Molecular typing methods are generally useful tools for resolving various issues related to the classical epidemiology of human pathogens, including M. tuberculosis
. Spoligotyping is definitely a “gold standard” method for detecting a Beijing family strain of M. tuberculosis
]. The Beijing genotype was identified for the first time in strains isolated in the Beijing area of China in 1995, giving rise to the name of the genotype [1
Beijing family strains having a characteristic spoligotype pattern (loci 1–34 are absent; loci 35–43 are present) have been identified in many areas. Moreover, in an M. tuberculosis
molecular epidemiological investigation in China, Beijing family strains were the most prevalent lineage in China (74.08%), based on the SpolDB4.0 spoligotype database and SpotClust results [25
]. In this study 211 strains were Beijing family strains, accounting for 81% of all isolates examined.
Interestingly, in the first molecular epidemiology study in Beijing city, 89.4% of the M. tuberculosis
strains collected from 1992 to 1994 were identified as Beijing family strains [1
], and 85.1% of strains collected in Beijing from 2002 to 2005 were also identified as Beijing family strains [24
]. Is this study of isolates collected in 2009, 81% of the strains were Beijing family strains. Since there is no statistically significant difference between these three percentages, as determined using the X2
0.05) we conclude that the predominance of Beijing family strains has not altered for almost twenty years.
There is some debate over the use of RD181 for classifying isolates. An early study showed that RD181 can be used to divide Beijing isolates into ancestral (RD181 intact) and modern (RD181 deleted) strains [26
]. Faksri et al., however, suggested that RD181 alone does not provide sufficient discrimination to define ancestral or modern Beijing lineages, but with RD181 bing intact or deleted ancestral Beijing family strains can be divided into two subgroups [27
]. Results from this study are consistent with this report in that ancestral Beijing family strains could be divided into RD181 intact strains (9 strains) and RD181 deletion strains (28 strains).
During the process of evolution in Beijing family strains, numbers of VNTR repeats can increase or decrease. Changes in the numbers of repeats in VNTRs can differ with locus. It has been shown that expansion and contraction in the number of repeats of VNTRs can occur during the evolutionary process [28
]. Many previous studies have shown that VNTR loci can be good markers for phylogenetic estimation [29
]. In this study, we found that the number of Mtub21 repeats was different between ancient and modern Beijing family strains, with the number of repeats being much lower in non-Beijing family strains. In agreement with other reports, we conclude that Mtub21 is an evolutionarily informative VNTR locus [30
Many previous studies have suggested that VNTR loci vary in their ability to discriminate Beijing genotype strains from geographically distant areas [11
). Some highly polymorphic VNTR loci have different powers of discrimination among strains from different regions; for example the discriminatory power of ETRE among strains from Jiangsu [22
] and MIRU26 among strains from Shanghai [23
] is much greater than among strains from Beijing. We compared the discrimination power of VNTR loci in this study with those in a previous study on Beijing strains published four years ago [24
]. Jiao et al. typed 72 Beijing family strains isolated in Beijing city from 2002 to 2005 using 24 VNTR loci. It is interesting that the discrimination power and alleles of some loci were different from our results. The differences are likely due to the different populations in the distinct geographic areas where the M. tuberculosis
strains were isolated or to differences in sampling times.
This study suffers from one main limitation. Our study based on a comparison of strains from Beijing, Shanghai and Jiangsu provinces suggests that VNTR results for M. tuberculosis strains differ according to geographic region. However, more data from other provinces is needed to precisely identify the nature of differences between different regions of China.