Antimicrobial and Antibiofilm Effects of Bithionol against Mycobacterium abscessus

Abstract

Mycobacterium abscessus (M. abscessus) is a multidrug-resistant nontuberculous mycobacterium (NTM) that is responsible for a wide spectrum of infections in humans. The lack of effective bactericidal drugs and the formation of biofilm make its clinical treatment very difficult. The FDA-approved drug library containing 3048 marketed and pharmacopeial drugs or compounds was screened at 20 μM against M. abscessus type strain 19977 in 7H9 medium, and 62 hits with potential antimicrobial activity against M. abscessus were identified. Among them, bithionol, a clinically approved antiparasitic agent, showed excellent antibacterial activity and inhibited the growth of three different subtypes of M. abscessus from 0.625 μM to 2.5 μM. We confirmed the bactericidal activity of bithionol by the MBC/MIC ratio being ≤4 and the time-kill curve study and also electron microscopy study. Interestingly, it was found that at 128 μg/mL, bithionol could completely eliminate biofilms after 48h, demonstrating an outstanding antibiofilm capability compared to commonly used antibiotics. Additionally, bithionol could eliminate 99.9% of biofilm bacteria at 64 μg/mL, 99% at 32 μg/mL, and 90% at 16 μg/mL. Therefore, bithionol may be a potential candidate for the treatment of M. abscessus infections due to its significant antimicrobial and antibiofilm activities.

Keywords: Mycobacterium abscessus; antimicrobial activity; biofilm; bithionol; drug screening.

Figure 1

Time–kill curves of (A) bithionol and (B) clarithromycin against M. abscessus 19977 in 7H9 medium supplemented with 1% glucose at different drug concentrations. Drug concentrations are indicated by different symbols.

Figure 2

Determination of antibiofilm activity of bithionol. M. abscessus 19977 was cultured in Sauton’s medium in a 24-well plate at 37 °C for 6 days and biofilms formed on the liquid–air interface. Clarithromycin, amikacin, moxifloxacin, and bithionol at final concentrations of 8, 16, 32, 64, and 128 μg/mL were added to wells and incubated at 37 °C for 48h. Then, the biofilms were collected and CFU counts were determined.

Figure 3

SEM images of bithionol treatment of M. abscessus. (A) M. abscessus control group treated with PBS; (B) M. abscessus bacteria treated with bithionol at 2 μg/mL for 24 h. The bars of the left panel indicate 5 μm, while the bars of the right panel indicate 1 μm.