Heating weakens drug-tolerant mycobacteria in biofilms and fish granulomas

Tuberculosis (TB) remains difficult and slow to treat, in part because the bacteria can enter drug-tolerant states and form protective biofilms that shield them from antibiotics. In a pioneering pilot study led by corresponding author Milka Marjut Hammarén, researchers asked whether controlled heat — hyperthermia — could make these tolerant bacterial communities easier to kill. To study this, the team focused on Mycobacterium marinum, a close relative of the bacterium that causes human TB and a model that forms TB-like lesions. Rather than working in whole patients, the investigators used laboratory and ex vivo approaches to keep the experiments tightly controlled. They applied a newly developed in vitro biofilm minimum duration for killing (MDK) tolerance assay to screen combinations of heat and drugs against bioluminescent mycobacteria growing in biofilms. The approach allowed the team to track how long bacteria survived under different conditions, and to identify heat and drug pairings that reduced the time needed to kill tolerant cells. By using both cultured biofilms and infected tissue from adult zebrafish, the study bridged simple lab models and more complex, TB-like lesions.

The core of the work combined the in vitro biofilm minimum duration for killing (MDK) tolerance assay with direct verification using standard plating methods. Researchers screened hyperthermal treatments of bioluminescent mycobacteria in biofilms and then tested promising conditions on Mycobacterium marinum recovered from granulomas isolated from adult zebrafish with chronic-stage infection. The decisive result reported was that the combination of hyperthermia at 47°C for 30 minutes and the antimicrobial drug rifampicin at 400 μg/ml produced positive effects against cultured Mycobacterium marinum biofilms and against M. marinum taken from zebrafish granulomas. The author summary also notes similar benefits when heat was paired with streptomycin against TB bacteria, indicating that the heat-plus-drug approach may work with more than one antibiotic. Throughout the study, standard plating methods were used to verify that the heat and drug combinations reduced viability of tolerant mycobacteria compared with drug alone.

These findings suggest a possible new tool to help conventional TB therapy: using controlled hyperthermia to sensitize drug-tolerant, biofilm-associated mycobacteria to antibiotics. Because heat damages cells in a general way rather than acting on a single molecular target, this approach might lower the chance that bacteria survive by developing specific drug-resistance mutations. The study’s use of both in vitro biofilm assays and ex vivo zebrafish granulomas strengthens the claim that heat can act on bacteria in structured, tissue-like environments as well as in lab-grown biofilms. At the same time, the work is framed as a pilot study and an early-stage proof of concept; it shows that hyperthermia can enhance the effect of rifampicin at 47°C for 30 minutes and that similar pairings with streptomycin may also help, but it does not itself establish clinical methods or safety. The authors present these results as intriguing evidence that controlled heating could be developed alongside standard drugs to kill stubborn, tolerant bacteria more quickly and reduce relapse.