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Genetic takeover: How the bacteria behind Legionnaires’ disease use host cells

Scientists have used CRISPR-Cas9 screens to reveal more about how the bacteria behind Legionnaire's disease infects humans.

Any bacterium's ability to infect a person depends on its survival in the host (aka its victim). For Legionella pneumophila, which causes Legionnaires' disease -- a type of lung infection that spreads often though water systems in buildings -- commandeering host genes is one key to its survival.

"Legionella is not the most widespread or deadly pathogen," said Michael Bassik, PhD, assistant professor of genetics. "But what makes it interesting is that this bacterium harnesses many of the same biological tactics to infect its host as a number of more serious pathogens. It also has unique characteristics that mimic more serious defects in the infected cell which look like other diseases, such as neurodegeneration."

Bassik and a team of researchers have tapped into Legionella's mechanism of infection with the help of prominent gene-editing tool CRISPR-Cas9, uncovering new genetic clues -- from the host -- that reveal how the bacteria takes hold.

A paper detailing their findings appears in Cell Host & Microbe. Bassik and Shaeri Mukherjee, PhD, associate professor of microbiology and immunology at the University of California, San Francisco, share senior authorship.

Past Legionella research focused on the bacterial genes behind infection, but Bassik has flipped his approach, studying instead the human host genes that are involved in the infection process.

Specifically, Legionella infects macrophages, a type of white blood cell. Once the bacteria gets into the cell, it unleashes 300 to 400 proteins -- infectious worker bees sent to do the bacteria's bidding.

"These proteins rewire the host's cellular trafficking machinery in a very interesting way," said Bassik. "Normally when a macrophage engulfs something, such as bacteria, cancer cells or protein aggregates, it gets trafficked to a compartment in the cell called the lysosome, where it gets digested and cleared. But in this case, the Legionella bacteria prevent that trafficking and instead remodel the pathway and even uses part of the cell's machinery to form a protective capsule."

Inside that protective bubble, the bacteria replicate, grow, and eventually take over the cell and kill it. "And so we're interested in not only understanding how the bacteria do this, but what host genes are commandeered in order to allow it to happen -- especially given that these macrophages defense processes are involved in a wide range of diseases."

To answer that question, Bassik and the team of scientists used CRISPR to create something called a genetic screen. The idea behind the screen is to turn off select genes in the macrophage and test if Legionella can still infect the altered cell.

"With these genes effectively silenced, we're looking to see if the cell becomes resistant to Legionella infection or, conversely, more susceptible," said Bassik. "Ideally, we want to find things that protect against infection, since those could potentially be drug targets."

Using this set up, the researchers found a handful of host genes that weren't previously known to play a role in Legionella infection. There was even one, C1orf43, that scientists had yet to classify at all.

"It's always cool to be the first to find a function of a gene," said Bassik. "This gene had been annotated through the Human Genome Project, but no one knew what it did. Our screen showed that it helps regulate how bacteria are taken up into the cell."

There's a good chance, according to Bassik, that their findings are not only telling for Legionella, but more broadly for pathogenic bacteria and macrophage function. It will take more work to show that definitively, but for now, the study does demonstrate that these kinds of CRISPR screens can contribute new knowledge of the mechanisms of bacterial infection. The genes, Bassik said, could possible even point to new avenues for drug targets.

"When scientists look for new antibiotics, they're often targeting the bacteria itself," he said. "I don't want to overstate the medical relevance of what we've done -- this is really only the beginning -- but the results of our study suggest that it could be possible to target the genes of the host cell for therapeutic effects against bacteria. We're excited about that possibility."

Photo by Drew Hays

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