The present application discloses a synthetic mixture of oligosaccharides for treating a microbiota of a mammal, preferably a human, to reduce or eliminate the activity and/or the proportion of a microbe in the microbiota that is associated with the development or maintenance of a disease or an unhealthy condition of the mammal, e.g. wherein the microbiota is a gut microbiota, or wherein the disease is a cardiovascular disease, gastrointestinal tract disorder, obesity, diabetes, autism, or atherosclerosis, or wherein the microbiota is a skin microbiota, such as where the unhealthy condition is a chronic wound caused by diabetes, wherein the mixture comprises at least 6 oligosaccharides selected from the following: one or more GlcNAc-containing oligosaccharides, and/or one or more fucosylated oligosaccharides, and/or one or more sialylated oligosaccharides, and/or one or more mannose containing oligosaccharides, and/or one or more GalNAc-containing oligosaccharides, and/or one or more sulfated oligosaccharides.