Manganese scavenging is required for virulence of the cytosolic pathogen Listeria monocytogenes
This study demonstrates that the cytosolic pathogen *Listeria monocytogenes* relies on high-affinity manganese transporters (MntH and MntABC) to scavenge manganese, a process essential for its resistance to oxidative stress, intracellular replication, and overall virulence.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Every living thing, from the smallest bacterium to a human being, needs a steady supply of certain metals to survive. Just as our bodies require iron to carry oxygen in our blood, bacteria need manganese to power the enzymes that keep their cells running and to protect them from the damaging effects of oxygen. But in the wild, these metals are not freely available. When a bacterium invades a host, the host's immune system fights back by actively hiding these essential nutrients, creating a state of starvation known as nutritional immunity. It is a biological tug-of-war where the host tries to starve the invader, and the invader must evolve clever ways to scavenge what little remains.
One such invader is Listeria monocytogenes, a foodborne bacterium that causes serious illness. Unlike many other bacteria that hide inside a protective bubble within a host cell, Listeria is a cytosolic pathogen, meaning it escapes that bubble and lives freely in the main fluid of the cell, the cytosol. Scientists have long known that this environment is hostile and difficult for bacteria to survive in, but the specific role of manganese starvation in this hidden niche remained a mystery. While it was clear that other bacteria needed to scavenge manganese to survive in the host, it was not known if this cytosolic pathogen faced the same challenge or if it had a different strategy for survival.
Researchers set out to solve this puzzle by creating a version of Listeria that could not grab manganese. They identified two specific systems the bacterium uses to import this metal, which function like specialized pumps on the cell surface. One system, called MntH, works best in acidic conditions, while the other, MntABC, operates differently. To see if these systems were truly essential, the scientists engineered a strain of Listeria where both pumps were disabled. In a standard, nutrient-rich laboratory dish, this double-mutant bacterium grew just fine, suggesting it could get by with whatever trace metals were floating around. However, when the researchers removed the extra manganese from the food, the mutant bacterium stopped growing entirely. It became clear that without these two specific pumps, the bacterium could not survive when manganese was scarce.
The team then looked inside the cells to see what was happening. They found that the mutant bacterium had significantly lower levels of manganese compared to the normal bacteria. This lack of metal made the mutant extremely sensitive to oxidative stress, a type of cellular damage caused by reactive oxygen species. In the lab, when the researchers exposed the bacteria to a chemical that mimics this stress, the mutant struggled to survive, especially when manganese was low. This confirmed that the bacterium relies on these pumps not just for growth, but to protect itself from the chemical warfare waged by its environment.
The most critical tests, however, took place inside living cells. When the researchers infected immune cells called macrophages with the mutant bacteria, the results were stark. The normal bacteria multiplied rapidly inside the cells, but the mutant bacteria, unable to scavenge manganese, failed to replicate. They grew for a short time after entering the cell but then stopped completely. The scientists tried to fix this by adding extra manganese to the cell culture, and the mutant bacteria immediately began to grow again, proving that the defect was indeed a lack of metal. Furthermore, when the mutant bacteria were tested in a model that mimics how they spread from cell to cell, they were completely unable to form the characteristic plaques of infection that the normal bacteria created.
Finally, the researchers tested the bacteria in live mice to see if this weakness translated to real-world virulence. They injected the mice with the mutant strain and watched what happened. The mice infected with the normal bacteria lost significant weight and carried a high load of bacteria in their livers and spleens. In contrast, the mice infected with the mutant bacteria remained healthy, losing no weight, and the bacteria were nearly undetectable in their organs. The mutant strain was so weakened that it could not establish an infection at all.
These findings reveal that the cytosol, the open space inside a host cell where Listeria lives, is a place of severe manganese starvation. The bacterium cannot rely on passive uptake or other backup systems; it must actively and aggressively scavenge manganese using its two dedicated transporters to survive. Without these tools, the bacterium is defenseless against the host's nutritional defenses and cannot cause disease. This study establishes that for this specific pathogen, the ability to steal manganese is not just helpful, but absolutely essential for its survival and its ability to make people sick.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.