Listeriolysin O - Pore formin' infection enabler

We've got a bit of a listeriosis outbreak on our hands up here in Canada right now. Three (elderly) people dead, over a dozen confirmed cases across the country. Given Ontario's past history with foodborne bacterial infections, this is big news. Health inspectors have apparently found Listeria monocytogenes, the gram-positive bacterium responsible for listeriosis, in a Toronto meat plant, but have not as of yet determined if the strain matches that which is responsible for the outbreak.

Based on what I've read (Wikipedia, obviously) listeriosis is fairly uncommon and largely associated with infants, old people, and those with compromised immune systems (due to HIV infection, being on certain medications, diabetes, etc.). It's actually more common in animals, particularly domesticated ruminants. Wikipedia also points out that although L. monocytogenes was first described in 1926, it wasn't identified as a cause of foodborne disease until 1981 (in Halifax, Nova Scotia, of all places). Listeriosis often presents as a flu-like illness (fever, puking, feeling like you're gonna puke, and getting the runs) that eventually subsides or else worsens into such fun things as septicemia (blood poisoning), meningitis/encephalitis (brain inflammation), corneal (eye) ulcer, or pneumonia. Spontaneous abortion or stillbirth may occur in pregnant women.

Anywho, let's try and bring some toxicology into the mix, shall we? Heck, it's actually microbiology than anything, but we're going to be talking about a toxin. Listeriolysin O (LLO) is a protein toxin secreted by L. monocytogenes that helps the bacterium get inside cells, where it can reproduce like crazy while remaining hidden from the the immune system. The entire process starts with the bacterium tricking the cell into engulfing it into a container called a phagosome. At this point, LLO creates a pore (hole) in the phagosome, permitting the bacteria to escape its container into the cytoplasm of the cell, where it can start dividing like a mofo.

Here's the really cool part. LLO is activated only under the more acidic conditions it encounters within a phagosome, so once it forms a pore and spills out along with the bacterium into the more basic cytoplasm, it's activity is reduced. This means that LLO won't form pores in the plasma (outer) membrane of the cell, which would likely kill it, thus ensuring the maintenance of a nice little cell incubator for the bacterium to multiply in.

- http://en.wikipedia.org/wiki/Listeriolysin_O