Listeriolysin O - Basically why listeriosis sucks so bad

Every so often, the narcissistic tendencies of Listeria monocytogenes get to be too much for this annoying Gram-positive bacterium to bear, and it decides to make the news by causing a listeriosis outbreak. Typically this involves a major food manufacturer having to recall a product that has become contaminated with the bug. The media loves bacterial narcissism.

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. It's actually more common in animals, particularly cows and stuff. 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), encephalitis/meningitis (brain/brain membrane inflammation), corneal (eye) ulcer (Google Image or bust!), or pneumonia.

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 [1]. The entire process starts with the bacterium tricking white blood cells into engulfing it into a container called a phagosome. At this point, LLO creates a 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 [1]. 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 Listeria to multiply in.

[1] Schnupf P, Portnoy DA. Listeriolysin O: a phagosome-specific lysin. (2007). Microbes Infect. 9(10):1176-1187.