High Level of Cross-Resistance between Kanamycin,Amikacin, and Capreomycin among Mycobacterium tuberculosis Isolates from Georgia and a Close Relation with Mutations in the rrs Gene

The aminoglycosides kanamycin and amikacin and the macrocyclic peptide capreomycin are key drugs for the treatment of multidrug-resistant tuberculosis (MDR-TB). The increasing rates of resistance to these drugs and the possible cross-resistance between them are concerns for MDR-TB therapy. Mutations in the 16S rRNA gene (rrs) have been associated with resistance to each of the drugs, and mutations of the tlyA gene, which encodes a putative rRNA methyltransferase, are thought to confer capreomycin resistance in Mycobacterium tuberculosis bacteria. Studies of possible cross-resistance have shown variable results. In this study, the MICs of these drugs for 145 clinical isolates from Georgia and the sequences of the rrs and tlyA genes of the isolates were determined. Of 78 kanamycin-resistant strains, 9 (11.5%) were susceptible to amikacin and 16 (20.5%) were susceptible to capreomycin. Four strains were resistant to capreomycin but were susceptible to the other drugs, whereas all amikacin-resistant isolates were resistant to kanamycin. Sequencing revealed six types of mutations in the rrs gene (A514C, C517T, A1401G, C1402T, C1443G, T1521C) but no mutations in the tlyA gene. The A514C, C517T, C1443G, and T1521C mutations showed no association with resistance to any of the drugs. The A1401G and C1402T mutations were observed in 65 kanamycin-resistant isolates and the 4 capreomycin-resistant isolates, respectively, whereas none of the susceptible isolates showed either of those mutations. The four mutants with the C1402T mutations showed high levels of resistance to capreomycin but no resistance to kanamycin and amikacin. Detection of the A1401G mutation appeared to be 100% specific for the detection of resistance to kanamycin and amikacin, while the sensitivities reached 85.9% and 94.2%, respectively.Although the first-line anti-tuberculosis (anti-TB) drugs rifampin (RMP; rifampicin), isoniazid (INH), ethambutol (EMB), pyrazinamide (PZA), and streptomycin (SM) were discovered several decades ago, they are still used today in standard short-course regimens for the treatment of TB. These regimens are, however, ineffective for the treatment of multidrug-resistant (MDR) TB (defined as resistance to at least the two most powerful anti-TB drugs, RMP and INH), leading to the use of less effective and more toxic second-line drugs (SLDs). Injectable drugs such as kanamycin (KAN), amikacin (AMK), and capreomycin (CAP) are the key SLDs for the treatment of MDR-TB (17). The emergence of extensively drug-resistant TB, defined as MDR-TB with additional resistance to any fluoroquinolone and at least one of the injectable drugs (10), once again underlines the importance of fast and reliable testing for susceptibility to these antibiotics.Mutations in the 3′ part of the 16S rRNA gene (rrs), particularly at positions 1401, 1402, and 1484 (1, 7, 11, 12), have been associated with resistance to each of the drugs. It has also been suggested that mutations in the tlyA gene are responsible for resistance to CAP (8). Additionally, reports of cross-resistance among various aminoglycosides and CAP have been variable (1, 4, 6, 16). Most of the previous investigations were done with laboratory-generated mutants and with only a limited number of clinical isolates. In this work, we investigated the correlation between mutations in the rrs and tlyA genes and the in vitro resistance to the three injectable drugs of clinical Mycobacterium tuberculosis isolates.