## 3 Results
### Gut Microbiota Composition  
#### 3.3.1 Age  
A marked difference in diversity was found between infants compared to the other age groups (Figure 1A). Infants showed a significant lower alpha diversity, in particular when compared to those aged 6-17 and >18 years (Table 4).  
As for bacterial composition, the phyla Proteobacteria and Actinobacteria were more abundant in <1 year of age, and the Firmicutes and Bacteroidetes phyla were more abundant in other age groups. In detail, the Christensenmelaceae, Rikenellaceae, and Succinivibrionaceae families were absent in subjects <1 year of age, while a high prevalence of Actinomycetaceae was observed within this group (Figure S1A). Moreover, Enterobacteriaceaeaceae abundance (0.30025 (0.0005-1)) was higher when compared to subjects aged >1-5 (0.002875 (0.00025-0.26)), 6-17 (0.00375 (0-0.67675)), and \(\geq\)18 years (0.004 (0-0.32125)). An opposite trend was observed with Prevotellaceae (0 (0-0.5625)), which was less abundant in subjects <1 of age than in age groups 1-5 (0.3544 (0.00-0.8035), \(p<0.05\)) and 6-17 years  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Alpha Diversity** & **<1 year (9)** & **1–5 years (16)** & **6–17 years (36)** & **>18 years (15)** \\  & **Median** & **Median** & **Median** & **Median** \\  & **(min–max)** & **(min–max)** & **(min–max)** & **(min–max)** \\ \hline \multirow{2}{*}{Shannon index} & 1.703 & 3.411 & 3.703 & 3.631 \\  & (0.6059-3.019) & (2.2730-4.131) c & (2.1940-4.962) a ** & (1.5140-4.589) c  \\ \hline \multirow{2}{*}{Observed OTUs} & 39.00 & 151.00 & 189.00 & 177.00 \\  & (19.00–86.00) & (79.00–258.00) b  & (94.00–459.00) a  & (82.00–305.00) a  \\ \hline \multirow{2}{*}{Faith’s phylogenetic diversity} & 4.601 & 12.850 & 16.560 & 17.540 \\  & (0.9572–12.04) & (7.2710–20.82) c  & (7.3410–34.15) a  & (8.4570–25.67) a  \\ \hline \multirow{2}{*}{Chao1 index} & 39.50 & 189.50 & 228.20 & 231.00 \\  & (19.00–95.07) & (98.60–313.30) b  & (105.50–610.00) a  & (100.40–406.90) b  \\ \hline \end{tabular}
\end{table}
Table 4: Alpha diversity indices in the age groups.  
Figure 1: Differences between age groups. (**A**): UniFrac distances among infants and other age groups. The samples were visualized using principal coordinate analysis (PCoA). Differences with other age groups are statistically significant. (**B**): The plot of top 15 relative bacterial abundances at genus level. The statistical significances are shown in Table S1A.  
(0.2610 (0.00-0.9328), \(p<0.001\)). Ruminococcaceae abundance (0.00025 (0-0.002)) was also lower than in the age groups 1-5 (0.29280 (0.06200-0.8785), \(p<0.01\)), 6-17 years (0.33000 (0.0010-0.9120), \(p<0.001\)), and \(>\)18 years (0.33000 (0.0010-0.9120), \(p<0.001\)). Finally, _Eubacterium coprostanoligenes, Eubacterium rectale_ group, _Lachospirace_ NK group, _Roseburia_, _Ruminococcaceae_ UCG-010, _Ruminococcaceae_ UCG-013, _Christensenelaceae_ R-7 group, and _Rikenellaceae_ RC9 were absent in subjects aged \(<\)1 year.  
The organisms most frequently present at the genus level are shown in Figure 1B; the related statistical significance in the abundance comparison between infants versus all other groups is reported in Table S1A.