How a bacterium becomes a permanent resident in a fungus as a bacterium that ends up by chance inside a different host cell typically faces many challenges. It must survive, reproduce, and be passed on to future generations, or it will disappear. Additionally, to avoid harming its host, it can’t take too many nutrients or grow too quickly. If the host and the new resident can’t cooperate, the relationship will end.
| Date | October 2, 2024 |
| Source | ETH Zurich |
| Summary | In biology, the concept of one organism living within another is often quite successful. Researchers have now uncovered insights into how such a symbiotic relationship, where one cell resides within another, can form and stabilize. |
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How a bacterium becomes a permanent resident in a fungus
To explore how the endosymbiosis relationship might start, a research team led by Julia Vorholt, Professor of Microbiology at ETH Zurich, initiated such partnerships in the lab. The team closely studied the early stages of potential endosymbiosis, and their findings have recently been published in Nature.
Forcing Coexistence
In this study, Gabriel Giger, a doctoral student in Vorholt’s lab, developed a technique to inject bacteria into cells of the fungus Rhizopus microsporus without causing damage. Giger used two types of bacteria for the experiment: E. coli and bacteria from the genus Mycetohabitans. While Mycetohabitans naturally form endosymbiotic relationships with another Rhizopus species, the strain used in the experiment does not engage in endosymbiosis in nature. Giger observed what happened under the microscope as these bacteria were forced into cohabitation with the fungus.
Following the injection of E. coli bacteria, both the fungus and the bacteria continued to grow, but the bacteria grew so quickly that the fungus triggered an immune response, encapsulating the bacteria to protect itself. As a result, the bacteria could not be passed on to the next generation of fungi.
Bacteria Enter Spores
The outcome was different when Mycetohabitans bacteria were injected. As the fungus formed spores, some of the bacteria managed to enter the spores and were passed on to the next generation. “The fact that the bacteria were successfully transmitted to the next generation of fungi through the spores was a breakthrough in our research,” Giger noted.
When Giger allowed these spores, containing bacteria, to germinate, he found that they germinated less frequently and that the fungi grew more slowly compared to those without bacterial inhabitants. “Initially, the endosymbiosis reduced the overall fitness of the fungi,” he explained. However, by continuing the experiment over several fungal generations and selecting fungi whose spores contained bacteria, the fungi adapted and produced more viable spores with resident bacteria. Genetic analysis showed that the fungus had undergone changes during the experiment to accommodate its bacterial partner.
The researchers also discovered that together, the host and its bacterial resident produced biologically active molecules that could help the fungus obtain nutrients or defend against threats like nematodes and amoebae. “What initially posed a disadvantage can later become an advantage,” emphasized Vorholt.
Delicate Relationships
The study highlighted how fragile early endosymbiotic systems are. “The initial decline in the host’s fitness could lead to the quick collapse of such a system in nature,” said Giger. “For new endosymbiotic relationships to stabilize, there must be a mutual benefit to living together,” Vorholt added. The potential resident must possess traits that favor endosymbiosis. For the host, incorporating another organism provides an opportunity to gain new characteristics, even though it requires adjustments. “In the long run, evolution shows how successful endosymbiosis can become,” the ETH professor concluded.
FAQ:
1. What is endosymbiosys?
Endosymbiosis is a remarkable biological process where one organism lives inside another. This unique relationship often benefits both organisms involved. Even within our bodies, traces of ancient endosymbiotic events remain—mitochondria, the energy producers of our cells, result from a long-ago endosymbiosis. In ancient times, bacteria entered other cells and remained, eventually giving rise to mitochondria, which became crucial to developing plant, animal, and fungal cells.