But fish don’t have lungs…?

Tuberculosis is the world’s oldest and most deadly disease. It’s caused primarily by Mycobacterium tuberculosis, which is commonly known as the tubercle bacillus, or TB. Despite the advent of antibiotics nearly 100 years ago, an estimated two billion people are infected with TB and tuberculosis kills around two million people every year. Drug tolerance in TB, after it has infected humans, appears to be key to its success.

Now it seems we are beginning to understand how this works. In a recent paper published in the journal Cell, Lalita Ramakrishnan and colleagues report that Mycobacterium marinum bacteria infecting zebrafish embryos respond to antibiotic treatment in an exactly the same way as M. tuberculosis bacteria infecting humans. Because M. marinum is 99% identical to TB genetically and infects transparent zebrafish embryos, it is an excellent model for studying tuberculosis. Tracking the M. marinum bacteria during infection and antibiotic therapy of the zebrafish shows that not all of the bacteria are killed by antibiotic treatment; some acquire drug tolerance and hide inside immune cells known as macrophages, then use these immune cells to spread around the body.

A zebrafish embryo infected with fluorescent M. marinum bacteria (highlighted with an arrow)