Holospora is a genus of endosymbiotic bacteria that only grow and divide within one of the two nuclei of their Paramecium caudatum hosts. For our study, we focus on Holospora undulata, a species that infects the micronucleus of specific P. caudatum strains, while other P. caudatum strains are resistant. In previous studies, it was shown that the presence of Holospora in Paramecium benefits the host, especially under stress conditions: certain genes that protect Paramecium against both osmotic stress and excess heat are expressed in the presence of Holospora. At the same time, infection comes at a significant growth-rate cost to the Paramecium host, and we expect that there are host defense mechanisms that are activated. Holospora have two forms: a reproductive form and an infectious form. During infection of P. caudatum, H. undulata are engulfed and released from the phagosome into the cytoplasm, where they begin to move to the micronucleus. Once the Holospora have successfully entered the micronucleus, binary fission occurs to produce reproductive forms of the bacteria. Upon starvation, the reproductive forms transform into infectious forms until they are able to break out of the micronucleus, to find a new host. The movement of Holospora within the host suggests that factors such as signaling and membrane trafficking mediate Holospora infection. Currently we are focused on determining what genes are associated with membrane trafficking and what stages of infection the resistant strains of Paramecium are blocked at. Total mRNA (RNA seq) from infected and uninfected Paramecium will be sequenced to find the total set of genes that are up-regulated during various stages of H. undulata infection. To find the genes that are up-regulated at specific time points during infection, we are staging infections and isolating mRNA though the infection cycle.