Boromycin Kills Mycobacterial Persisters without Detectable Resistance

Area of Science:

• Microbiology
• Antibiotic Resistance
• Drug Discovery

Background:

• Boromycin, a polyether macrolide, is active against Gram-positive bacteria and acts as a potassium ionophore.
• Gram-negative bacteria's outer membrane hinders boromycin's access to the cytoplasmic membrane.
• Mycobacterium tuberculosis also possesses an outer membrane, posing a question about boromycin's efficacy.

Purpose of the Study:

• To investigate the activity of boromycin against Mycobacterium tuberculosis.
• To determine if boromycin can overcome drug tolerance in mycobacterial persister cells.
• To assess the potential of targeting mycobacterial ion gradients for chemotherapy.

Main Methods:

• Determined the minimum inhibitory concentration (MIC50) of boromycin against M. tuberculosis.
• Assessed bactericidal activity against growing and non-growing persister bacilli.
• Measured membrane potential, intracellular ATP levels, and cytoplasmic protein leakage upon boromycin exposure.
• Investigated the effect of KCl on boromycin's antimycobacterial activity.
• Evaluated cytotoxicity (CC50) and hemolytic activity (HC50).
• Assessed spontaneous resistant mutant frequency.

Main Results:

• Boromycin demonstrated potent inhibition of M. tuberculosis growth (MIC50 = 80 nM).
• It exhibited strong bactericidal activity against both growing and drug-tolerant persister bacilli.
• Boromycin caused rapid loss of membrane potential, reduced ATP levels, and cytoplasmic protein leakage.
• Antimycobacterial activity was blocked by KCl, confirming its role as a potassium ionophore.
• Boromycin displayed low cytotoxicity and hemolytic activity, with a high selectivity index (>300).
• Spontaneous resistant mutants were not isolated, suggesting a very low mutation frequency (<10(-9)/CFU).

Conclusions:

• Boromycin is a potent antimycobacterial agent effective against drug-tolerant persister cells.
• Targeting mycobacterial transmembrane ion gradients represents a promising chemotherapeutic strategy.
• This approach may be valuable for eliminating persister bacilli and slowing antibiotic resistance development.