By contrast, the contribution of rpoB carrying Q513L mutation Erlotinib clinical trial to RMP-resistance was not that evident. The insertion of this gene into an M. tuberculosis H37Ra laboratory strain did not result in a significant level of RMP-resistance, however the insertion of the same gene was responsible for resistance to RMP of two M. tuberculosis clinical strains (MIC 12.5 and 50 μg/ml) when used as hosts. As identified in various clinical studies, the level of RMP-resistance of M. tuberculosis isolates carrying the Q513L mutation varies from 2 to 200 μg/ml [14, 20, 21, 23, 38]. The collected results suggest that rpoB

carrying Q513L mutation is able to cause resistance to RMP only in selected tubercle bacilli. It is likely that this mutation can result in RMP-resistance

in strains with low cell wall permeability since this exclusion barrier is responsible for natural resistance of some MAIC strains [26, 27]. We also cannot exclude the possibility that other mechanisms support RMP-resistance of strains carrying Q513L mutation. The drug resistance of M. tuberculosis can be also connected to the overproduction of a drug target due to accumulation of point mutations in a promoter region [40–42]. To test whether overproduction of rpoB carrying a given mutation result in higher MIC for RMP compared to a strain expressing the same gene under control of the natural promoter, rpoB genes were cloned under control of the P hsp promoter and introduced into M. tuberculosis host. The P hsp promoter, commonly used in genetics studies of mycobacteria controlling the groEL gene (Rv0440) in M. tuberculosis, has already been Navitoclax clinical trial reported as highly active in mycobacterial cells growing in vitro [24, 25]. A recent microarray study showed that the expression level

of groEL in M. tuberculosis cells growing in log phase is high, but not higher than rpoB [43]. However, the arresting of M. tuberculosis growth results in 3.6-fold induction of groEL with a decrease of rpoB expression in the same conditions [44]. We have not observed higher RMP resistance next when mutated rpoB genes were expressed under control of P hsp promoter in comparison to the natural promoter. It is possible that the natural level of RpoB is high enough to saturate RMP (if its concentration in cell is low). On the other hand, the extra expression of rpoB cannot help in cells accumulating high RMP level. However, to elucidate this problem an alternative expression system and precise control of protein expression would be required. The natural resistance to RMP in some M. avium and M. intracellulare strains is known to be as a result of an efficient cell wall permeability and exclusion barrier [26, 27], suggesting that these elements may be also important in M. tuberculosis. Changes in cell wall composition could affect permeability [45] decreasing the intracellular concentration of drug.