Sally James

Seattle Science Writer

Blog post

Scientists shovel dirt in book for parents

Picture of the cover of Rob Knight, Jack Gilbert, and Jack A. Gilbert's book entitled, "Dirt is Good".

Should you wash the pacifier, if it falls on the ground, or should you lick it off and give it back to your child?

Two scientists tackle this – and hundreds of other questions about dirty and clean in the new book, “Dirt is Good: The Advantage of Germs for Your Child’s Developing Immune System.” Hint – They fall on the side of licking and returning the pacifier, with qualifications.

Both Jack Gilbert and Rob Knight are parents. They’ve been sleepless in the nursery, and worried about vaginal versus c-section births. They’ve framed the science as a series of parent questions from pregnancy and childbirth itself all the way up the playground years. Science nitpickers will find plenty of details about what is called the microbiome – the ecology of all the microscopic creatures who share our skin and our guts. (And our hair, and our armpits, and …)

Gilbert is a PhD and professor of surgery and director of the Microbiome Center at the University of Chicago. Knight is a PhD and professor of pediatrics and computer science at the University of California, San Diego. Gilbert and Knight are not medical doctors, and don’t give medical advice. Their co-writer, Sandra Blakeslee, wrote for the New York Times for many years.

These three bring academic heft to their answers, often quoting specific research projects that have established certain science principles. There are more than 180 references in the book’s appendix.

But scattered in the pages, there is room for humor.

“Should I wash dishes in the dishwasher or by hand? What’s best for microbial health?”

“Is it okay for my child to touch poop?”

Before we look at the answers to these questions, let’s just explain that the last few years have seen an explosion of discoveries about the benefit of healthy microbes in the human gut. People who are infected with one microbe, C. difficile, are sometimes cured by a transplant of feces from a healthy person.  Conditions such as depression and autism have been studied by comparing the microbes living in the intestines of patients with and without those issues. But the explosion of research has led to a corresponding explosion of hype.

Parents may feel especially confused. Perhaps you have heard that exposing children to a certain amount of dirt may help protect them from developing allergies. There is truth to that, and this book goes into pages of details.

On the dish question, the scientists explain that the extra-hot drying cycle of some dishwashers is probably too much cleanliness. They recommend hand washing dishes. As they write, “You want your kid to be exposed to more, not fewer, bacteria in daily life.” On the poop question – they are generally saying it is OK but there are important limits.

Should I take my child to a farm? Should we get a dog?

With careful but clever explanations, the scientists say “yes” to both the farm visits and the dog. But they may surprise you with some other answers. They don’t routinely advise eating yogurt, for example, even though it has the reputation of carrying “good” microbes, often called probiotics. Probiotics is the opposite of antibiotics, which kill bacteria. Probiotics often contain what are called “good” bacteria.

But the book explains that there is no evidence that probiotics always make people’s guts healthier. The whole field of what is known as probiotics is filled with claims that promise one outcome, but frequently don’t deliver it.

Should I give my child probiotics if he has diarrhea?

In this example, the scientists say “Yes.”   Studies do show the value of giving an infant with diarrhea a probiotic. The precise answer to the question gives an education in how the relatively young science of microbial health defines “good” bacteria, and how there is evidence for infants that shows certain probiotics help in that situation.  But they also explain exceptions and nuance.

Reading the whole book may leave you with an awestruck appreciation for what is not known. The true dimension or “population” of microbes that constitutes the healthiest possible gut is not established yet for most people. There are layers and layers of complexity surrounding what we eat, because some of what we eat is “food” for the bacteria themselves and other things we eat are broken down into elements that we actually absorb to power our own cells.

In the 14 chapters and hundreds of questions, the authors slowly educate us to a awesome admiration for the colonies that share our body.  Each person is a planet. Each part of the body carries its own colony of microbes, which thrives in a dynamic communal way, competing with each other for certain resources, but also driving off invaders in some cases that might make the person-planet sick.

For a deeper dive into what is called the “hygiene hypothesis” and how exposure to more diversity in childhood may keep children from allergies and asthma, see this New England Journal of Medicine article.

The more we know about the complicated interplay of these creatures, the more questions we may have. It isn’t an easy time to wash the dishes.


Review of book by pediatrician Perri Klass of The New York Times.

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