Freeze-Fracture Study of the Endosymbiont of Blastocrithidia culicis1

The two unit membranes which envelope the endosymbiont of the trypanosomatid protozoon, Blastocrithidia culicis, were studied using the freeze-fracture technique. The distribution of the intramembranous particles on both fracture faces of the inner and outer membrane of the endosymbiont was analyzed in the replicas. The protoplasmic face of the inner membrane (PFi) had a higher density of membrane particles than that observed on the extracellular face (EFi), a pattern typical of plasma membranes. The extracellular face of the outer membrane (EFo) presented a density of membrane particles much higher than that observed on the P face of the outer membrane (PFo) a distribution significantly different from that found in the inner membrane of the endosymbiont and in the plasma membrane of the protozoon, but similar to that observed in Gram-negative bacteria. The data obtained support the idea that the endosymbiont of trypanosomatids represents a Gram-negative bacterium-like microorganism enveloped by two unit membranes and lacking a peptidoglycan layer and which lives in direct contact with the cytoplasm of the protozoon.