A genetically modified bacterium comprising one or more heterologous nucleic acid molecules together encoding at least three different proteins, each protein comprising an enzymatic domain and a bacterial microcompartment-targeting signal polypeptide, wherein said enzymatic domains each catalyse a different substrate to product conversion in the same metabolic pathway, and wherein said bacterium is free of bacterial microcompartments (BMCs), wherein each of said proteins is a fusion protein comprising an enzymatic domain and a bacterial microcompartment-targeting signal polypeptide that are not present in the same protein in nature, wherein said bacterium a) does not produce BMCs naturally and has not been genetically engineered to provide BMC production capability; or b) is a strain that naturally comprises the genes necessary for the expression of BMCs, and wherein expression of said BMCs is inducible by the presence of one or more inducer molecules, but wherein said bacterium is present in a culture medium in which the level of said inducer molecule(s) is too low to induce the expression of said BMCs; or c) is a strain that naturally comprises the genes necessary for the expression of BMCs but wherein said bacterium is a BMC null mutant, and wherein in said bacterium each BMC-targeting signal polypeptide comprises a sequence selected from the group consisting of LETLIRTIL (SEQ ID NO:2), LETLIRNIL (SEQ ID NO:3), LRQIIEDVL (SEQ ID NO:4), IEEIVRSVM (SEQ ID NO:5), IEQVVKAVL (SEQ ID NO:6), VEKLVRQAI (SEQ ID NO:7), VEEIVKRIM (SEQ ID NO:9), IESMVRDVL (SEQ ID NO:10), VQDIIKNVV (SEQ ID NO:11), IRQVVQEVL (SEQ ID NO:12), VRSVVEEVV (SEQ ID NO:13) and ARDLLKQIL (SEQ ID NO:14), or a variant sequence having at least 88% identity thereto. wherein said bacterium a) does not produce BMCs naturally and has not been genetically engineered to provide BMC production capability; or b) is a strain that naturally comprises the genes necessary for the expression of BMCs, and wherein expression of said BMCs is inducible by the presence of one or more inducer molecules, but wherein said bacterium is present in a culture medium in which the level of said inducer molecule(s) is too low to induce the expression of said BMCs; or c) is a strain that naturally comprises the genes necessary for the expression of BMCs but wherein said bacterium is a BMC null mutant, and a) does not produce BMCs naturally and has not been genetically engineered to provide BMC production capability; or b) is a strain that naturally comprises the genes necessary for the expression of BMCs, and wherein expression of said BMCs is inducible by the presence of one or more inducer molecules, but wherein said bacterium is present in a culture medium in which the level of said inducer molecule(s) is too low to induce the expression of said BMCs; or c) is a strain that naturally comprises the genes necessary for the expression of BMCs but wherein said bacterium is a BMC null mutant, and wherein in said bacterium each BMC-targeting signal polypeptide comprises a sequence selected from the group consisting of LETLIRTIL (SEQ ID NO:2), LETLIRNIL (SEQ ID NO:3), LRQIIEDVL (SEQ ID NO:4), IEEIVRSVM (SEQ ID NO:5), IEQVVKAVL (SEQ ID NO:6), VEKLVRQAI (SEQ ID NO:7), VEEIVKRIM (SEQ ID NO:9), IESMVRDVL (SEQ ID NO:10), VQDIIKNVV (SEQ ID NO:11), IRQVVQEVL (SEQ ID NO:12), VRSVVEEVV (SEQ ID NO:13) and ARDLLKQIL (SEQ ID NO:14), or a variant sequence having at least 88% identity thereto.