Symbiotic bacteria build tubular networks 

Networked membrane systems increase bacterial surface area, enhancing nutrient exchange between bacteria and their hosts 

To the Point

• Bacteria within cells: Grain weevils harbor symbiotic bacteria in their cells—a relationship that is crucial for the animals' survival.
• Architectural feature: The bacteria form complex, interconnected membrane structures.
• Important resource: These membrane networks significantly increase the surface area, improve nutrient exchange with the host insect, and enable the bacteria to efficiently obtain essential sugars.

Sitophilus weevils are pests that attack cereal crops such as wheat, rice, and corn in fields and silos. The weevils have symbiotic bacteria, known as Sodalis pierantonius, that live in large numbers in specialized insect cells. The bacteria provide the weevils with essential nutrients that cannot be found in cereals. This relationship is mutually beneficial: the bacteria use the sugar produced during digestion, and in return, they provide the insect with essential nutrients such as vitamins and certain amino acids.

Although scientists recognized the importance of these exchanges, the process itself remained unknown. To study these interactions, the research team used electron microscopy and a sample preparation method that preserves membranes more effectively. This method allowed the research team to observe unique tubular patterns forming complex membrane structures built by the bacteria. To study these structures' architecture and composition, the scientists developed observation and analysis methods using 3D microscopy and the SOLEIL Synchrotron particle accelerator.

Tubenets - veritable exchange networks built by the bacteria

Analysis revealed that these structures form a complex network of tubes, each with a diameter of 0.02 micrometers (µm) and several micrometers in length that connect the bacteria with numerous interconnections. Similar to how microvilli in the human intestine increase the surface area for absorbing nutrients during digestion, these tubular structures increase the surface area of the bacteria for exchanging nutrients with host cells. This enables the bacteria to better assimilate sugar. In exchange, the bacteria produce essential nutrients for the cell. The research team named these structures "tubenets," combining "tube" and "network" to reflect their shape.

Although scientists are familiar with structures that increase the surface area for nutrient absorption in multicellular organisms, such as intestines and plant roots, this is the first time that this type of structure has been identified in bacteria. Similar structures may exist in other types of bacteria and allow them to acquire nutrients more efficiently.