Compositions and methods for modulating growth of a genetically modified gut bacterial cell

A genetically modified bacterial cell comprising:

• a heterologous carbohydrate-utilization gene set that provides the genetically modified cell with an ability to utilize as a carbon source a porphyran,
• wherein the heterologous carbohydrate-utilization gene set comprises one or more nucleic acids that each encode one or more proteins selected from the group consisting of proteins having 90% or more sequence identity to SEQ ID NOs:
• 1-34.

A genetically modified Bacteroides cell comprising:

• a heterologous carbohydrate-utilization gene set that provides the genetically modified Bacteroides cell with an ability to utilize as a carbon source a porphyran,
• wherein the heterologous carbohydrate-utilization gene or gene set comprises one or more nucleic acids encoding a porphyranase.