A large body of work has demonstrated that leptospires regulate and modify gene expression in response to environmental cues, as encountered during disease transmission, including changes in temperature, osmolarity, concentration of iron, the presence of serum, and interaction with macrophages. However, since leptospires are not readily amenable to genetic manipulation, the functional and biological significance of these differentially expressed genes often remains unclear. More recently, it is been shown that proteins, in response to changing environmental conditions, can be modified further by specific post-translational modifications. Indeed, both saprophytic and pathogenic leptospires have comprehensive systems to modify proteins. The paucibacillary nature of spirochetal infections, combined with the challenges associated with acquiring pathogens free from contaminating host proteins makes the study of these bacteria in a mammalian host-adapted state inherently difficult. As an alternative approach, we developed a model in which leptospires are cultivated in a dialysis membrane chamber (DMC) implanted within the peritoneal cavity of rats, where they are exposed to the environmental cues encountered during host infection. This strategy has been successfully applied to compare the transcriptome of L. interrogans cultivated within DMC with that of leptospires grown under standard in vitro conditions. In addition to determining the relative expression levels of ‘‘core’’ housekeeping genes under both growth conditions, we identified 166 genes that were differentially-expressed by L. interrogans in response to mammalian host signals. The proteome of DMC (dialysis membrane chamber) cultivated leptospires was compared to that of in vivo derived leptospires and with leptospires cultivated in vitro at 30°C or 37°C by 2-dimensional difference in gel electrophoresis (2-D DIGE). Our analysis indicates that the abundance of leptospire-proteins is modulated the expression of a range of proteins which are differentially expressed in response to mammalian host signals, and not temperature alone . In addition, we confirm that in several proteins there is a change in the presence of modify the expression of the post-translational modifications trimethyllysine and acetyllysine in response to environmental cues encountered during persistent renal colonization in a reservoir host of infection. These results provide novel insights in the proteome level changes, including to differential protein and post-translational modifications, expressed in response to mammalian host signals which can be used to further define the unique equilibrium that exists between pathogenic leptospires and their reservoir host of infection.