Even tiny organisms can be brutal – not only eliminating potential competitors for resources but also using their neighbours as a source of nutrition. This is the conclusion reached by an international group of researchers from ETH Zurich, the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and with contributions from other institutions. The researchers have just published their findings in the journal, Science.
Poison tipped spear
The researchers became aware of the bacteria's behaviour when they observed under a microscope two distinct species of rod-shaped bacteria from the sea in tiny growth chambers. "We noticed that cells of one species began to disintegrate when they came into contact with cells of the other species," says Astrid Stubbusch, first author of the study and a doctoral student in the Microbial Systems Ecology group of ETH Zurich and Eawag and the Geobiology group of ETH Zurich.
While both species belong to the same genus of Vibrio bacteria, one produces a special secretion system known as the type 6 secretion system (T6SS) - a highly sophisticated weapon that bacteria use to fight other bacteria for resources.
Martin Ackermann, Professor of Microbial Systems Ecology at ETH Zurich and Eawag, imagines the T6SS as a spear with a poisoned tip. The predatory bacteria fire the spear. It penetrates the neighbouring cell but does not cause any fatal injuries. "It is the poison that the spear carries into the neighbouring cell that kills it," says Ackermann. The killer cells themselves are immune to the poison. "Cells of the same species also fire at each other without killing themselves."
Hungry cells feed slowly
In the lab, the research team investigated how differences in nutrient availability affected the bacterial behaviours. For experimental conditions rich in alginate - a carbon source which can only be consumed by the victim cells, but not the killer's cells - the researchers observed that the well-fed victim cells changed shape and slowly leaked their contents for the killer cells to consume. When both the killer and victim cells were well-fed, the victim cells quickly disappeared as their cellular contents rapidly dissolved into the environment.
"This difference in dissolution time could mean that the killer cells load their spears with different toxins," says co-author Glen D'Souza. When the killer cells find food in their environment, they kill the victim cells quickly eliminating the competition for food.
However, if the killer cells are starved, they will steal nutrients from their neighbours and "ensure that the victim cell in the neighbourhood empties slowly so that they can absorb as many nutrients as possible," says D'Souza.
A global ecosystem cell strategy
To find out how widespread this newly discovered bacterial behaviour is and what ecological relevance it might have, the team combed through huge DNA sequence databases that scientists have collected from a wide variety of habitats.
These analyses showed that bacterial species in the genus Vibrio that have the T6SS genes often lack genes for metabolising complex carbohydrates such as alginate. In other words, many killer bacteria are genetically optimised to live off easily usable molecules, such as the cell sap of neighbouring cells. "This appears to be an evolutionary trade-off," explains Cara Magnabosco, ETH Zurich professor of Geobiology. "And we found even more species with the potential to express this killer behaviour across the bacterial tree of life and within all the environments we investigated."
The authors suspect that this killer bacterial behaviour plays a previously overlooked role in nutrient cycling across ecosystems by enabling the direct transfer of nutrients between cells. Magnabosco says, "By slowly lysing their neighbours, the killer cells are able to keep important nutrients that would normally be lost to the environment or other trophic levels within the microbial food web. The quantification of these fluxes is an important next step that we look forward to investigating this across ecosystems."
