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The obligate intracellular parasite Chlamydia trachomatis has a reduced genome and is thought to rely on its mammalian host cell for nutrients. Although several lines of evidence suggests C. trachomatis utilizes host phospholipids, the bacterium encodes all the genes necessary for fatty acid and phospholipid synthesis found in free living Gram-negative bacteria. Bacterial-derived phospholipids significantly increased in infected HeLa cell cultures. These new phospholipids had a distinct molecular species composition consisting of saturated and branched-chain fatty acids. Biochemical analysis established the role of C. trachomatis-encoded acyltransferases in producing the new disaturated molecular species. There was no evidence for the remodeling of host phospholipids and no change in the size or molecular species composition of the phosphatidylcholine pool in infected HeLa cells. Host sphingomyelin was associated with C. trachomatis isolated by detergent extraction, but may represent contamination with detergent insoluble host lipids rather than being an integral bacterial membrane component. C. trachomatis assembles its membrane systems from the unique phospholipid molecular species produced by its own fatty acid and phospholipid biosynthetic machinery utilizing glucose, isoleucine and serine. [ABSTRACT FROM AUTHOR]