Effect of intestinal microecology on postnatal weight gain in very preterm infants in intensive care units

Preterm infants can suffer from immature immune system and physiological characteristics. The establishment of the neonatal intensive care units (NICU) and the extensive development in advanced life support for preterm infants have generally improved their survival rate. Very premature infants need to be admitted to the NICU after birth and receive respiratory, nutritional, antibiotic and other support treatment. In 2019, the incidence of preterm infants was reported by the World Health Organization (WHO) to be about 10.6%, with an average of 14.8 million premature births per year. Among these births, more than 1.1 million happen in China, with an incidence of 6.9% [1]. A common phenomenon is that these infants have different degrees of feeding intolerance and growth retardation, and it is challenging for them to establish a normal intestinal flora [2]. Healthy intestinal microecology plays an indispensable role in the neonatal intestinal development, maintaining the integrity of the intestinal mucosa and nutritional status of the host [3]. Therefore, maintaining the colonization of the normal intestinal microflora is undoubtedly a key factor contributing to the overall health of newborns. Extrauterine growth retardation (EUGR), considered as a body weight less than the 10th percentile of the corresponding gestational age or a weight loss between birth and a given time of  >  2SD, does not only affect the growth of infants, but also affects the occurrence and development of diseases, leading to prolonged hospitalization period and long-term development retardation, finally affecting the brain development and the occurrence of metabolic diseases in adulthood. In this work, we monitored very preterm infants with a gestational age of less than 32 weeks at the NICU of our hospital and investigated the association between growth and the intestinal microecology in order to find a new way to improve the extrauterine growth of very preterm infants.

In the neonatal period, the intestinal microflora undergoes special dynamic changes, and its colonization and composition are affected by the body weight, gestational age, delivery mode, feeding mode as well as the living environment and drug use (such as antibiotics) [8, 9]. In this study, a total of 22 very preterm infants were included, among which 10 had growth retardation at 4 weeks after birth (10/22), and the incidence of EUGR was about 36–45%. In Tokyo area of Japan, the incidence of EUGR was reported to be 8.4% [10]. In 2016, Griffin et al. [11] analyzed the data of the California Perinatal Quality Care Collaborative from 2005 to 2012 and found that the incidence rate of EUGR at discharge of very low birth weight (VLBW) infants was 52.7 and 44.4% in the 1000–1249 g and 1250–1500 g weight groups, respectively. In 2017, Park et al. [12] reported that at 40 weeks of corrected gestational age, the incidence of EUGR was 58.4%. In China, the incidence of EUGR was higher. A multicenter survey of 572 VLBW infants in 15 hospitals across the country in 2015 showed that the incidence of EUGR at discharge was 80.9%, and the cases with a weight  <  3rd percentile accounted for 63.6%. The high incidence of EUGR reveals the challenging nature of the nutritional status of VLBW infants in China during hospitalization [13]. The lower the gestational age, the lower the birth weight and the higher the incidence of EUGR [14]. In our study, the incidence of EUGR was shown to be similar to that of foreign countries, which might be due to better management of preterm infant in the NICU. All the included very preterm infants were admitted to the NICU immediately after birth, and the intestinal microflora composition was detected by performing 16S rRNA high-throughput sequencing. The results showed a dynamic change in the diversity of intestinal flora of very preterm infants over time. The OTU number of the infants in the control group was significantly higher than that in the EUGR group, especially at 2 weeks after birth. The intestinal microflora in the EUGR group had a decreased diversity, but the difference compared with the control group was significantly reduced in the 4th week. Very preterm infants are faced with great challenges after birth, and their intestinal environment is very immature at first. Besides, the invasive operation of the NICU in the early stage of birth and the particularity of the NICU environment, which includes a more strict disinfection and sterilization system and a massive use of broad-spectrum antibiotics, result in a special composition of environmental microorganisms. Therefore, very preterm infants in the NICU have particular colonization model of intestinal bacteria, species types and diversity [15] (Additional file 2: Table S3–S4).

We performed a comparison between the composition of microbial community in the EUGR and control groups and found that Enterococcus was the dominant intestinal microflora in the two groups. However, the proportion of Enterococcus in the EUGR group was significantly lower than that in the control group, and the proportion of pathogenic bacteria, such as Streptococcus, was significantly increased. The LEfSe analysis showed that in the 2nd week after birth, Streptococcaceae and Streptococcus in the EUGR group and Enterococcaceae and Enterococcus in the control group were the most significantly different bacterial species between the two groups. Although the contents of Bacteroidetes, Bacteroidales and Stenotrophomonas maltophilia were also significantly different between the two groups, they had low proportions in the community composition of the two groups, which has no clear clinical significance. Streptococcus is a common pathogen that causes early-onset neonatal infection. Group B streptococcus (GBS) is the most common cause of early septicemia and meningitis in neonates. The mortality of early-onset GBS infection in full-term infants is 2–3%, while it reaches 20% in preterm infants and 30% in preterm infants with a gestational age of less than 33 weeks [16]. An analysis of 1,04,186 very preterm infants admitted to 312 NICUs in the United States from 1997 to 2011 showed that the rates of early-onset and late-onset GBS infection were 10.2 and 11.8%, respectively, such that early-onset infection increased the risk of death [17]. In China, the conducted studies showed that the incidence of neonatal infection in GBS-positive pregnant women was 29.8%, which was significantly higher than that in GBS-negative pregnant women (13.2%). Our results indicated that the proportion of Streptococcus significantly increased in the EUGR group at 2 weeks after birth, which suggests that growth restriction is more related to the disease status. Schwiertz et al. [18] analyzed the stool microbial diversity of 29 preterm infants at the NICU and 15 full-term infants within 4 weeks after birth and found that it took preterm infants 10 days to reach homeostasis. Jacquot et al. [19] showed that almost no Bifidobacterium was detected in preterm infants within 8 weeks after birth. Our results also indicated that intestinal microflora was close to steady state at 4 weeks after birth, but no Bifidobacterium was found in either group, which may be related to the special environment of the NICU.

EUGR represents a risk factor for neurodevelopmental abnormalities [20, 21]. Since the intestinal microflora maintains a bidirectional interaction with the central nervous system (CNS) through the gut-brain axis, its effect on newborns is not limited to the intestinal tract. Metabolites from the intestinal microflora disorders can destroy the blood–brain barrier (BBB), producing harmful components, which can then enter the brain more easily causing brain damage [22]. The intestinal colonization of Bifidobacterium can weaken the hypothalamic–pituitary–adrenal (HPA) axis response, and this inhibitory effect occurs in the early stage of life, which indicates that the original microbial exposure is necessary to inhibit the neural regulation of the HPA axis [23]. A recent study conducted by Bercik et al. [24] showed that after transferring the feces of the donor mice to the recipient mice, the recipient mice showed a similar behavioral phenotype to the donor mice, suggesting that the intestinal microbes can communicate with the brain through certain mechanisms.

The establishment of intestinal microecology in very preterm infants undergoes dynamic changes over time and differs between preterm infants with EUGR and those with normal growth. The diversity and richness of the intestinal microflora in preterm infants in the NICU are significantly insufficient. Early detection of the intestinal microflora and early intervention can improve the growth of very preterm infants. Probiotics have a high strain specificity in the intestinal tract, and different probiotics have different functions; questions such as the colonization rate and time (short-term or long-term), suitable strains and doses remain to be investigated. In future work, we will investigate the types of probiotics that can effectively reduce the growth retardation of very preterm infants when they are early administrated.