A recent Danish study has discovered that the gut microbiome of babies contains thousands of previously unknown viruses, providing new insights into the role of gut microbiomes in the development of chronic illnesses later in life. The research, published in Nature Microbiology, analysed the faecal makeup of 647 healthy 1-year-olds and found that the baby gut contains around 10,000 viral species – 10 times more than the number of bacterial species in the average child.

The newly identified viruses belong to a total of 248 viral families, with 232 of these families previously unknown. Professor Dennis Sandris Nielsen explained, “This means that, from early on in life, healthy children are tumbling about with an extreme diversity of gut viruses, which probably have a major impact on whether they develop various diseases later on in life.”

Around 90% of the detected viruses were bacteriophages, which attack potentially harmful bacteria, while the remaining 10% were eukaryotic, attaching themselves to human cells. Nielsen noted that the average child is infected with 10 to 20 of these eukaryotic viruses at any given moment, without becoming sick. “My guess is that they’re important for training our immune system to recognize infections later. But it may also be that they are a risk factor for diseases that we have yet to discover,” he said.

Shiraz Shah, the study’s first author and a senior researcher at Copenhagen Prospective Studies on Asthma in Childhood, suggested that the high viral loads in infants’ guts are due to their still-maturing immune systems, which need bacteriophages as a backup defence. He elaborated, “Our hypothesis is that because the immune system has not yet learned to separate the wheat from the chaff at the age of one, an extraordinarily high species richness of gut viruses emerges, and is likely needed to protect against chronic diseases like asthma and diabetes later on in life.”

The researchers believe that understanding the role of bacteria and viruses in a well-trained immune system could help prevent many chronic diseases. The study contributes to our understanding of the gut microbiome’s role in infant health and immune system development, potentially leading to new treatments and preventive measures for chronic diseases linked to early-life gut microbiome dysbiosis.