Saccharibacteria Nanosynbacter lyticus type strain TM7x exhibits a remarkably compact genome and an extraordinarily small cell size. This obligate epibiotic parasite forms a symbiotic relationship with its bacterial host, Actinomyces odontolyticus, strain XH001. Due to its limited genome size, TM7x possesses restrained metabolic capacities, predominantly staying outside its host cells to sustain this symbiotic lifestyle. To comprehend the evolutionary strategies of Saccharibacteria, a thorough understanding of the physical interaction between TM7x and XH001 is imperative. Unexpectedly, we serendipitously found the formation of intrabacterial lipid droplets, a feature scarcely observed inside prokaryotic cells. Additionally, the binding with TM7x led to a substantial alteration in the membrane fluidity of the host cells. Employing label-free non-invasive Raman spectroscopy, we unveiled numerous phenotypical differences between the host cells alone and the TM7x/XH001 co-culture, especially in regard to saturated fatty acids.
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