**1. Introduction**

Worldwide, tuberculosis (TB) is a major public health problem. Despite a slight decrease in TB incidence rates in recent years (1.6% per year in the period 2000−2018 and 2.0% between 2017 and 2018), the situation remains extremely tense. A total of 1.5 million people died from the disease and more than 9 million new cases were detected in 2018 worldwide [1]. Currently, seven lineages of *Mycobacterium tuberculosis* are described, which can cause disease and demonstrate specific phylogeographic patterns [2]. Of them, lineage 2 and lineage 4 are the most widely dispersed, affecting humans across the world. Lineage 2 (or East Asian lineage) is arguably the most widespread

and the Beijing genotype family is its major component (13% of global *M. tuberculosis* population; predominant in East Asia and Northern Eurasia) [3,4].

Multiple clinical and epidemiological studies demonstrated a strict association of Beijing genotype members with a high level of drug resistance combined with a large number of compensatory mutations, as well as enhanced pathogenicity, which lead to increased transmissibility and rapid progression of infection [5–7]. However, virulence studies provide less conclusive results, showing a variety of phenotypes. The latter is confirmed in animal models [8,9] and in vitro models of macrophage infection [10,11]. This is due to the fact that hypervirulence is not a characteristic feature of the Beijing genotype, but is specific only for certain genetic sublineages, often associated with disease outbreaks in some regions [12,13]. One of the most detailed examples is the spread of the virulent *M. tuberculosis* Beijing B0/W148 cluster in the Russian territory [14].

Phylogenetically, members of lineage 2 may be assigned to at least two large branches, termed ancient and modern sublineages, and 11 populations belong to these branches [15]. Modern Beijing sublineage strains are prevalent worldwide, leading to speculation that this sublineage has hypervirulent features [16]. In the present study, we aimed to investigate a strain belonging to a more homogenous group within modern Beijing called Central Asia Outbreak (CAO) clade. This clade is a part of the Central Asian population. The latter was initially designated as CC1 or Central Asian [6] and then as East Europe 1 [17] and Central Asian and Russian [18]. It largely correlates with the 94-32 cluster and M2 subtype according to multilocus variable-number tandem-repeat analysis (MLVA) and mycobacterial interspersed repetitive unit (MIRU) typing, respectively [19,20]. The members of the population are characterized by a high level of drug resistance and comprise about one-fourth of the pathogen population in Uzbekistan, Tajikistan, Kyrgyzstan, and Kazakhstan [21–23]. Additionally, these strains distributed in Russia and other former Soviet Republics [7]. It should be noted, that this population is heterogeneous and includes at least two large clades: CladeA and the previously mentioned CAO. CladeA strains are prevalent in Russia. In turn, CAO isolates are not often identified in Russia and are usually associated with the spread of resistant TB forms in the former Soviet Central Asia [7,21].

In the current study, we examine genetic and phenotypic characteristics of the Rostov strain of *M. tuberculosis* belonging to the CAO clade of the Beijing genotype. This strain was attributed to the pre-extensively drug-resistant (XDR) tuberculosis group. The growth rate and virulence for mice of the Rostov strain were compared with the same characteristics of the *M. tuberculosis* H37Rv strain.
