She continued to improve clinically and was discharged to complete a four-week course of antibiotics. Incidentally, her CD4 count was measured to be 250. This bacterium grows well at refrigerator temperatures, and has been found in many foods, including raw vegetables, unpasteurized milks, soft cheeses and processed meats. There is growing evidence that outbreaks of listeriosis are due to food-borne transmission, as illustrated by a recent outbreak that has been traced to hot dogs and cold cuts that resulted in 70 cases of listerosis and 9 deaths. Listeria contamination has now been the cause of multiple food recalls and this experience emphasizes the need for food irradiation.
Listeria has an interesting intracellular lifecycle.
The bacterium is phagocytosed by epithelial cells and hepatocytes and is enclosed in a compartment called a phagolysosome. The acidic pH of this organelle triggers expression of listeriolysin O by Listeria , which lyses the phagolysosome and allows release of the bacteria into the cytoplasm. Once in the cytoplasm Listeria organizes the cell's actin filaments into a comet tail-like structure, which it uses to propel itself to the cell membrane. The bacteria then push against the cell membrane and form protrusions, called filopods, that can be ingested by adjacent cells. Two other pathogens, Shigella flexneri and rickettsia, seem to also hijack the cell's actin machinery in a manner similar to Listeria .