Zurich researchers have discovered a new life form of Listeria monocytogenes, an opportunistic pathogen responsible for serious food poisoning. These bacteria can reproduce and proliferate as so-called L-forms. The methods to detect these bacteria should now be adapted.
For over 100 years, it was known that bacteria may lose their cell wall and can still survive. However, it was believed that this phenomenon was merely an artefact and that bacteria without cell walls do not remain viable. Recent research of a group headed by ETH Zurich Professor Martin J. Loessner, which has just been published in Molecular Microbiology, shows that bacteria without a cell wall can be a stable form of bacterial life. Astonishingly, not only can Listeria survive without a cell wall, they are even able to reproduce and proliferate.
From cheese to the brain
Listeria (Listeria monocytogenes) are pathogens causing dangerous and often fatal cases of food-borne infections, and are frequently found in milk products such as vacherin soft cheese. The bacteria invade the human body through the epithelial cells of the intestine and spread from cell to cell., which renders them invisible to the immune system. Listeria can cross both the blood-brain barrier and the placenta barrier. Having reached the brain, they cause severe inflammation of the brain, which can be fatal. Listeria can also endanger fetuses and pregnant women.
Membrane instead of a cell wall
Listeria cells normally appear as small rods. If they shed their cell wall, e.g. through contact with certain antibiotics such as penicillin, they become spherical and enlarge greatly. These cell wall deficient cells are surrounded by a single membrane only. As an intermediate stage between this L-form and the rod-shaped parental cells, there is an intermediate stage from which the bacteria can rebuild their cell wall. However, once Listeria has reached the complete L-form status, there may be no way back.
The change from the normal form to the L-form is accompanied by many changes in cell metabolism and gene activity. Almost 280 of the genes of normal and L-form Listeria showed differing activity. While genes responsible for stress regulation were activated in the L-forms, genes for metabolism and energy balance were strongly repressed. The researchers interpret this as the bacterial response and active adaptation to its new lifestyle. Loessner says “L-form Listeria really have a very stressful life.”
“Culturing” the L-forms of bacteria is not easy. They need to be “bred” in a liquid medium and do not normally form colonies, so plating on a petri dish is not possible. Although L-form Listeria cells are capable of reproducing themselves, this can take time: formation of a visible colony within tubes containing a soft medium takes at least six days, compared to 16 to 20 hours for normal cells.