A recent study by researchers from the University of Basel show that the bacteria responsible for causing the infectious disease tularemia use “nano-spearguns” to free themselves after being imprisoned by cells from the host’s immune system.
Tularemia is a highly infectious disease that chiefly affects rabbits and rodents, but can attack certain other mammals as well, including humans. It is caused by the bacterium Francisella tularensis and can spread to humans not just as an airborne infection but also through intermediary hosts such as bloodsucking ticks and fleas. Tularemia is a serious disease that can have a fatal outcome.
“The mortality rate can be as high as thirty percent,” says Professor Petr Broz from the Biozentrum of the University of Basel. “Inhaling as little as a dozen Francisella bacteria is sufficient to get infected.”
Since F. tularensis can spread through the air, it could be used as a biological weapon.
The bacterium with the recyclable speargun
The high mortality rate for tularemia is connected to the bacterium’s ability to combat the hosts immune system.
When a team of infection biologists – including Professor Marek Basler and Professor Petr Broz – studied a close relative of F. tularensis, they found that it used a “nano-speargun” to escape from the phagocytes sent out by the host’s immune system and that it could use components from a used speargun to assemble a new speargun.
It is the type VI secretion system (T6SS) that works like a speargun for the bacterium. The phagocytes “ingest” the bacteria and enclose them in small vesicles before digesting them. To avoid this destiny, the bacteria in the study used T6SS to disrupt the digestive vesicles and escape.
Interestingly, the bacterium was then capable of recycling its speargun.
“After firing the speargun, it is immediately disassembled into its individual components. The bacterium then uses these components again to assemble a new T6SS,” says Professor Basler. “With this weapon, the bacteria puncture the vesicle membrane, in which they are enclosed, and deliver toxic proteins into the cytosol of the immune cell.”
The study has been published in Nature Communications.
Maj Brodmann et al, Francisella requires dynamic type VI secretion system and ClpB to deliver effectors for phagosomal escape, Nature Communications (2017). DOI: 10.1038/ncomms15853