Sixteen-month-old Stone Yeatts unearthed a small rock from the grassy backyard of his Bala Cynwyd home — and promptly put it into his mouth, sucking on it like a piece of candy. His sister Elfey, 3½, licked a similar find.
Some moms might have yelled "Yuck!" and yanked these dirt-laden treats the moment they hit the kids' tongues.
Not Jay Grebe Yeatts. On her watch, dirt is no big deal. After a few seconds, she calmly asked Stone to spit out the prize. She was concerned he might choke.
"I think a lot of things are gross," allowed the 34-year-old mother of five who was hosting a toddler playdate. "I just can't worry about them. I don't have time to sterilize this, scrub that."
The other moms agreed. Kathy Freyhof, 32, of Wayne, watched year-old Emma dig out mud-laced acorns, and Teresa Frizell, 38, of Bala Cynwyd, followed 13-month-old Melody Jones as she crawled all over the place while the resident dog ran around. Besides, Yeatts said, speaking for the trio, "it's not going to kill them or seriously hurt them in any way."
Research on the microbiome — mostly 40 trillion microorganisms of bacterial cells that populate the human body — increasingly backs up what parents like Grebe have known all along in their, well, guts.
"The notion that most bacteria, or germs, are intrinsically bad — and must be killed by any means possible — is widespread," say authors and leading microbiome scientists Jack Gilbert and Rob Knight in their new book Dirt Is Good (St. Martin's Press, $25.99), written with New York Times science reporter Sandra Blakeslee. "But it's wrong, dangerously wrong. … What they encounter in those early years serves as a critical inoculation for their well-being. This is why dirt is so good."
Of course, playing in dirt contaminated by lead or chemicals is not good. A recent Inquirer investigation revealed the serious health concerns that some children who live in Philadelphia's river-ward neighborhoods face from high levels of lead in their blood because of contaminated soil, a legacy of the area's industrial past.
Provided that kind of contamination is not an issue, Gilbert and Knight, co-founders of the Earth Microbiome Project, a massive scientific collaboration to analyze the global microbial community, and American Gut, a major crowd-sourced, citizen-scientist project, argue that early exposure to germs (including those dirty rocks) may play a key role in reducing allergies and asthma. Microbes may even affect obesity, autoimmune diseases such as multiple sclerosis and rheumatoid arthritis, and how the young brain performs, associations that need more examination.
"It's an enormously complex ecosystem," Gilbert, a professor of surgery at the University of Chicago, said in an interview that included Knight, a professor of pediatrics and computer science and engineering at the University of California-San Diego. "When that ecosystem becomes disrupted, it can actually disrupt how your child's health and well-being develop."
Take asthma. Several studies suggest that children raised around farm animals are less likely to get asthma and allergies. A 2016 New England Journal of Medicine study, in which Gilbert was involved, compared Amish children, who routinely help with farm animals, with Hutterite children, who do not. Despite similar lifestyle and diet, the Hutterites, a German speaking-sect that shares its religious roots with Mennonites and Quakers, suffer from asthma at about seven times the Amish rate.
The researchers argue that exposure to the barnyard — and the associated bacteria and viruses — made the difference.
Growing up with a dog is associated with an average 13 percent reduction in the likelihood of developing asthma, and the linkage is even stronger for those who grow up on farms, according to Dirt Is Good, which uses a parent-friendly Q&A format to share the latest research.
"That exposure triggers your immune system to become less inflammatory, less likely to overreact to stimulus," Gilbert said.
Michael Silverman, an infectious-diseases physician at Children's Hospital of Philadelphia, seconded the benefits of getting down and dirty. "For most of our history we've been exposed to a lot more dirt and germs than we are now," he said. "Our immune system and body have evolved to get all of those exposures."
One emerging area of research involves the young brain.
"We're starting to see that the bacteria in the gut is associated with cognitive performance," Gilbert said. "Bacteria influences the immune system, and the immune system can affect how the brain develops. … We're also seeing bacteria that produce neurotransmitters. When the balance is disturbed, it also can affect the neurocognitive performance and development of the brain in the child."
In one Gilbert project, stool samples taken from low-income children with learning disabilities were introduced into lab animals. "The microbacterium associated with that environment are causing the animals to suffer poor neurological performance," he said.
It all can sound like one more thing for parents to worry over.
"We have to be careful about blaming everything on our microbiome," Silverman said. "It's a new area, and there is a lot to learn."
Gilbert and Knight are leading the way.
"We're only really scraping the surface of potential therapies that could be enabled," Gilbert said. In the not too distant future, a child's microbiome might be tweaked in the first three years, when it's most malleable, to improve health. Targeted therapies and designer probiotics are already in the pipeline, the authors say.
In the meantime, trials show that the probiotic L. rhamnosus GG can significantly reduce the severity of diarrhea in children. And a fecal matter transplant (yes, it's what you're thinking) is effective in treating recurrent Clostridium difficile colitis. Trials are underway with other diseases.
Silverman emphasized that the impact of all those germs — or even the use of probiotics — is a complex question.
"One probiotic or microbial exposure that's good for me may not be good for you," he said. "Genetics affects how we might respond."
He also cautioned that children with compromised immune systems due to conditions such as cancer needed to be extra careful about exposure to germs. Asthma or allergy sufferers, of course, should avoid allergens.
Pediatrician Dina DiMaggio of the American Academy of Pediatrics "tries not to sweat the small stuff." Still, she encouraged washing that dropped pacifier with soap and water before giving it back to the baby, and wiping off that public toilet seat, then covering it with toilet paper. In the kitchen, she recommended thoroughly scrubbing countertops after cooking raw meat, fish, or eggs to protect children from harmful bacteria.
Said Knight: "You want to think about the relative risks," meaning that the potential benefits of encountering some dirt could outweigh the downsides.
In Terri Gordon's family, most germs are welcome. The New Jersey schoolteacher and mother of two young boys tries to avoid giving her children antibiotics (her husband, a doctor, agrees with her that they're not needed for most ear infections), isn't overly concerned about the cleanliness of public restrooms, and doesn't bother to wipe the shopping cart before touching it.
"If a cookie falls on the floor," said Gordon, 33, "we say, `Just eat it. It's an immunity cookie.' "
Gilbert and Knight's work is rooted in the hygiene hypothesis, which suggests that too-clean environments do not effectively challenge the immune system. "A wide range of exposure to non-pathogenic microbes is quite good," Knight said.
Indeed. When his daughter was born by C-section, he swabbed the baby's mouth and face with his partner's vaginal fluids.
At the same time, he emphasized the importance of keeping kids current on the standard vaccination schedule.
Meanwhile, Gilbert got gasps when his young child's toy fell in a public urinal and he washed it off rather than trashing it. Of course, this is the man who says he licks his hand when he lectures on the microbiome, touches the floor, and then licks it again.
For the rest of us, Gilbert and Knight offer these less-gross tips:
"Balance," Gilbert says, "is everything, making sure we balance hygiene with exposure."
Contact Lini S. Kadaba at Lkadaba@gmail.com. Follow @exinkygal.