A Preemies Gut: The Surprising Differences



Preterm infants have a tough start

Preterm infants – babies born before 37 weeks of pregnancy – face many challenges. Because their bodies are not as far developed as those of term babies – babies born after 37 weeks of pregnancy – they have difficulties handling their new environment.  In particular, infants born before 32 weeks of pregnancy and those of very low birth weight, might have a tough time breathing, staying warm, and digesting food. To help these infants cope with their new surroundings outside the womb, they are taken care of in a Neonatal Intensive Care Unit (NICU).

The formation of the gut microbiome after birth

One of the challenges that preterm infants face is the colonization of their gut with microbes. In the first moments of life, babies become colonized with microorganisms – microscopically small organisms such as bacteria, archaea, and fungi. These microbes live in and on the human body, and in the environment, and are transferred during birth and through skin-skin contact.

The microbial gut colonization of babies is a natural process in which the newborn’s intestine  gradually becomes inhabited by large communities of microbes, the human gut microbiome. These gut bacteria have many functions, such as helping us digest our food and building a strong immune system. Any newborn baby goes through this colonization process immediately after birth and in the following months. However, this process does not always go smoothly for preemies, whose guts and immune systems might not yet be ready to handle these microbes.

How the newborn’s gut becomes colonized with microbes and which microbes will stick around depends on a lot of factors, such as how the baby is born (vaginally or by Cesarean section), how the baby is fed (with breast milk or formula), the environment in which the baby stays, and the use of antibiotics. Many of these factors are different for preterm babies, who spend their first weeks or months of life in the NICU.

A different gut colonization process for preemies

The microbial colonization of the newborn’s gut often follows a different path in preemies compared to term babies. Most preemies will receive antibiotics immediately after birth to reduce their chance of infections, but this practice can also disrupt the natural process of microbial gut colonization and result in lower bacterial diversity in the preterm gut.

Preterm infants may receive formula because their mothers might not yet produce enough breast milk. And because most preemies spend the majority of their time in a NICU incubator in the hospital, they become colonized with very different microbes than full term babies who go home with their parents to get cuddled, touched, and exposed to a home’s normal – and healthy – non-sterile environment.

Not surprisingly, therefore, the microbes found in the gut and stools of preterm infants differ from those of term babies. Preterm neonates are colonized with fewer Bifidobacteria, the bacteria that feed on the sugars in breast milk. On the other hand, their feces contain more skin bacteria and Enterobacteriaceae, such as Escherichia, Enterobacter, and Klebsiella species.

Some of the microbes colonizing preterm infants appear to come directly from their hospital environment. In a 2017 study, bacterial strains found in the feces of preterm infants were identical to those found on NICU sinks and surfaces, such as Enterococcus, Pseudomonas, and Staphylococcus species.

Gut troubles for preemies

Preterm infants, especially those born before 32 weeks of pregnancy, are at increased risk for many life-threatening health problems. Two of the most common problems that preemies have to face are sepsis and necrotizing enterocolitis (NEC). Both conditions are associated with underdeveloped tissues, disturbed colonization, and lower bacterial diversity of the preterm infant intestinal tract.

In neonatal sepsis, gut bacteria can cross the immature intestinal wall, which might not yet be tightly sealed. Once these bacteria enter the infant’s blood, the baby’s underdeveloped immune system cannot clean them up and a severe bloodstream infection might follow. The presence of Bifidobacteria in the preterm infant gut was found to be protective against this type of sepsis.

NEC is a severe inflammation of the gut of preterm, low-birthweight infants that is fatal in one in three cases. Studies that compared the gut microbiome of neonates with or without NEC found that the disease is often associated with a relative increase in the amount of Proteobacteria, and a decrease in Bacteroidetes and Firmicutes.

Nutrition is important

In addition to factors such as antibiotic use and hospital environment, the development of the preterm microbiome is greatly influenced by the duration of pregnancy, birth weight, time since birth, and nutrition. Breast milk feeding, which is associated with higher amounts of Bifidobacterium, has been found to help overcome the immaturity of the preterm gut, increase the diversity of the gut, and to be protective against NEC. Finding the optimal nutritional strategy for preterm babies is therefore believed to be a key factor in giving these tiny newborns the best chance of survival.