New findings in mice could pave the way for novel approaches to protect human babies from gastrointestinal infections.
Gastrointestinal infections are a common cause of diarrhea and vomiting and are triggered by germs including viruses, bacteria, and parasites. Bacteria - such as Salmonella and Escherichia coli - are estimated to be responsible for between 10 and 20 percent of cases of gastroenteritis in children.
Most cases of Salmonella and E. coli infection get better without treatment within 4 to 10 days. However, in young children with weak immune systems, there is a greater risk of complications, some of which include dehydration, kidney failure, and even death.
The University of Michigan Medical School explored mice of different ages to evaluate whether the gut bacteria Clostridia may play a role in protecting the mice from the life-threatening consequences of Salmonella and E. coli. The findings could lead to new approaches to protect human infants from gastrointestinal infections. The researchers' results were published in Science.
"Any parent knows that newborns are very susceptible to infections in the first year of life, including enteric, or gut, infections," says Gabriel Nunez, the study's senior author and a University of Michigan pathology professor. "This work suggests that the lack of protective bacteria in the gut microbiota is a mechanism for that susceptibility, perhaps more than the age of the immune system."
Clostridia are a group of more than 100 species of harmless bacteria that grow in the gut. The research indicates that Clostridia is absent in newborn mice but begins to grow days later. The absence of Clostridia makes young mice vulnerable to invading bacteria, which is similar to the pathogens that infect human babies.
With the knowledge that newborn mice have no natural gut bacteria, the researchers saw an opportunity to observe how gut bacteria microbes from mice of different ages would affect newborn mice's vulnerability to infection when transplanted into their guts.
Advanced DNA analysis techniques were used to detect the types and measure the amounts of bacteria in the guts of the mice.
Clostridia reduced harmful bacteria, lowered risk of death
Nunez and colleagues used newborn and adult germ-free mice to carry out a series of experiments. Gut microbe samples were extracted from the feces of 4-day-old, 12-day-old, and 16-day-old normal mice.
The team found that the 16-day-old mice had the most diverse range of gut microbes - including Clostridia and Bacteroides bacteria - which were not observed in the younger mice that were still receiving all their nutrition from their mother's milk.
Germ-free mice received a transplant of bacteria from either 4-day-old or 16-day-old normal mice. They were then exposed to a strain of Salmonella that would infect the gut only. Of the mice that received the 4-day-old microbes, half of them died. However, none of the mice that received the 16-day-old microbes died.
Next, the investigators tried the same experiment using Citrobacter rodentium - a strain of bacteria similar to the E. coli strains that affect humans. Many of the mice that received the 4-day-old microbes died. The surviving mice were then given a transplant of the 16-day-old microbes, which rapidly decreased the amount of C. rodentium in their guts.
The researchers subsequently wanted to find out what would happen if germ-free mice were given the microbes from a newborn mouse but which contained extra doses of either Clostridia or Bacteroides bacteria. When the groups of mice were exposed to C. rodentium, only the mice that had received Clostridia were able to resist infection.
Similarly, 90 percent of mice that received Clostridia were still alive a week later and after being exposed to Salmonella, compared with those who had not received Clostridia.
Salmonella and E. coli also affect adults, although to a much lesser extent, so the researchers decided to evaluate the consequences of giving vancomycin - an antibiotic that destroys Clostridia and Bacteroides - to adult mice. The Salmonella and C. rodentium thrived in this weakened gut environment.
Protection from infection significantly increased by adding succinate
Nunez and collaborators also compared the roles of the body's immune system and gut microbes in fighting infection. They achieved this by studying mice with impaired immune systems. The mice were given 4-day-old microbes and exposed to Salmonella. The mice were able to fight off infection despite their weakened immune systems, but only when they received an extra dose of Clostridia.
The final experiment assessed the impact of adding succinate to the drinking water of mice that had received 4-day-old microbes with added Clostridia. Succinate is a salt that bacteria that thrive on oxygen produce as a byproduct. Adding succinate to the mix provided the mice with even better protection against Salmonella, which suggests that Clostridia - that live in the absence of oxygen - feed off the waste products of bacteria that need oxygen, which flourish in the guts of newborns.
Clostridia begin to grow in the gut and prevent the growth of Salmonella and C. rodentium during the time that weaning mice transition from receiving the milk of their mother to being fed solid food. The team is beginning to look at what role breast milk plays in establishing a newborn's gut microbiome and how weaning to solids transfers microbes from the outside world to the child's gut.
"Normally, we acquire Clostridia strains in our guts when we begin to eat solids, but this work suggests a window of vulnerability to enteric pathogens in the early stages of life."
Future work by the University of Michigan Medical School will test the particular strains of Clostridia that are most effective. If, in further studies, the protective role of added Clostridia for newborns is confirmed, there might be potential for a clinical trial in humans to test a combination of strains, Nunez concludes.