Chlamydia pneumoniae is an extremely common respiratory pathogen associated with acute and chronic inflammatory diseases such as community-acquired pneumoniae, bronchitis, asthma and arthrosclerosis. Protective immunity against the infection of Chlamydial species is impacted by the release of interferon gamma (IFN-γ) from host cell, which affects tryptophan. IFN-γ induces expression of the tryptophan catabolizing enzyme indoleamine 2, 3 dioxygenase (IDO1) in human cells, which disrupts the biochemical pathways of C. pneumoniae, which is a tryptophan auxotroph. We sought to investigate the role of tryptophan sensitivity, using an in vitro model that tests the effects of human IFN-γ (50 U/mL and 100 U/mL) on human and ancestral animal C. pneumoniae strains cultured, in human lung epithelial BEAS-2B cells. The ancestral animal C. pneumoniae isolate was quite resistant to IFN-γ treatment of BEAS-2B cells with only minor reductions in viable infectious yield (≤1 log reduction when cultures were treated with 100 U/ml IFN-g) and little evidence of aberrant inclusion sizes. However, the human C. pneumoniae isolate was highly sensitive to IFN-γ culture conditions with significantly smaller inclusion sizes during the infection and a 4 log reduction in the viable infectious yield. We confirmed via immunoblot analysis that neither strain was able to alter the IFN-γ induction of IDO1 in this cell culture model, and the lack of sensitivity observed for the ancestral animal strain to IFN-γ treatment of host cells does not relate to a modulation upstream of IDO1 protein induction. Furthermore, the addition of tryptophan to IFN-γ treated cultures resulted in significant recovery of viable bacteria for human and animal C.pneumoniae strains. These studies confirm the importance of tryptophan in the C. pneumoniae developmental cycle and may suggest the ability of human C. pneumoniae to use the available tryptophan, under the control of IFN-γ induced IDO as a sensing mechanism to regulate its developmental cycle in order to survive immune attack and cause or exacerbate chronic diseases.