Effect of ribosome-targeting antibiotics on streptomycin-resistant Mycobacterium mutants in the rpsL gene

Streptomycin (Sm) was the first antibiotic used against Mycobacterium tuberculosis, the aetiological agent of tuberculosis (TB). However, point mutations in the rpsL gene can generate resistance to Sm, which is why spontaneous resistance to this antibiotic emerges so rapidly during treatment. Here we examine the interaction between Sm resistance and sensitivity to other ribosome-targeting antibiotics. Levels of resistance of rpsL mutants to the ribosome-affecting antibiotics chloramphenicol, tetracycline, gentamicin and erythromycin were tested, both singly and in combination. For this purpose, Mycobacterium smegmatis was used, which is commonly used in laboratory experiments as a model for TB. Generally, Sm-resistant mutants were as sensitive to the ribosome-affecting antibiotics as the wild-type strain. Combinations of different ribosome-affecting antibiotics were occasionally more potent than either of the single drugs, with better inhibition of both wild-type and mutant strains. Combining different ribosome-affecting drugs could represent an additional strategy in treating mycobacterial infections, including those resistant to newer drugs such as isoniazid or ethambutol.