Wash your hands all the time. Sterilize your kitchen counters. Take antibiotics whenever you have a cough or an itchy throat. Avoid dirt and pets and farms. Do everything you can to keep away those germs.
Such antimicrobial views used to be broadly accepted and beaten into us further by many in the medical community and by diverse marketing efforts. Sure, we have known about the important role of beneficial microbes in the lives of various plants and animals for many many years. And we certainly knew about some “good microbes” living in and on us. But somehow humans seem to have been generally viewed as – well – bigger than all this. Better. Able to go about our daily lives without depending on others. Especially things so – well – small.
But in recent times, there has been a slow and now accelerating progression towards embracing our microbial connections. Probiotics and prebiotics are everywhere. There are new developments every day it seems on microbiomes (the sum total of the microbes that live in or on an organism) and their role in the human condition. I go back and forth about how I feel about this. On the one hand – I am studying microbial communities on plants and animals (including humans) because I believe they play many roles – including many yet to be discovered. On the other hand, I spend perhaps a bit too much time trying to dampen down the hype in the field. I blog about “overselling the microbiome” and I try myself to not overstate things. My biggest fear in giving a talk at TEDMED on the microbes among us which has now been picked up by TED was that I would somehow overstate the importance of the human microbiome.
In the midst of all of this building appreciation about our microbial overlords, there has been one area where humans were somehow still viewed as separate – the womb. A pervasive view among the medical community and the wider world of scientists and the public has been that we humans are sterile in the womb and that we only get colonized by microbes after birth. And on top of that there has been the impression that microbial colonization is somehow a mostly random, environmentally based process.
But these notions of colonization semi-randomly after birth are also beginning to change. Now, in a fascinating piece in PLOS Biology Funkhouser and Bordenstein discuss how in the animal world maternal transmission of microbes to offspring is pretty much the rule. And they also discuss how this seems to be the case in humans too. We are in fact not (apparently) sterile in the womb. And vaginal birth and breastfeeding can be viewed largely as delivery mechanisms for microbes (and the food for the microbes).
Check out their Figure 1 for a summary of some of the details they have collated about maternal transmission of microbes in humans.
Of course, in retrospect this all makes sense. Many key properties of animals actually come from symbioses with microbes. And for the mutually beneficial interactions it would make further “adaptationistic” sense that mothers would want to deliver good microbes to their offspring. Now of course, just because someone can make up a just-so story about how such a transmission should be beneficial does not mean that (a) it is beneficial or (b) such a transmission would evolve. But as they document in their paper maternal transmission of microbes (good and bad mind you) is a pervasive feature of animal life. So why should such things not happen in humans?
In fact, I would argue that the argument of Funkhouser and Bordenstein does not go far enough. Why limit the concept to animals. Certainly, as we learn more and more about the importance of microbial communities in the life of plants, we might expect to find similar delivery and feeding mechanisms to be pervasive in the plant world too. And of course, there are other multicellular organisms out there beyond plants and animals that also depend on microbes for many functions. Consider many fungi – which have extensive microbial interactions. And kelp (a brown algae – not closely related to plants) for which the microbial interactions are now being examined in more and more detail (e.g., see Michelou et al. 2013). And of course many microbes themselves likely have evolved detailed mechanisms for transmitting symbiotic (other) microbes to their offspring.
So what does this all mean? Of course, just because microbes are delivered to offspring does not mean that those microbes are “good” for us – as “bad” and “neutral” microbes can also be delivered too. However, this perspective certainly means that for studies of humans we really need to delve much deeper into the ways that microbes are transmitted from parent to offspring and how activities like breastfeeding affect the establishment of microbial communities. And, it does give everyone a convenient excuse if something goes wrong with their microbial community. You can just blame mom. But if things go well, perhaps everyone should remember to thank their mother. After all, “Mom Knows Best.”
Full citation of the referenced paper:
Thanks to Keith Bell for (though a bit aggressively) making me continually reconsider the notion that humans are sterile in the womb.
UPDATE 8/22 12:52 PM PST:
Check out the blog post by author Seth Bordenstein about the paper: The Story Behind “Mom Knows Best: The Universality of Maternal Microbial Transmission”