Home Sweet Microbial Home
Germs affect our health, influence our development, and may even shape our evolution
by Amy McDermott
The jewel wasps are elegantly named. Their metallic exoskeletons shine like precious stones. Light glints off of each mother wasp as she injects her eggs into the living body of a fly maggot, and glitters across the dark carapaces of her offspring, as they emerge fully-grown. The jewel wasps are parasites; they hijack fly larvae as nurseries for their young.
But the wasps have also been hijacked, although you wouldn’t know by looking.
All four species (Nasonia giraulti, N. longicornis, N. oneida, and N. vitripennis) are match-head small with shiny black bodies and six orange legs. They look pretty much the same, but a microscopic invader, invisible to the naked eye, divides these wasps into separate species.
The invader is Wolbachia —a genus of bacteria living in the ovaries and testes of jewel wasps, passed from parent to offspring in the jelly-like fluid of the egg. Each species of jewel wasp carries a unique Wolbachia infection, which is incompatible with other Wolbachia strains. When two different wasp species interbreed, their hybrid offspring inherit incompatible infections and die.
If Wolbachia is eliminated with antibiotics, all four wasps can successfully interbreed. The hybrids survive. Genetically, the jewel wasps are similar enough to reproduce; bacteria divide them into separate species.
We usually think of germs as drivers of disease, but as the jewel wasps illustrate, they can also drive evolution. Since the birth of genetics, scientists have looked to DNA to answer questions on the origin of species, but what if genes—the ones inherited from mom and dad, stored in the nucleus of every cell—aren’t the whole story? What if our microbes make us human too?
The microbiome is the collection of archaea, bacteria, viruses, and yeast living in and on each of us. We pick up our first set of microbes from our mother, while still inside the womb, and continue to accrue germs throughout our lives. Some of the microbes we encounter are helpful, others are harmful; the set we carry is not random.
Each species (including humans) has a characteristic set of microbes. And each individual is a patchwork of different microbial habitats. Your skin hosts different germs than your gut, which hosts different germs than your reproductive tract. Hell, even your left and right hand host different species of bacteria.
Some bacterial communities tend to vary between individuals, while others are more conserved. For example, if I went out and swabbed the mouths of New Yorkers, I would likely find a similar set of bacteria in everyone’s saliva samples. But if I tested everyone’s palms, I would find very different bacterial communities on the hands of men and women. Curiously, women have more species of bacteria on their palms than men do.
The cities of microbes on and in us play a variety of roles in our health. Bacteria in the human gut, for instance, help to digest food and release nutrients we otherwise would not be able to access. These 300 to 500 species also outcompete potentially harmful bacteria, or keep their populations at bay. By competing well, gut microbes benefit the body as a whole. And when the bacterial community is changed or disrupted, illness often results.
Microbes live on our skin and in our guts, but they still don’t feel like us. Under a microscope we see their tiny wiggling appendages and their micro-scale battles, and we think of them as tiny, brainless, different.
But in fact, bacteria are also a part of our bodies. They're not only on us, and in us, they are us.
We share an intimate history with them, which is stamped like a thumbprint in every one of our human cells. Our mitochondria—the essential energy-generating structures in our cells (without which we would not survive)—were once free-living bacteria too.
One and a half billion years ago, when Earth was covered in a soup of shallow seas, free-living mitochondria were swallowed by other free-living cells, but were not digested. They survived, and over time the mitochondria and their hosts exchanged genes and synchronized their replication.
Today, we see evidence of the mitochondrion’s independent history in its DNA. Unlike other cellular structures, mitochondria have their own genes, separate from the DNA in the nucleus of our cells. Mitochondrial genes are very similar to those of the proteobacterium Rickettsia, a free-living bacteria, that's still alive today. Perhaps the ancient mitochondrion would have resembled Rickettsia one and a half billion years ago.
We host specific microbes in our cells and on our bodies, which are critical for health and development. Now research suggests they might also shape our behavior.
In a 2010 study, molecular microbiologists at Tel Aviv University, Israel, split a population of fruit flies (Drosophila melanogaster): one group was fed on starch, the other on molasses. After a single generation, the molasses-fed flies preferred one another, as did the starch-bred flies. However, when both groups were treated with antibiotics, their gut bacteria were destroyed, and the flies freely interbred, regardless of their previous diet. The set of bacteria living in a fly’s gut influenced its sex pheromones, and therefore its preference for mates. Flies fed on different diets had different gut communities, produced different amounts of pheromone, and preferred mates who smelled familiar.
Now, imagine if the dietary separation had continued for millennia, with starch flies and molasses flies divided by sexual attraction. Over time, genetic changes would multiply in the two groups, until they were physically unable to reproduce. They would become separate species, because they had different varieties of gut bacteria.
Of course fruit flies are not humans, but they can teach us a lot about ourselves. They are a model organism, a widely studied species (particularly in genetics), which is easy to breed and maintain in the laboratory. Humans and flies have the same types, or families, of genes. So by studying fruit flies, we gain insight into ourselves as well.
We shape our microbial communities through diet and environment; they shape us too.
Germs affect our health, influence our development, and may even change our behavior (and mate-choice). Without them, we would not be ourselves. Our bodies are their evolving ecosystems, home-sweet-microbial-home.
Amy McDermott is a budding science writer, and graduate student at Columbia University, where she studies marine conservation biology. She can be reached at firstname.lastname@example.org