by Sarah Scoles

Francisella in General

The bacteria named Francisella was first identified in 1911, when it was ounfd in some ground squirrels in California who were exhibiting plague-like symptoms.

Animals typically do not harbor and then distribute Francisella to people or each other, though. The reservoirs for the bacteria are most likely freshwater and their populations of amoebae. People who become ill because of Francisella cannot spread bacteria to each other, so most people who contract the bacteria do so because of bug bites, licking the dead bodies of animals that died of Francisella, breathing in aerosolized bacteria, or swallowing contaminated food or water. Though the chances of your coming down with and dying of Francisella are small, a person need only breathe in 10 bacteria and that person could end up dead.

Thus, it's important to know what makes Francisella such an effective germ, or what gives it such virulence, as we say "in the industry." That's part of what my co-author, Brooke, and her labmates spend their days doing at the Emory Vaccine Center. Today, I'm writing about their paper--"Subversion of Host Recognition and Defense Systems by Francisella spp." (Jones, Napier, Sampson, et al., 2012)--and about what makes Francisella so effective in ruining the lives of ground squirrels everywhere.

What does Francisella have to do to infect a human?

The bacteria must first get into your body by first getting into your cells. Then it must get into cytosol (aka cytoplasm), or the mileu inside cells. Then it must replicate. Then it must replicate. Then it must replicate. Must. Replicate.

And during all that, the bacteria must avoid alerting the body to how there are a bunch of rogue bacteria inside it. Because if the body finds out, the body will send out the SWAT team (your immune system's responses), and why would Francisella want to deal with that when it doesn't have to?

And the point of this paper is that Francisella is so good at flying below the immune system's radar and taking down enemies one at a time (with a silencer) that it doesn't have to fight the whole horde.

Mixing metaphors?

There will be a lot of that in this post.

Francisella's tactics are divided up into those they employ while inside your body but still outside your cells (extracellular) and those they employ once they have infiltrated your cells (intracellular).

What are the extracellular obstacles, and how does Francisella overcome them?

Before Francisella can get inside a cell, where it can replicate, it has to beat two bosses: Complement and cationic microbial peptides (CAMPs). Everyone's worst nightmares.

Complement are a group of signaling proteins that alert the body upon detection of bacteria. Francisella, though, disarms these signaling molecules by splitting them up into their contituent fragments--fragments that have the molecular effect of actually helping Francisella enter the cell.

It's like a spy that cuts a security guard's arm off and then holds the severed hand up to the fingerprint-detecting door lock. That way, the alarm (the immune system) doesn't go off, and there are no more security guards (complement).

Cationic microbial peptides (CAMPs) are bad news for bacteria. But you may notice the first word of their name: cationic. It means that they are positively charged. Francisella, to evade these peptides, actually changes the charge of its surface, by moving ionic (charged) phosphate groups around. When its surface is positive, it repels the positively charged peptides. I think this is amazing and wish that I personally could change my surface charge.

For peptides that do get in (anionic or neutral ones), Francisella has pumps that can flush toxic compounds out.

What are intracellular obstacles, and how does Francisella overcome them?

Once Francisella knocks out its extracelluar enemies, and successfully keeps them quiet so that they do not call for any immune system back-up, they still have to get inside the cell. The cell actually engulfs them in "pseudopod loops" and draws them into the cell. Nom, nom.

And you thought you wouldn't hear the word 'pseudopod' after 10th grade (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1231130/bin/zii0090552330001.jpg)

Once Francisella is inside the museum, it still has to get to the room where the Van Gogh is and take Starry Night off the wall without setting off the alarm system.

In the cell, Francisella is first inside a phagosome, which is the membrane left from the pseudopod that drew it in: it is encircled and separated from the actual working parts of the cell. Before it can do its full job, Francisella must get to the cytosol, or the liquid filling the cell, which means it must escape the phagosome. In doing so, Francisella will encounter three difficulties, all meant to degrade and destroy the bacteria so that it cannot replicate and take over the host (you).

1. Phagosomal acidification: The phagosome lowers its pH, making the environment more acidic, so that bacteria cannot replicate. However, Francisella (BAM) generally escapes before acidification on the phagosome occurs.

2. Reactive oxygen species (ROS): These are molecules containing oxygen that want to react with other molecules. In reacting with, say, bacteria, they can damage the bacteria's structure. Francisella, however, blocks the assembly of these species, blocks the production of those that are assembled, and detoxifies them if they are produced. And then it wipes the blood from its hands.

3. Antimicrobial peptides: These molecules are made in the host to destroy bacteria; however, Francisella escapes the phagosome quickly to avoid these grenades or alters its membrane (as mentioned above) to avoid the attraction of cationic antimicrobial peptides (CAMPs).

4. Pattern recognition receptors (PRRs): Toll-like receptors (TLRs) are narks that alert the immune system. Francisella slinks stealthily past them because it has structural integrity that prevents its DNA from leaking out and alerting the authorities.

Well, once it gets past those problems in the phagosome and moves out into the cytosol, it's free and clear, right?

Celebration and rejoicing! Francisella is in the Van Gogh room! The laser alarm system hasn't gone off! TIME TO REPLICATE!

But three more things: Francisella has to make sure non-phagosomal PRRs, other cytoplasmic signaling molecules that tell the body to send out the immune troops, don't come on the scene; it must kill the cell; and it may induce autophagy (being encircled by a part of the cell again) in order to get out of the cell again and move to a new one.

But what about food?

This bacteria is a growing boy. In order to make more of itself and kill you, it has to get lots of nutrients. But the body knows what Francisella needs, and it doesn't want Francisella to have it. So it stops making as much. The body lowers its levels of iron and tryptophan, specifically, because that's what the bacteria want.

Francisella goes inside of cells to get its iron, and it makes its own tryptophan! Though this is energy-intensive, it's a solution. I personally wish I could make my own food.

The Point

Francisella has come up with novel ways not only to avoid being killed directly but also to avoid setting off the body's alarm bells. It systematically disables the alarms and slips silently into and out of the body's cells, slitting their throats and making more of itself, all without your immune system knowing that anything is going on. Because of its stealth mode, it is incredibly effective and virulent. Understanding why, though, allows microbiologists to investigate how to circumvent Francisella's efforts.

Thanks to Brooke for letting me write this post on a microbiology paper! It was a whole new world to realize how intricate and thorough bacteria can be about getting past our defenses. It's a scary world out there.

(To read about what it was like to read and then analyze a microbiology paper when I am a microbiology layperson, see this post.)

Jones CL, Napier BA, Sampson TR, Llewellyn AC, Schroeder MR, & Weiss DS (2012). Subversion of Host Recognition and Defense Systems by Francisella spp. Microbiology and molecular biology reviews : MMBR, 76 (2), 383-404 PMID: 22688817