Due to its wide distribution, the bacteria have many opportunities to contaminate food, which are the main vehicles of transmission for humans.
Listerias are a group of Gram positive bacteria that contain several species that are widespread in the environment. There are many classes, but only one of them, Listeria monocytogenes, is pathogenic to humans. She is responsible for cases of listeriosis such as those observed in the recent outbreak in Andalusia.
It is a facultative anaerobic bacterium, which means that it is capable of surviving in the presence and in the absence of oxygen.L. monocytogenes has been isolated from species of mammals, birds, fish, and crustaceans. Although it does not produce spores, it is quite resistant in the outdoor environment and its main habitat is the soil and plant matter. Its ability to grow at temperatures close to 0 ⁰C allows it to multiply even in the fridge.
Due to its wide distribution, the bacteria have many opportunities to contaminate food, which are the main vehicles of transmission for humans. We are talking about unpasteurized milk, cheeses and other dairy products, fresh vegetables, smoked fish, and meats like chicken and turkey.
Listeria is one of the most virulent foodborne pathogens: it can kill 20-30% of those infected. The reason for this lethality is found in its interesting biology.
The gateway to our body is the gastrointestinal tract. Listeriosis is a mild disease in most people, but it is dangerous in older people or people with a previous illness.
Pregnant women are especially prone to bacteremia (the presence of bacteria in the blood), usually without serious consequences for them. The problem is that the bacteria can cross the placenta and cause a fetal infection, which can be fatal. L. monocytogenes has a certain predilection for the central nervous system, which is why encephalitis is common.
A master of camouflage and adaptation
Macrophages are cells of the immune system that, when a pathogenic microorganism enters our body, are activated, chase and destroy it. This is possible thanks to vesicles that contain enzymes that digest the bacteria. In other words, the macrophage eats the microbe.
This biological process by which some specialized blood cells ingest and destroy foreign particles, such as microorganisms, is called phagocytosis.
But L. monocytogenes is a peculiar bacterium. Not only is it capable of resisting macrophage voracity, it can even invade other non-phagocytic cell types such as epithelial cells, liver cells, and even neurons. Once inside, the bacteria can grow, reproduce, and move around inside the cell.
This is what is called a facultative intracellular pathogen. For many years Listeria has been one of the models most studied by microbiologists and cell biologists to understand how a bacterium can resist phagocytosis and multiply inside cells.
What they found was a fascinating story.
L. monocytogenes can invade cells. For this, it has a protein on its surface called internalin, which, by joining with another protein in the cell, promotes a cascade of reactions between the bacteria and the cell that ends at the entrance of the microorganism. In this way, a vacuole or phagosome is formed inside the cell that contains the bacteria inside.
Stages of intracellular life of Listeria monocytogenes. The figure in the center is based on the surrounding electron micrographs. Daniel A. Portnoy, Victoria Auerbuch, and Ian J. Glomski-Portnoy, DA, et al. 2002, CC BY
The bacteria then have to quickly escape from that phagosome, before it fuses with other cellular digestive vacuoles (lysosomes) to destroy it. For that, L. monocytogenes has a whole arsenal of toxins capable of breaking the membrane of the vesicle that surrounds the bacteria and escaping into the cytoplasm of the cell.
There, in the cytoplasm, the bacteria feel free and can move and multiply wherever they want. For a microorganism, the inside of a cell is like a sea full of nutrients and, moreover, a safe place. There it is hidden from the attack of the host's immune system, from the antibodies that block it, from the macrophages that want to eat it, and from antibiotics.
Once in the cytoplasm, L. monocytogenes moves through the interior of the cell. For this, it uses a protein that it has at one of its ends and that is capable of assembling actin microfilaments (a protein that forms the cytoskeleton of the cell) on the surface of the bacteria.
The assembly of these microfilaments at one of the poles of the bacterium forms a structure that pushes the microorganism in one direction. It is as if an actin rocket is formed, a comet tail that moves the bacteria through the cytoplasm.
But the story does not end here. Thanks to this free movement, L. monocytogenes can pass from one cell to another, spreading the infection throughout the tissue.
This video shows real and spectacular images of how these bacteria move through the interior of a cell:
A cellular Trojan horse
The presence of the bacteria in phagocytic cells such as macrophages, which can move through the bloodstream, allows it to cross the placenta and access other organs such as the liver and brain. L. monocytogenes uses its ability to resist and multiply intracellularly like a Trojan horse to reach other sites within the body.
The extraordinary biology of L. monocytogenes does not end there. In addition to the interior of organisms and cells, it can live in the soil and in plant debris. To survive in such different environments, you need to adapt and know where you are.
L. monocytogenes, like other bacteria, communicates with its companions and with the external environment through a system of signals called quorum sensing. . The bacterium receives different signals depending on where it is, so that it can regulate the expression (turn on or turn off) of different virulence genes. It expresses different genes depending on whether it is in the intestine, in the blood, or within a cell. A master of camouflage and adaptation.
The study of the biology of L. monocytogenes as a model of a facultative intracellular pathogen has allowed us to better understand how a bacterium can communicate with the cell and adapt to living inside it.
Fortunately, L. monocytogenes is not one of those antibiotic-resistant "superbugs," so its treatment with antimicrobials such as penicillin or ampicillin is often effective.
Learn more about listeriosis at the Centers for Disease Control and Prevention.
Recommendations to prevent food-borne gastrointestinal infections (such as Listeria monocytogenes).You guys know I’m a mix of practical plus woo. On the practical side, beyond those studies referenced in the Gaia article, I didn’t find any studies that said there was a definite connection between high vibrational music and healing. I did find some studies that showed some hope but nothing definitive, so I’m not going to link them.