A complex infection
Immature intestines at risk for destructive infection, inflammation
Steve McElroy, M.D., assistant professor of Pediatrics at Vanderbilt University Medical Center, recalls the case of a baby girl born at 27 weeks and weighing 1,000 grams (2.2 pounds).
Like most premature babies, she had a history of eating intolerance. Her belly swelled and became tense, and she went into respiratory failure that required her to be intubated. When surgeons performed exploratory surgery, they discovered that only about 3 inches – 5 percent -- of her small bowel were viable. The cause was necrotizing enterocolitis.
“The end result was we had to close this baby’s belly up, hand her to her parents and withdraw from care,” McElroy recalls.
Necrotizing enterocolitis (NEC) affects 1 percent to 3 percent of babies admitted to intensive care nurseries. Primarily a disease of prematurity, it affects one in 10 infants born weighing less than 1,500 grams (3.3 pounds).
The mortality rate is between 20 percent and 50 percent, which means about 2,700 infants die from necrotizing enterocolitis every year in the United States. Babies who survive have significantly increased risks of feeding problems, liver failure and developmental delays.
“The process happens very rapidly, and you can’t predict who it’s going to happen to,” says McElroy. “It often happens within 12 hours, and is a very devastating disease for these babies.
“We’ve known about this disease about as long as there have been neonatologists,” he continues. “We’re not any better at treating it today than we were back in the 1970s. Part of that is because it’s a very complex disease.
“While we have been studying it for half a century, we don’t fully understand it or why some babies get it and why some get it worse than others. It’s a disease process that’s ripe for investigation.”
Necrotizing enterocolitis begins with an infection.
A single layer of epithelial cells line the intestine and separate us from the outside world where bacteria live and outnumber us 10 to 1.
At birth, a newborn’s intestine is relatively sterile. Once a baby starts taking milk – either formula or breast milk -- bacteria proliferate and become the large “microbiome” inside our intestines.
For unknown reasons, something causes mild mucosal damage at the surface of these epithelial cells. When that happens, the normal bacteria that reside in the intestines adhere to the epithelium, causing inflammation and damage to the bowel.
Tumor necrosis factor (TNF) is one of the key players in NEC. In the 1990s researchers found that plasma levels of TNF were much higher in babies who develop the disorder than in age-matched controls.
“We also know if you inject mice with high doses of TNF, you will cause bowel necrosis that looks very much like NEC,” McElroy said.
TNF is a potent inflammatory cytokine that helps to regulate cell survival during normal development. However, in inflammatory diseases such as NEC, TNF increases inside the epithelial cells and induces additional cytokines which set up a perfect storm of inflammation within the epithelium.
This in turn increases membrane permeability, allowing for bacterial invasion into the epithelial cells and eventually into the bowel wall. Once inside the bowel wall, bacteria feed on the infant’s tissues causing fermentation, which produces gas.
Trapped inside the bowel wall, the abnormal air forms a characteristic pattern of pockets that on X-ray films resembles a chain-link fence.
“We as neonatologists can’t pick up this disease until this point. So, by the time we can say a baby has NEC, it already has so much disease process that bacteria has eaten away at the baby’s intestine,” says McElroy. “This leads to necrosis (death) of the bowel.
“But perhaps even more frustrating is that the damage often happens in a pattern similar to leopard markings. You have areas of necrotic lesions all throughout the bowel – not just one section – which makes it difficult for the surgeons to know what areas to remove.”
Only maternal breast milk is known to protect against NEC. Compared to breastfed-only babies, formula-fed infants are six to 10 times more likely to develop NEC, and babies fed both formula and mother’s milk are three times more likely to suffer from the disorder.
“We know breast milk is a very good thing to give these babies. We just don’t know what part of it is such a great thing,” McElroy says.
Many breast milk components have been studied, including epidermal growth factor (EGF). Investigators know there is a role of EGF in NEC. Mice that are deficient in the EGF receptor will develop intestinal necrosis similar to the human disorder.
“EGF promotes bowel healing by increasing the replication and migration of epithelial cells,” McElroy explains. “However, expression of EGF receptor is lower in premature infants compared to adults. EGF is important in preventing NEC, and premature infants have lower levels of the major receptor of EGF than adults.
“This coupled with the fact that TNF causes a disruption of normal EGF receptor signaling may be one of the reasons they are more susceptible to the disease,” he continues. “That is why our lab focuses on the roles that TNF, EGF and their receptors play in the mechanisms of gastrointestinal injury and repair during development.”
Some studies outside the United States show that probiotics (helpful bacteria) decrease the incidence of NEC in premature babies. The first study of this kind was done in Colombia, South America, where babies were given a combination of two probiotics.
“They were able to demonstrate an almost 50 percent decrease in the incidence of NEC from the year prior to the year it was being used,” says Mario A. Rojas, M.D., associate professor of Pediatrics at Vanderbilt.
These findings stimulated similar research in other countries, but there are very few under way in the United States thus far, in part due to the fact that probiotics are currently sold as nutritional supplements.
To receive federal funds to study probiotics in a specific disease, the U.S. Food and Drug Administration requires that the investigator submit an investigational new drug application, which can delay and increase the expense of the study.
“I’ve been trying to do research with this ever since this study came out in the 1990s,” Rojas says. “We feel there is enough biomolecular and clinical data to support a well-designed multicenter trial, but funding has been very difficult to obtain in the U.S.”
So Rojas has turned his attention to probiotic studies outside of the United States. He has currently enrolled 250 patients in a double-blinded randomized controlled trial in nine medical centers in his native Colombia. The focus of the study is a probiotic called Lactobacillus reuteri, which has been shown to be effective in the treatment of babies with colic.
The gastrointestinal (GI) tracts of babies admitted to the intensive care nursery are colonized by Gram-negative (pathogenic) bacteria very quickly. Premature babies have immature GI tracts that allow pathogens to move from the mucosa into the bowel wall, where they produce inflammation and damage, and then into the bloodstream. The result is sepsis, an overwhelming, body-wide infection that is life-threatening.
Probiotics compete with the “bad” bacteria to colonize the gut. Lactobacillus reuteri also produces a substance, “reuterin,” which can kill pathogens directly.
“If we are able to avoid predominance of Gram-negative pathogens in the GI tract by administering Lactobacillus reuteri early on, we can decrease the risk of … damage to the mucosa and bowel wall,” Rojas says. “We’re trying to prove that in our trial. In developing countries there is a predominance of Gram-negative sepsis; we suspect probiotics can also decrease these infections.”
Rojas hopes to enroll 1,100 babies in this multi-center randomized controlled trial in which babies are randomly assigned to groups receiving probiotic or an inactive “placebo.”
“We’ve been very careful designing this study in order to have the power to demonstrate a difference between placebo and probiotic,” he said. “Probiotics are a low-cost, high-impact intervention that can be used at a global level.
“If we can demonstrate Lactobacillus reuteri administration to preterm infants is safe and efficacious, then we have an excellent opportunity to decrease global infant mortality.”
Current research is also investigating the use of these helpful bacteria in acute and chronic diarrhea of infants and children, and in inflammatory bowel disease and colon cancer in adults.
“We need to overcome the barriers to doing research with probiotics in the United States,” Rojas says. “We should have done these studies a long time ago. The cost of doing research in the U.S. has become a tremendous barrier for low-cost interventions like probiotics.”