Table 2.

Studies of gut bacterial assembly in preterm infants without necrotizing enterocolitis (NEC).

Study and location	Sequencing technology	No. Of subjects without NEC	No. of specimens	Conclusions about pattern of bacterial community assembly
Zhou et al. [16], Brigham and Women's Hospital, Boston, MA, USA	16S rRNA gene sequencing	26	111	Increasing proportion of Clostridia over time, balanced by slowly diminishing proportion of Gram-negative genera, with little effect of antibiotics on this trend
Ward et al. [17], Cincinnati, OH, and Birmingham, AL, USA	Metagenomic sequencing	89	185	Clostridia class increases over time (specifically veillonella and C. freundii), with consistently high Proteobacteria (specifically E. coli)
Shaw et al. [18], St Mary's Hospital, Queen Charlotte's and Chelsea Hospital, London,UKa	16S rRNA gene sequencing	44	369	Bifidobacteria and klebsiella increased in proportionality, and Gram-positive bacteria decreased in proportionality, over time
Gibson et al. [19], St Louis Children's Hospital, St Louis, MO, USA	Metagenomic sequencing	84	401	Some of these subjects and specimens were also analyzed in La Rosa et al. [21]. Notably, metagenomic sequencing recapitulated the 16S sequence analysis of this cohort in these two companion publications.
La Rosa et al. [20], St Louis Children's Hospital, St Louis, MO, USA	16S rRNA Gene sequencing	58	922	Bacterial classes proceed from Bacilli to Gammaproteobacteria to Clostridia in these infants, but these populations are prone to changes in content. When infants near 33–36 weeks postconceptional age (i.e., an interval that is equivalent to the 3rd to the 12th week of age, in view of the wide range of gestational ages in this cohort), the populations converge on a consensus community, with ∼40% of the bacteria being obligate anaerobes (especially Clostridia and Negativicutes), and an equal percentage being Gammaproteobacteria. There was little or no effect of use of postnatal antibiotics, mode of delivery, or breast milk, and the community composition at this point.
Warner et al. [21], St Louis Children's Hospital, St Louis, MO; Children's Hospital at Oklahoma University, Oklahoma City, OK; Kosair Children's Hospital, Louisville, KY, USA	16S rRNA gene sequencing	120	2720	Includes the 58 subjects without NEC and their 922 stools in La Rosa et al. [21]. Patterns in NICUs in Oklahoma City and in Louisville recapitulate those in St Louis cohort

NICU, neonatal intensive care unit.

^aBased on data from Supplemental Table 1 in [18].