Microbial community differences between NEC and control infants, days 4 to 9 samples. (A) This box plot indicates the relative abundance of the genus Propionibacterium in 18 control samples and 9 samples prior to NEC onset during days 4 to 9 of life. None of the infants who later developed NEC, but 10 (56%) of the control samples had detectable amounts of Propionibacterium (using Fisher’s exact test for the presence or absence in NEC vs control samples, P = 0.009). (B) This box plot indicates the Chao1 richness index in samples from days 4 to 9 of life. NEC cases tended towards lower diversity than controls, but the comparison was not significant (Kruskal–Wallis, P = 0.086). For each box plot, the middle bar represents the median, the outer horizontal lines of the box represent the 25th and 75th percentiles. The dots overlaying the plots indicate the values of individual samples. Dots are distributed horizontally in order to prevent overlapping. NEC: necrotizing enterocolitis.