A bacterial endosymbiont with a highly reduced genome forms extensive membrane contact sites with host organelles

Bacterial endosymbionts that establish long-term associations inside of host cells often lose a huge number of genes and become dependent on their hosts for essential cellular functions. One such example is the mealybug endosymbiont Tremblaya princeps: its ~139-kb genome encodes only ~120 protein coding genes and lacks all recognizable genes for lipid and fatty acid synthesis, cell wall components, membrane transport, protein translocation, and transmembrane proteins, raising fundamental questions of how Tremblaya's cellular envelope is formed and maintained. Here, using a combination of transmission electron microscopy (TEM) and focused ion beam scanning electron microscopy (FIB-SEM) of an entire endosymbiont-containing insect cell, we identified extensive and specific contacts between host cell mitochondria and Tremblaya. These contacts occurred at close membrane appositions and contained electron-dense bridge-like structures closely resembling classical eukaryotic membrane contact sites. Whole-cell segmentation showed that approximately 53% of host mitochondria were in contact with Tremblaya, with some mitochondria extensively wrapped by Tremblaya-associated membranes. Mitochondria in contact with Tremblaya were larger and contained proportionally greater cristae volume and surface density compared to non-contact mitochondria, exhibiting differences in both external and internal mitochondrial architecture. As membrane contact sites are well known to mediate lipid and metabolite exchange between membranes, these findings suggest that direct interactions with host organelles may be a source of vital membrane and cellular components for endosymbionts that have lost the autonomous ability to produce and transport these molecules.