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Tiny fatty droplets in our cells are part of the immune system and help fight off bacterial infections. Until now the droplets were thought to be among the most vulnerable parts of the cell.

Lipid droplets are found in the cells of all complex organisms. They store fats and other lipids, which are essential nutrients. In humans, specialised cells called adipocytes store body fat in the form of lipid droplets.

For many years, biologists thought lipid droplets were “just an inert structure, just a storage site”, says Robert Parton at the University of Queensland in Australia. But in fact they also contain proteins, which carry out a wide range of functions. “We now have whole conferences looking just at the lipid droplets and all the associated processes.”

When a bacterium infects an animal cell, it often feeds on the lipid droplets. “It’s a nice source of fat inside the cell,” says Parton.

But it seems animal cells have found a way to turn the tables. “In retrospect, it sort of makes sense that in millions of years of battles between us and pathogens, the cell has also responded,” says Parton.

In a series of experiments on mice and on human cells, Parton’s team found that lipid droplets carry an array of proteins that are involved in the immune response. When dangerous bacteria enter the cell, chemical alarm signals are released, and these activate the immune proteins on the lipid droplets – which kill any bacteria that approach the droplet.

The cell has taken one of its most vulnerable components and weaponised it, says Parton. “It’s using it like a honey trap,” he says. “It’s producing these proteins, putting them on the lipid droplets, and then killing the bacteria.”

The evidence so far only shows that the lipid droplets can fight bacteria – there is no sign of them fighting viruses.

Furthermore, some bacteria can evade the lipid droplet attacks, and these are often pathogens we are familiar with. “We had Salmonella in there, and there was no killing as far as we could tell by this mechanism,” says Parton. It will be crucial to find out which bacteria can survive, and how they do so.

In the long run, Parton hopes to harness the immune activity of the lipid droplets to help treat infectious diseases.

“The ultimate is to be able to use this knowledge to combat the resistance of bacteria, and have it as part of our arsenal,” he says.