**7. Concluding remarks**

Over the last decades there has been a substantial increase in our understanding of the molecular bases of *M. tuberculosis* biology and its interaction with the host. However, the clinical and epidemiology consequences of *M. tuberculosis* infection are still poorly understood. Despite the restricted variability and clonality of the MTBC population, various studies make evident that circulating strains vary in terms of their genomic makeup and differ with respect to virulence and immunogenicity properties. The differences observed in the interactions between pathogen and host and in disease manifestation indicate that variation must play a role in disease and in clinical outcome, even though the extent of the impact of this strain diversity is still unclear (Coscolla & Gagneux, 2010). Thus, the precise role of bacterial factors and the importance of strain diversity in pathogenicity and tuberculosis disease remain elusive, partly due to the complex interplay between host and pathogen that is compounded by additional environmental factors. The strain-to-strain variation also has important consequences for the development of efficient control strategies. The development of new diagnostics tools, drugs and vaccines must somehow incorporate analysis of the differences that characterize host responses and strains, highlighting the importance of continued studies regarding the genetic makeup of circulating strains. The use of modern genetic and molecular tools, including the availability of massive sequencing techniques, can contribute significantly to our understanding of *M. tuberculosis* variability and its possible association with biological properties. Only by realizing the need to incorporate this added level of complexity to the study of tuberculosis, will we be able to tackle the intricacies of this disease and achieve an adequate level of control on a global scale.

#### **8. Acknowledgment**

Support was obtained from the CCITB, Colciencias and the StopLATENT-TB network (Collaborative Project) supported by the EC under the Health Cooperation Work Programme of the 7th Framework Programme (G.A. no. 200999) (http://cordis.europa.eu/fp7/dc/index.cfm).

#### **9. References**

50 Understanding Tuberculosis – Deciphering the Secret Life of the Bacilli

(Kaufmann, 2010). New and alternative drugs are also required to shorten the current duration of chemotherapy, to act against persistent bacilli and to counteract the spread of drug-resistant strains that frustrate global eradication programs. Due to renewed efforts in recent years, several novel drugs have been identified and are under clinical evaluation or being developed, many of which involve novel targets and mechanisms (Coxon & Dover, 2011). The discovery of novel drugs has involved different approaches that include the use of genomics to identify targets, whole-cell screening and re-engineering of known chemical molecules (Koul *et al.*, 2011). Given the observed strain variability it is nonetheless possible that some of these drugs might vary in efficiency in different strain backgrounds, as was made evident for DGC and Cyp121 in the Haarlem lineage (Cubillos-Ruiz et al., 2010). Thus the heterogeneity among different strains and lineages, as well as of the host-pathogen interaction, must be taken into account when developing novel diagnostics and therapeutic strategies. Extensive analysis of circulating *M. tuberculosis* populations will be required to address the efficacy of treatment and vaccination in different genetic backgrounds. The advent of novel massive sequencing techniques to generate genomic data for multiple strains will undoubtedly allow examination of whole genomes and make such analyses

Over the last decades there has been a substantial increase in our understanding of the molecular bases of *M. tuberculosis* biology and its interaction with the host. However, the clinical and epidemiology consequences of *M. tuberculosis* infection are still poorly understood. Despite the restricted variability and clonality of the MTBC population, various studies make evident that circulating strains vary in terms of their genomic makeup and differ with respect to virulence and immunogenicity properties. The differences observed in the interactions between pathogen and host and in disease manifestation indicate that variation must play a role in disease and in clinical outcome, even though the extent of the impact of this strain diversity is still unclear (Coscolla & Gagneux, 2010). Thus, the precise role of bacterial factors and the importance of strain diversity in pathogenicity and tuberculosis disease remain elusive, partly due to the complex interplay between host and pathogen that is compounded by additional environmental factors. The strain-to-strain variation also has important consequences for the development of efficient control strategies. The development of new diagnostics tools, drugs and vaccines must somehow incorporate analysis of the differences that characterize host responses and strains, highlighting the importance of continued studies regarding the genetic makeup of circulating strains. The use of modern genetic and molecular tools, including the availability of massive sequencing techniques, can contribute significantly to our understanding of *M. tuberculosis* variability and its possible association with biological properties. Only by realizing the need to incorporate this added level of complexity to the study of tuberculosis, will we be able to tackle the intricacies of this disease and achieve an

Support was obtained from the CCITB, Colciencias and the StopLATENT-TB network (Collaborative Project) supported by the EC under the Health Cooperation

more feasible (Lin & Ottenhoff, 2008).

adequate level of control on a global scale.

**8. Acknowledgment** 

**7. Concluding remarks** 


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**3** 

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**IS***6110* **the Double-Edged Passenger** 

**1. Introduction** 

**1.2 ISs families** 

1992).

elements (Gordon et al., 1999, Table 1).

**1.1 Insertion sequences in** *Mycobacterium tuberculosis* **complex** 

Insertion sequence (IS) is a short DNA mobile genetic element coding for proteins involved in the transposition activity, which allows it to spread within the genome. ISs are widely distributed in prokaryotes and can be grouped into different families established by Mahillon & Chandler (1998) based on structural characteristics and transposase similarities. In the genus *Mycobacterium* have been located and identified more than 46 ISs from different species, mostly on the basis of sequence similarities (Brosch et al., 2000). In the genome of the members of the *Mycobacterium tuberculosis* complex (MTBC) has been possible to find dispersed IS elements that could be included in various of the following families attending to their characteristics: IS*3*, IS*5*, IS*21*, IS*30*, IS*110*, IS*256*; IS*1535*, IS*L3* and other IS-*like*

The ISs can induce duplications, deletions, and rearrangements in the bacteria genome, all of them essentials changes for the genome plasticity of the members of MTBC (Mahillon & Chandler, 1998). Not all of the ISs described in *M. tuberculosis* are active and have the availability of transpose from one site to another in the genome, some of the elements are defective copies. Furthermore, some of them have a limited host range (Brosch et al., 2000). The Table 1 shows the ISs described in *M. tuberculosis* that will be briefly presented below.

The IS*3* family represents an extensive set of insertion elements in bacteria. The features that characterize this family are their length between 1200 and 1600 bp, and their inverted repeats (IRs) between 20 and 40 bp long, as well as the presence of two overlapping open reading frames (ORFs: *orf*A and *orf*B) (Mahillon & Chandler, 1998; McAdam et al., 2000). After the insertion, a duplication of 3 or 4 bp occurs at the insertion point (Mendiola et al.,

María del Carmen Menéndez1, Sofía Samper2,4,

*1Autonoma University of Madrid, School of Medicine 2IIS Aragon, Hospital Universitario Miguel Servet 3University of Zaragoza, School of Medicine* 

Isabel Otal3,4 and María Jesús García1

*4CIBER Enfermedades Respiratorias* 

