“…When the 16S rDNA was targeted for sequencing, the hypervariable regions targeted included V4 (Diener et al, 2021;Lambeth et al, 2015;Allin et al, 2018;Gaike et al, 2020), V3-V4 (Egshatyan et al, 2016;Nuli et al, 2019;Zhao et al, 2019), V3 (Bhute et al, 2017;Chavez-Carbajal et al, 2020), V1-V3 (Ericson et al, 2020) or V3-V5 (Zhang et al, 2013) regions, while five studies did not specify the target regions (Karlsson et al, 2013;Zhong et al, 2019;Wu et al, 2020;Wang et al, 2021). The two studies using qPCR to assess the microbiota had used ten and six pairs of specific bacterial 16S rRNA primers, respectively, to target Atopobium cluster, Bacteroides fragilis, Bifidobacterium, Clostridium coccoides, Clostridium leptum, Clostridium perfringens, Enterobacteriaceae, Enterococcus, Lactobacillus and Prevotella (Chen et al, 2019) and Akkermansia, Bacteroides, Bifidobacterium, E.coli, Faecalibacterium and Lactobacillus (Ghaemi et al, 2020). Following sequencing, the microorganisms were classified and reported as operational taxonomic units (OTUs) (Zhang et al, 2013;Lambeth et al, 2015;Egshatyan et al, 2016;Bhute et al, 2017;Allin et al, 2018;Nuli et al, 2019;Zhao et al, 2019;Gaike et al, 2020;Chavez-Carbajal et al, 2020;Ericson et al, 2020), metagenomic clusters (MGCs) (Karlsson et al, 2013;Wu et al, 2020), microbiota (log10 cell/g) (Chen et al, 2019;Ghaemi et al, 2020), metagenome (Wang et al, 2021), metagenomic linkage group (MLGs) (Zhong et al, 2019) or amplicon sequence variants (ASVs)…”