The baby???s microbiome continues to grow during??breast-feeding. In??a study of 16 lactating women published last year, Katherine M. Hunt of the University of Idaho and her colleagues reported that the women???s milk had up to 600 species of bacteria, as well as sugars called oligosaccharides that babies cannot digest. The sugars serve to nourishcertain beneficial gut bacteria??in the infants, the scientists said. The more the good bacteria thrive, the harder it is for harmful species to gain a foothold.