A method for producing at least one oligosaccharide of interest by total fermentation through genetically modified host cells, the oligosaccharide of interest comprising a terminal galactose-(1->4)-glucose disaccharide, and the method comprising the steps of: a) obtaining (i) a bacterial host cell being capable to produce oligosaccharides comprising a terminal galactose-(1->4)-glucose disaccharide, and comprising - at least one recombinant nucleic acid sequence encoding for a protein having a beta-1,4-galactosyltransferase activity and being able to galactosylate a free glucose monosaccharide to intracellularly generate lactose, - at least one recombinant nucleic acid sequence encoding a glycosyltransferase that is selected from at least one of the following: a fucosyltransferase, a sialyltransferase, a glucosamyltransferase, a galactosyltransferase, and which bacterial host cell is capable of generating the oligosaccharide without the exogenous addition of lactose, or (ii) a first bacterial host cell comprising a recombinant nucleic acid sequence encoding for a protein having a beta-1,4-galactosyltransferase activity and being able to galactosylate a free glucose monosaccharide to intracellularly generate lactose, and a second bacterial host cell comprising at least one recombinant nucleic acid sequence encoding a glycosyltransferase that is selected from at least one of the following: a fucosyltransferase, a sialyltransferase, a glucosamyltransferase, a galactosyltransferase; b) culturing said bacterial host cells (i) or (ii) from step a), wherein said culturing is performed from a carbon source that is selected from at least one of the following: glucose, sucrose, glycerol, molasses, corn syrup, galactose, syngas, carbon monoxide, methanol, succinate, pyruvate, raffinose, wherein the host cell is capable of generating free glucose within the cell from said carbon source, and without externally supplying lactose during execution of the method, thus producing said oligosaccharide of interest. a) obtaining (i) a bacterial host cell being capable to produce oligosaccharides comprising a terminal galactose-(1->4)-glucose disaccharide, and comprising - at least one recombinant nucleic acid sequence encoding for a protein having a beta-1,4-galactosyltransferase activity and being able to galactosylate a free glucose monosaccharide to intracellularly generate lactose, - at least one recombinant nucleic acid sequence encoding a glycosyltransferase that is selected from at least one of the following: a fucosyltransferase, a sialyltransferase, a glucosamyltransferase, a galactosyltransferase, and which bacterial host cell is capable of generating the oligosaccharide without the exogenous addition of lactose, or (ii) a first bacterial host cell comprising a recombinant nucleic acid sequence encoding for a protein having a beta-1,4-galactosyltransferase activity and being able to galactosylate a free glucose monosaccharide to intracellularly generate lactose, and a second bacterial host cell comprising at least one recombinant nucleic acid sequence encoding a glycosyltransferase that is selected from at least one of the following: a fucosyltransferase, a sialyltransferase, a glucosamyltransferase, a galactosyltransferase; (i) a bacterial host cell being capable to produce oligosaccharides comprising a terminal galactose-(1->4)-glucose disaccharide, and comprising - at least one recombinant nucleic acid sequence encoding for a protein having a beta-1,4-galactosyltransferase activity and being able to galactosylate a free glucose monosaccharide to intracellularly generate lactose, - at least one recombinant nucleic acid sequence encoding a glycosyltransferase that is selected from at least one of the following: a fucosyltransferase, a sialyltransferase, a glucosamyltransferase, a galactosyltransferase, and which bacterial host cell is capable of generating the oligosaccharide without the exogenous addition of lactose, or - at least one recombinant nucleic acid sequence encoding for a protein having a beta-1,4-galactosyltransferase activity and being able to galactosylate a free glucose monosaccharide to intracellularly generate lactose, - at least one recombinant nucleic acid sequence encoding a glycosyltransferase that is selected from at least one of the following: a fucosyltransferase, a sialyltransferase, a glucosamyltransferase, a galactosyltransferase, (ii) a first bacterial host cell comprising a recombinant nucleic acid sequence encoding for a protein having a beta-1,4-galactosyltransferase activity and being able to galactosylate a free glucose monosaccharide to intracellularly generate lactose, and a second bacterial host cell comprising at least one recombinant nucleic acid sequence encoding a glycosyltransferase that is selected from at least one of the following: a fucosyltransferase, a sialyltransferase, a glucosamyltransferase, a galactosyltransferase; b) culturing said bacterial host cells (i) or (ii) from step a), wherein said culturing is performed from a carbon source that is selected from at least one of the following: glucose, sucrose, glycerol, molasses, corn syrup, galactose, syngas, carbon monoxide, methanol, succinate, pyruvate, raffinose, wherein the host cell is capable of generating free glucose within the cell from said carbon source, and without externally supplying lactose during execution of the method, thus producing said oligosaccharide of interest.