Abstract: Compositions and methods are provided for modulating growth of a genetically modified bacterial cell present in a human organ, for modulating growth of a genetically modified bacterial cell in an organ (e.g., gut), for displacing at least a portion of a population of bacterial cells in an organ, and for facilitating gut colonization by a genetically modified bacterial cell. Also provided are genetically modified bacterial cells, e.g., cells that include a heterologous carbohydrate-utilization gene or gene set that provides for the ability to utilize as a carbon source a rare carbohydrate of interest that is utilized as a carbon source by less than 50% of bacterial cells present in a human microbiome.