ABSTRACT utilizes the Dot/Icm type IV translocation system to proliferate within a vacuole in a wide variety of natural amoebal hosts and in alveolar macrophages of the human accidental host. Although utilizes host amino acids as the main sources of carbon and energy, it is not known whether synthesis of amino acids by intravacuolar contributes to its nutrition. The and genes encode enzymes for the shikimate pathway that generates the aromatic amino acids Phe, Trp, and Tyr. Here we show the and mutants of to be defective in growth in human monocyte-derived macrophages (hMDMs) but not in spp. The and mutants are severely attenuated in intrapulmonary proliferation in the A/J mouse model of Legionnaires' disease, and the defect is fully complemented by the respective wild-type alleles. The two mutants grow normally in rich media but do not grow in defined media lacking aromatic amino acids, and the growth defect is rescued by inclusion of the aromatic amino acids, which are essential for production of the pyomelanin pigment. Interestingly, supplementation of infected hMDMs with the three aromatic amino acids or with Trp alone rescues the intramacrophage defect of the but not the mutant. Therefore, the shikimate pathway of is differentially required for optimal growth within human macrophages, which are auxotrophic for Trp and Phe, but is dispensable for growth within the spp. that synthesize the aromatic amino acids.