Mycobacteria, a group of pathogenic bacteria that cause diseases like leprosy and tuberculosis in humans, are associated with red blood cells at lung infection sites.

M. tuberculosis and other mycobacteria implicated in lung disease are now known to live in macrophages - white blood cells that engulf and kill pathogens. Mycobacteria turn up in blood and sputum coughed up by sick patients.

Red blood cells, although also found in the sputum of tuberculosis patients, have not yet been specifically studied in disease progression. But, "our research will change the conventional common sense that mycobacteria grow intracellularly", said Yukiko Nishiuchi, Associate Professor at Hiroshima University in Japan.

The team obtained lung tissue samples from a human patient infected with Mycobacterium avium subsp. hominissuis (MAH). Microscopic examination revealed red blood cells were co-located with mycobacteria in both the capillary vessels and granulomas (clumps of immune cells) in human lung tissues.

To assess the relationship of the mycobacteria to human red blood cells, the team monitored their growth with and without the blood cells. They found that MAH grew more in the presence of red blood cells, multiplying at a rate dependent on blood cell concentration.

Their exponential growth was even faster than the growth of MAH inside macrophages - typically targeted as parasitic hosts by mycobacteria. The findings, published in the journal Microbiology Spectrum, showed that pathogenic mycobacteria attach to human red blood cells, then capitalise on the relationship to multiply.

Mycobacteria had previously been found outside macrophages at infection sites; the new findings suggest that the presence of those extracellular mycobacteria may be a result of the relationship with red blood cells.

While red blood cells are best known for their role in transporting oxygen between lungs and tissues, they also play two roles in mycobacterial infections. They play a defensive role against infections by capturing pathogens and transferring them to macrophages in the liver and spleen to eliminate them.

Nishiuchi said the study shows that red blood cells may also get targeted as host cells for mycobacteria. However, how these roles play out might determine the outcome of an infection.

If the red blood cells' defence role is going well, the TB or other mycobacterial disease is controlled. But red blood cells overwhelmed by an attack of mycobacteria may help spread them throughout the body, Nishiuchi noted.