Heterogeneity assessment by 16S rRNA target region. Observed species (operational taxonomic unit, OTU) richness (a) and Shannon diversity index (SDI) values (b) in cases and controls are subgrouped by 16S rRNA target region (V1-V3 vs. V3-V5). Data is represented in box and whisker plots with median and whiskers representing 10–90th centiles. Significant differences were observed in SDI between controls of V1-V3 compared to controls of V3-V5 (*p < 0.05) and controls of V1-V3 compared to NEC V3-V5. No other significant differences were observed. c, d Depict weighted and unweighted UniFrac distances in PCoA plots of NEC and controls subgrouped 16S rRNA target region (V1-V3 vs V3-V5), and notable clustering was observed. The proportion of variation explained along each axis is listed in parenthesis with the axis labels. e Represents differences in proportion of sequences based on 16S rRNA target regions; V3-V5 targeting resulted in a significant increase in proportion of sequences of Proteobacteria and significant decrease in proportion of sequences of Firmicutes compared to studies targeting V1-V3.