Encyclopedia of Ayurvedic Medicinal Plants

Kingdom:         Bacteria

Division:           Firmicutes

Class:   Bacilli

Order:  Bacillales

Family: Listeriaceae

Genus:  Listeria

                 Listeria is a bacterial genus containing six species. Named in honour of Joseph Lister, Listeria species are Gram positive bacilli and are typified by L. monocytogenes, the causative agent of Listeriosis.

          Listeria monocytogenes is a bacterium commonly found in soil, stream water, sewage, plants, and food. Each bacterium is Gram-positive and rod-shaped. Listeria are known to be the bacteria responsible for listeriosis, a rare but lethal food-borne infection that has a devastating mortality rate of 25%(Salmonella, in comparison, has a less than 1% mortality rate). They are incredibly hardy and able to grow in temperatures ranging from 4°C (39°F), the temperature of a refrigerator, to 37°C (99°F), the body's internal temperature. Furthermore, listerosis's deadliness can be partially attributed to the infection's ability to spread to the nervous system and cause meningitis. Finally, Listeria has a particularly high occurrence rate in newborns because of its ability to infect the fetus by penetrating the endothelial layer of the placenta.

Pathogenesis

        Listeria uses the cellular machinery to move around inside the host cell: it induces directed polymerization of actin by the ActA transmembrane protein, thus pushing the bacterial cell around.

           Listeria monocytogenes for example, encodes virulence genes which are thermoregulated. The expression of virulence factor is optimal at 37 degrees Celsius and is controlled by a transcriptional activator, PrfA, whose expression is thermoregulated. At low temperatures, the PrfA transcript is not translated due to structural elements near the ribosome binding site. As the bacteria infects the host, the temperature of the host melts the structure and allows translation initiation for the virulent genes.

Mechanism of Infection

         The majority of Listeria bacteria are targeted by the immune system before they are able to cause infection. Those that escape the immune system's initial response, however, spread though intracellular mechanisms and are therefore guarded against circulating immune factors (AMI).

           To invade, Listeria induces macrophage phagocytic uptake by displaying D-galactose receptors that are then bound by the macrophage's polysaccharide receptors (Notably, in most bacterial infections it is the host cell, not the bacteria, that displays the polysaccharide).  Once phagocytosed, the bacteria is encapsulated by the host cell's acidic phagolysosome organelle.  Listeria, however, escapes the phagolysosome by lysing the vacuole's entire membrane with secreted hemolysin,  now characterized as the exotoxin listeriolysin O.The bacteria then replicate inside the host cell's cytoplasm.