transmission events, and that some may actually represent remote transmission. In 1994, the World Health Organization Global Tuberculosis Programme and the International Union Against Tuberculosis and Lung Disease initiated the Global Project on Anti-Tuberculosis Drug Resistance Surveillance.6 As part of the project, M. tuberculosis isolates were collected from several provinces all over Thailand.7 We therefore took the opportunity to study the IS6110-RFLP of the isolates to provide a nationwide picture of the population structure of M. tuberculosis. However, due to the short period of sample collection and the fact that the isolates represented a small fraction of all smearpositive patients, it was anticipated that detection of recent transmission among the patients would be unlikely and that the degree of clustering would consequently be low.
Correspondence to: Prasit Palittapongarnpim, Department of Microbiology, Faculty of Science, Rama 6 Road, Bangkok 10400, Thailand. Tel: (66) 2-564-6700, ext 3444. Fax: (66) 2-564-6704. e-mail:
[email protected] Article submitted 11 May 2004. Final version accepted 24 September 2004.
RFLP typing of M. tuberculosis in Thailand
MATERIALS AND METHODS Over a 12-month period in 1997–1998, M. tuberculosis isolates from 1137 patients were collected from 59 diagnostic centres in 46 of 76 administrative provinces of Thailand for the drug surveillance study. The inclusion criterion was newly diagnosed smear-positive pulmonary TB patients without a history of treatment. A multi-stage stratified cluster sampling procedure was used. Information on the patients’ age, sex and history of previous anti-tuberculosis treatment was collected. Two specimens were collected from each patient and sent to the Tuberculosis Division’s National Reference Laboratory Centre for culture in Ogawa medium. Bacteria were identified by the Ziehl-Neelsen staining method, the niacin production test, the nitrate reduction test and the catalase test at 68C. Drug susceptibility testing was performed using the proportion method.7,8 Isolates from 828 (72.8%) patients were available for the RFLP study. The numbers of isolates from central, western, eastern, northern, north-eastern and southern Thailand were 154, 32, 59, 34, 438 and 111, respectively (Figure 1A). Samples from central Thailand included 50 samples from Bangkok. Seventy-three per cent of all isolates were from male patients. After culture, the cells were harvested and chromosomal DNA was extracted using an enzymatic lysis method. Two micrograms of DNA of each isolate were digested with PvuII and Southern hybridised with digoxigenin-labelled plasmid pDC73 containing a segment of IS6110. The methods are described in detail elsewhere.2
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The IS6110 hybridisation patterns were analysed using Gelcompar II version 1.5 (Applied Maths, Kortrijk, Belgium). Isolates were classified as members of the Beijing family or of the Nonthaburi group, based on 78% or more similarity to the previously described isolates.2 Isolates with identical IS6110-RFLP patterns were defined as clustered. 2 tests were performed for the association of clustering or RFLP patterns with other parametres. Linear regression was performed between the percentages of some RFLP patterns in each province and the distance of the province from Bangkok. Parametres significantly associated with RFLP patterns were included in a logistic regression model.
RESULTS Of the 828 isolates, 226 (27.3%) had only one copy of IS6110. In 194 (85.8%), the single IS6110 hybridised band was 1.45 kb long, while the bands in 10 and 22 isolates (4.4% and 9.7%) were 1.3 and 4.5– 5 kb long, respectively. One hundred and sixty-six isolates (20.0%) had two to five copies of IS6110. Respectively 172 (20.8%) and 118 (14.3%) isolates had RFLP patterns similar to the previously described Beijing family and Nonthaburi group.2 The RFLP patterns of the other isolates were heterogeneous. Some examples of the RFLP patterns and the frequency distribution of the number of IS6110 hybridised bands are shown in Figures 2 and 3, respectively. The patients were aged 15–89 years, with an average age of 44.7. The average age of patients harbouring the Beijing strain and of those with single-banded
Figure 1 A. Map of Thailand showing the number of isolates from each geographic region. B. Provinces where the isolates belonging to the biggest cluster were isolated. The RFLP patterns are shown in the inset. RFLP restriction fragment length polymorphism.
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Figure 3 Number of M. tuberculosis isolates with different numbers of copies of IS6110.
Figure 2 IS6110 hybridisation patterns of some isolates. The patterns in lanes 1 and 20 belong to the standard MT14323 strain. The samples in lanes 3, 6, 11, 12, 19 and 21 belong to the Nonthaburi family while the samples in lanes 9, 13 and 22 belong to the Beijing family.
isolates was 38.5 and 48.9 years, respectively. Patients aged 45 years were more likely to harbour the Beijing strain than those aged 45 (2, P 0.001). The reverse was true for single-banded isolates (2, P 0.001). The odds ratios (OR) of isolating singlebanded and Beijing strains from patients with different ages are shown in Table 1. The distribution of isolates belonging to the various groups was not homogeneous across the different regions in Thailand (2, P 0.05) (Table 2). The Beijing strain was more prevalent in Bangkok and central and western Thailand (2, P 0.001); the re-
verse was true for the single-banded isolate (2, P 0.001). The Nonthaburi strain was common in central Thailand (2, P 0.001). The Beijing strain is believed to have originated from China9 and likely to have entered Thailand mainly via Bangkok. The Nonthaburi strain is found mainly in Thailand. Its high incidence in central Thailand suggests that it may have originated there. The prevalence of both strains decreased significantly with distance from Bangkok (r 0.42 and 0.36, respectively, P 0.05). In contrast, the prevalence of singlebanded isolates increased significantly with distance from Bangkok (r 0.40, P 0.05). Respectively 99 (12.0%), 50 (6.0%), 57 (6.9%) and 89 (10.7%) isolates were resistant to isoniazid (INH), rifampicin (RMP), ethambutol (EMB) and streptomycin (SM), while 19 isolates (2.3%) were multidrugresistant (MDR, defined as resistance to at least INH and RMP). The rates of resistance to RMP, EMB and SM correlated significantly with the RFLP patterns (2, P 0.05) (Table 3). The rates of resistance to the three drugs were significantly higher among the Beijing strain and the isolates with heterogeneous banding patterns with 5 bands combined than in the other groups combined (2, P 0.05). There was no significantly higher prevalence of INH resistance or MDR in any groups of bacteria.
Table 1 Percentages of isolates with different IS6110-RFLP patterns among various age groups. The average age of the patients in each group of isolates is shown in the last row Groups of isolates Age, years 24 25–34 35–44 45–54 55–64 65 Average age
Single-banded (OR, 95%CI)
2–5 bands
Beijing (OR, 95%CI)
17.7 (0.67, 0.34–1.31) 16.9 (0.63, 0.37–1.07) 24.4 (1.00) 34.7 (1.65, 1.01–2.68) 33.3 (1.55, 0.94–2.55) 35.6 (1.71, 1.04–2.82) 48.9
13.9 19.3 19.2 20.1 20.0 25.8 47.0
32.9 (1.30, 0.73–2.32) 28.9 (1.08, 0.67–1.74) 27.3 (1.00) 16.0 (0.51, 0.29–0.88) 10.4 (0.31, 0.16–0.59) 10.6 (0.32, 0.17–0.60) 38.5
RFLP restriction fragment length polymorphism; OR odds ratio; CI confidence interval.
Heterogeneous Nonthaburi with 5 bands 16.5 14.5 12.2 17.4 16.3 9.8 43.7
19.0 20.5 16.9 11.8 20.0 18.2 43.9
Sub-total 9.5 20.0 20.8 17.4 16.3 15.9 44.7
RFLP typing of M. tuberculosis in Thailand
Table 2
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Number of isolates with different IS6110-RFLP patterns in various regions of Thailand Groups of isolates
Regions Northern Central Bangkok Western North-eastern Eastern Southern Total
Single-banded n (%)*
2–5 bands n (%)*
Beijing n (%)*
Nonthaburi n (%)*
Heterogeneous with 5 bands n (%)*
Total
10 (29.4) 14 (13.5) 10 (20) 7 (21.9) 134 (30.6) 18 (30.5) 33 (29.7) 226 (27.3)
9 (26.5) 15 (14.4) 3 (6) 7 (21.9) 115 (26.3) 7 (11.9) 10 (9) 166 (20.1)
6 (17.7) 29 (27.9) 21 (42) 10 (31.3) 73 (16.7) 13 (22) 20 (18) 172 (20.8)
3 (8.8) 34 (32.7) 6 (12) 6 (18.8) 48 (11) 11 (18.6) 10 (9) 118 (14.3)
6 (17.7) 12 (11.5) 10 (20) 2 (6.3) 68 (15.5) 10 (17) 38 (34.2) 146 (17.6)
34 104 50 32 438 59 111 828
* Percentage of total isolates in the region. RFLP restriction fragment length polymorphism.
The RFLP patterns were analysed for association with age, distance from Bangkok and drug resistance. Logistic regression revealed that the chance of isolating different groups of bacteria correlated independently with patients’ age (P 0.001), distance from Bangkok (P 0.001) and resistance to RMP (P 0.02) and SM (P 0.002), but not to EMB (P 0.19). All single-banded isolates and 94 isolates with two to five copies of IS6110 (51.6%) were clustered. They could also usually be differentiated by other typing methods.2,10,11 Among isolates that contained more than five copies of IS6110, 111 (26.4%) were in 42 clusters. The clustering rates of the Beijing and Nonthaburi families were respectively 22.1% and 22.8%, which was not significantly different from the rate among isolates with heterogeneous RFLP patterns (31.5%). The cluster rate was not associated with age, RFLP pattern, distance from Bangkok or drug resistance. However, it was higher among females than males (OR 1.9, 95% confidence interval 1.5–2.3, P 0.01). The biggest cluster comprised 13 isolates. They were from five provinces (Figure 1B) and were sensitive to all drugs. Nine isolates in the cluster were found in southern Thailand, while the rest were found in central Thailand. Fourteen more isolates had hybridisation patterns similar to this cluster, 11 of which were also from southern Thailand.
Table 3 Rate of resistance to each drug among the various groups of isolates Groups of isolates
INH
RMP EMB
Single-banded 2–5 bands Beijing Nonthaburi Heterogeneous with 5 bands Total
10.2 11.4 15.1 6.8 15.8 12.0
6.6 3.6 9.3 1.7 7.5 6.0
3.5 6.6 9.9 4.2 11.0 6.9
SM
MDR
4.9 6.6 15.1 14.4 16.4 10.7
2.7 1.2 4.1 0.0 3.4 2.3
INH isoniazid; RMP rifampicin; EMB ethambutol; SM streptomycin; MDR multidrug resistance, defined as resistance to at least INH and RMP.
Although clustered isolates are generally believed to be the result of recent transmission between clustered cases, several findings in the study did not support this conclusion. First, many clustered isolates (54.1%) had no isolates with identical RFLP patterns in the same province. Second, when the drug resistance profiles of 33 pairs of isolates with identical RFLP patterns were analysed, 18 pairs were sensitive to all drugs. Of the remaining 15 pairs, the drug resistance patterns of 14 pairs were different. Among these, seven pairs were also from different provinces. Although the drug resistance profile could change more rapidly than the IS6110-RFLP patterns and there were limitations in the use of administrative provinces to identify geographic separation, these data indicate that much of the clustering might not be due to transmission during or immediately before the period of study.
DISCUSSION This is the most extensive study to date of M. tuberculosis RFLP in Thailand, although the period of study is rather short and only one method of typing was used. The general genotypic population structure of M. tuberculosis in this study was slightly different from the previous report, which studied isolates from three referral hospitals in Bangkok and vicinity.2 However, the differences might not reflect the real changes in the bacterial population in Thailand, as the bacteria in this study were from a less biased sample collecting procedure. They were also from a much wider area, including a large number of isolates outside Bangkok and vicinity. This study suggests an increase in Beijing isolates and a decrease in single-banded isolates in Thailand. The higher prevalence of the Beijing strain among younger patients suggests that the strain was less common in the past, and agrees with the hypothesis that the strain is spreading. The higher prevalence in Bangkok and central Thailand confirms that Bangkok is the main portal for Thailand, and that the strain
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enters Thailand here, propagates rapidly and spreads from here to other parts of the country. The Beijing strain was also found to be more common in younger patients in Vietnam12 and in the Netherlands,13 which was also interpreted as evidence of the expansion of the strain. The higher prevalence of single-banded isolates among patients of older age and in provinces far from Bangkok suggests that the opposite might occur. We hypothesised that the single-banded isolates have been circulating in Thailand for such a long time that they are distributed fairly evenly in regions far from Bangkok, i.e., north, north-eastern and southern Thailand. They may be in decline relative to other strains, especially in Bangkok and central Thailand, where they may be replaced by more modern isolates such as the Beijing strain. The single-banded isolates were also reported to be more common among older patients in Malaysia.14 An alternative, and possibly complementary explanation is that Beijing strains may be more likely to cause disease immediately, while single-banded isolates tend to result in reactivation later in life and are therefore more common in older patients. Another possible compounding factor that was not addressed in this study is human immunodeficiency virus infection. The Beijing strain was found in association with MDR in several studies.13,15–19 Although it was not associated with MDR in the present study, probably due to the small number of MDR strains, they were more likely to be resistant to RMP and SM in combination with the heterogeneous isolates with 5 bands. The association of the Beijing strain with drug resistance was described in other studies, although the associated drugs were not the same. The Beijing strain was resistant to INH and SM in Vietnam12 and to EMB and SM in Russia.17 A recent study demonstrated alterations in the genes involved in the repair of DNA mutations unique to the Beijing strain, which might be related to the high rate of drug resistance.20 This conforms to the report that double mutations in the rpoB gene were relatively common in the Beijing strain.18 However, the in vitro rate of mutation-conferred resistance to RMP was not elevated in the Beijing strain.21 The mechanisms for the association between the Beijing strain and drug resistance are therefore still unclear. The degree of clustering of the RFLP patterns in this study (26.4%), which was similar to the previous study (26.9%),2 was rather surprising, given the widely varied geographic origins of the samples, the relatively small proportion of isolates collected and the relatively short period of sample collection. The observation of a larger cluster of 13 isolates from patients in two regions was also extraordinary. In low-incidence countries, IS6110-RFLP clustering is used to assess epidemiological relationships, which usually reflect
active or recent transmission.5,22 In high-incidence countries such as Thailand, the situation may be different. Although clustering still validly reflects the epidemiological relationships, transmission may be recent or remote. In this study the clustered isolates were frequently isolated from different provinces within a single year. The drug resistance patterns of the clustered pairs were usually different, suggesting that some clusters in this study did not reflect recent transmission between patients. It is possible that the period of transmission reflected by the clustering depends on several epidemiological parametres, such as the size of the area of study, the period of sample collection, the incidence of TB and the rate of migration. Therefore, the level of recent transmission in a nationwide study in a high-incidence country such Thailand should be inferred with caution. Acknowledgements This work was supported by the Department of Disease Control and the Thailand Tropical Research Programme. We thank Ms Arunee Thong-On and Ms Wiyada Arjratanakool for their technical support.
References 1 van Embden J D, Cave M D, Crawford J T, et al. Strain identification of Mycobacterium tuberculosis by DNA fingerprinting: recommendations for a standardized methodology. J Clin Microbiol 1993; 31: 406–409. 2 Palittapongarnpim P, Luangsook P, Tansuphaswadikul S, Chuchottaworn C, Prachaktam R, Sathapatayavongs B. Restriction fragment length polymorphism study of Mycobacterium tuberculosis in Thailand using IS6110 as probe. Int J Tuberc Lung Dis 1997; 1: 370–376. 3 Mongkoljit S, Saringearingul H, Smittipat N, Thong-On A, Palittapongarnpim P. Evidence for the transmission of tuberculosis in a prison in Nakhon Ratchasima, Thailand. Int J Tuberc Lung Dis 2001; 5: 586–587. 4 Sretrirutchai S, Silapapojakul K, Palittapongarnpim P, Phongdara A, Vuddhakul V. Tuberculosis in Thai prisons: magnitude, transmission and drug susceptibility. Int J Tuberc Lung Dis 2002; 6: 208–214. 5 Kato-Maeda M, Small P M. How molecular epidemiology has changed what we know about tuberculosis. West J Med 2000; 172: 256–259. 6 Pablos-Mendez A, Raviglione M C, Laszlo A, et al. Global surveillance for antituberculosis-drug resistance, 1994–1997. World Health Organization-International Union against Tuberculosis and Lung Disease Working Group on Anti-Tuberculosis Drug Resistance. N Engl J Med 1998; 338: 1641–1649. 7 Payanandana V, Rienthong D, Rienthong S, Ratanavichit L, Kim S J, Sawert H. Surveillance for antituberculosis drug resistance in Thailand: results from a national survey. Thai J Tuberc Lung Dis 2000; 21: 1–8. 8 The WHO/IUATLD Global Project on Anti-tuberculosis Drug Resistance Surveillance. Anti-tuberculosis drug resistance in the world. Report No 2. Prevalence and trends. Geneva, Switzerland: WHO, 2000. 9 van Soolingen D, Qian L, de Haas P E, et al. Predominance of a single genotype of Mycobacterium tuberculosis in countries of east Asia. J Clin Microbiol 1995; 33: 3234–3238. 10 McHugh T D, Gillespie S H. Nonrandom association of IS6110 and Mycobacterium tuberculosis: implications for molecular epidemiological studies. J Clin Microbiol 1998; 36: 1410–1413.
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11 Dale J W, Al-Ghusein H, Al-Hashmi S, et al. Evolutionary relationships among strains of Mycobacterium tuberculosis with few copies of IS6110. J Bacteriol 2003; 185: 2555–2562. 12 Anh D D, Borgdorff M W, Van L N, et al. Mycobacterium tuberculosis Beijing genotype emerging in Vietnam. Emerg Infect Dis 2000; 6: 302–305. 13 Borgdorff M W. Mycobacterium tuberculosis Beijing genotype, the Netherlands. Emerg Infect Dis 2003; 9: 1310–1313. 14 Dale J W, Nor R M, Ramayah S, Tang T H, Zainuddin Z F. Molecular epidemiology of tuberculosis in Malaysia. J Clin Microbiol 1999; 37: 1265–1268. 15 Bifani P J, Plikaytis B B, Kapur V, et al. Origin and interstate spread of a New York City multidrug-resistant Mycobacterium tuberculosis clone family. JAMA 1996; 275: 452–457. 16 Glynn J R, Whiteley J, Bifani P J, Kremer K, van Soolingen D. Worldwide occurrence of Beijing/W strains of Mycobacterium tuberculosis: a systematic review. Emerg Infect Dis 2002; 8: 843–849. 17 Toungoussova O S, Mariandyshev A, Bjune G, Sandven P, Caugant D A. Molecular epidemiology and drug resistance of Mycobacterium tuberculosis isolates in the Archangel prison in
18
19
20
21
22
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Russia: predominance of the W-Beijing clone family. Clin Infect Dis 2003; 37: 665–672. Tracevska T, Jansone I, Baumanis V, Marga O, Lillebaek T. Prevalence of Beijing genotype in Latvian multidrug-resistant Mycobacterium tuberculosis isolates. Int J Tuberc Lung Dis 2003; 7: 1097–1103. Kruuner A, Hoffner S E, Sillastu H, et al. Spread of drug-resistant pulmonary tuberculosis in Estonia. J Clin Microbiol 2001; 39: 3339–3345. Rad M E, Bifani P, Martin C, et al. Mutations in putative mutator genes of Mycobacterium tuberculosis strains of the W-Beijing family. Emerg Infect Dis 2003; 9: 838–845. Werngren J, Hoffner S E. Drug-susceptible Mycobacterium tuberculosis Beijing genotype does not develop mutationconferred resistance to rifampin at an elevated rate. J Clin Microbiol 2003; 41: 1520–1524. Alland D, Kalkut G E, Moss A R, et al. Transmission of tuberculosis in New York City. An analysis by DNA fingerprinting and conventional epidemiologic methods. N Engl J Med 1994; 330: 1710–1716.
RÉSUMÉ
Au cours des années 1997–1998, une surveillance nationale de la résistance aux médicaments antituberculeux a été menée en Thaïlande au sein d’un projet mondial. O B J E C T I F : Evaluer les types d’hybridation IS6110 ainsi que le niveau de regroupement en grappes que l’on s’attendait à être faible vu la courte période de recueil des échantillons. S C H É M A : On a disposé de 828 isolats bactériens pour recherche d’empreintes digitales par hybridation standard IS6110. R É S U L T A T S : Les types de polymorphisme de la taille des fragments de restriction ont varié selon les zones géographiques, l’âge des patients, la résistance à la rifampicine et à la streptomycine. La souche Beijing s’est avéré plus courante parmi les patients les plus jeunes et leur prévalence semble décroître avec l’éloignement de Bangkok, CONTEXTE :
alors que l’inverse s’avère vrai pour les isolats à bande unique. Si l’on exclut les isolats contenant cinq copies ou moins de IS6110, il y a regroupement en grappes dans 26,4% des cas. Le regroupement en grappes est plus courant parmi les femmes. Les isolats en grappes proviennent parfois de provinces différentes, et lorsqu’ils sont résistants aux médicaments, possèdent habituellement des profils différents de résistance. C O N C L U S I O N S : Ces résultats posent la question de savoir si on peut valablement conclure à une transmission récente lorsque l’on observe un regroupement en grappes des types d’hybridation IS6110 dans certains contextes de Thaïlande. Le niveau de transmission récente dans une étude au niveau national dans un pays où l’incidence de la tuberculose est élevée devrait être évalué avec prudence.
RESUMEN
Durante 1997 y 1998 se llevó a cabo en Tailandia un estudio nacional de vigilancia de la resistencia a los medicamentos antituberculosos, como parte de un proyecto mundial. O B J E T I V O : Evaluar los perfiles de hibridación con la secuencia IS6110 y la frecuencia de conglomerados, la cual se esperaba baja dada la corta duración de la recogida de muestras. M É T O D O : Se estudió la huella genética de 828 aislados clínicos de micobacterias mediante la hibridación estándar con la secuencia IS6110. R E S U L T A D O S : Los perfiles del polimorfismo de longitud de los fragmentos de restricción variaron dependiendo de la localización geográfica, de la edad de los pacientes y de la resistencia a rifampicina y a estreptomicina. La cepa ‘Beijing’ fue más frecuente en los pacientes más jóvenes y su prevalencia pareció disminuir a mayor MARCO DE REFERENCIA :
distancia de Bangkok ; el caso contrario se observó para los aislados con una sola banda. Excluyendo los aislados micobacterianos que contenían cinco copias de IS6110 o menos, el 26,4% de aislados se agrupó en conglomerados. Los conglomerados fueron más frecuentes en las mujeres. Los aislados clínicos de un conglomerado provenían en ocasiones de diferentes provincias y cuando presentaban farmacorresistencia su patrón de resistencia era diferente. C O N C L U S I Ó N E S : Estos resultados cuestionan la validez de la asociación entre transmisión reciente y la presencia de conglomerados en los perfiles de hibridación con IS6110 en algunos lugares en Tailandia. El nivel de transmisión reciente mediante un estudio de carácter nacional debe ser evaluado muy cuidadosamente en países con alta incidencia de tuberculosis.
