Network analysis reveals a potentially 'evil' alliance of opportunistic pathogens inhibited by a cooperative network in human milk bacterial communities

The critical importance of human milk to infants and even human civilization has been well established. Although the human milk microbiome has received increasing attention with the expansion of research on the human microbiome, our understanding of the milk microbiome has been limited to cataloguing OTUs and computation of community diversity indexes. To the best of our knowledge, there has been no report on the bacterial interactions within the human milk microbiome. To bridge this gap, we reconstructed a milk bacterial community network with the data from Hunt et al (2011), which is the largest 16S-rRNA sequence data set of human milk microbiome available to date. Our analysis revealed that the milk microbiome network consists of two disconnected sub-networks. One sub-network is a fully connected complete graph consisting of seven genera as nodes and all of its pair-wise interactions among the bacteria are facilitative or cooperative. In contrast, the interactions in the other sub-network of 8 nodes are mixed but dominantly cooperative. Somewhat surprisingly, the only 'non-cooperative' nodes in the second sub-network are mutually cooperative Staphylococcus and Corynebacterium, genera that include some opportunistic pathogens. This potentially 'evil' alliance between Staphylococcus and Corynebacterium could be inhibited by the remaining nodes who cooperate with one another in the second sub-network. We postulate that the 'confrontation' between the 'evil' alliance and 'benign' alliance in human milk microbiome should have important health implications to lactating women and their infants and shifting the balance between the two alliances may be responsible for dysbiosis of the milk microbiome that permits mastitis. A related study focusing on ecological analysis was reported at (http://www.eurekalert.org/pub_releases/2014-09/scp-ahb090214.php).