Title: Population dynamics and universal statistics of tumor-inhabiting bacteria.

Abstract: Bacteria inhabit different areas of the human body and perform essential functions. Increasing evidence of bacterial effects on cancer progression has brought interest to tumor-inhabiting bacteria. However, it is not understood how bacteria affect the tumor, nor how the tumor environment affects bacterial dynamics. Our experimental collaborators infect mouse tumors with DNA-barcoded bacteria, creating competition among thousands of clonal bacterial "species". We find that after an initial expansion period, clone sizes exhibit universal power-law statistics. These statistics are robust across experiments and collection times, and unique to bacteria grown in the tumor environment rather than in liquid culture. In this work, we develop a mechanistic theory of intra-tumor bacterial growth that includes an infection bottleneck, local growth constraints, global resource competition, and environmental noise. Our simple physical theory captures the dynamics and the statistics of the experiments and explains the uniqueness of the observations to the tumor environment.