They were here billions of years before complex life formed on earth. They still make up the vast majority of the biomass on our planet. And an estimated 99.999 percent1 of them have yet to be discovered. Bacteria are among the most important—and least understood—lifeforms on this planet. Thanks to decades of sustained scientific interest and effort, that’s beginning to change. We now know bacteria play an outsized role in the health and diversity of all life on earth, from towering redwood trees to the cells in our body. Here are some of the most remarkable ways they keep the biological world humming.





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Not all bacteria interact mutualistically with their hosts. Some behave more like a parasite, to varying degrees of horror and intrigue. Consider, one of the most common parasitic microbes on the planet. Bacteria from this genus are thought to infect approximately 40% of the insects on the planet. We’re talking hundreds of thousands—possibly even millions—of species. That’s an impressive feat, considering the fact thatcan only spread through female hosts.This limitation has forcedto employ some very creative tactics to ensure it always has a large selection to choose from.When they infect certain kinds of wasps, for instance,are able to induce a form of asexual reproduction called parthenogenesis, which allows infected females to mate without males. When they infect certain kinds of moths, they can induce feminization, which results in host males that develop as females instead. And wheninfects certain kinds of flies, they can even alter the chemical composition of male brains, forcing them to be less aggressive, and therefore less competitive, in their mating rituals. That’s right—we’re talking about mind controlling microbes. Thankfully, they’re not harmful to humans.One of the reasons bacteria are so successful is their ability to achieve symbiosis, or a mutually beneficial bond, with other organisms. Oftentimes, this evolutionary effort leads to extraordinary outcomes and abilities for the bacterial host. The Hawaiian bobtail squid () is a glowing example of this—literally. Its underside contains a two-chambered light organ that is full of luminous bacteria called, which feed off sugars and amino acid solutions produced by the squid. At night, the glow of these bacteria matches the moonlight reflected on the ocean’s surface, masking the squid’s silhouette and hiding it from predators.The relationship begins at birth, where the cilia of newborn squid begin drawing in a mix of bacteria. When just fivecells touch the squid, a whole host of squid genes are triggered. Some of these genes create a toxic environment for other microbes, while others produce a substance that attracts even more. Once inside, a cascade of further changes seals thewithin., alone, has the ability to colonize the bobtail squid, and it alone has the power to establish the squid’s light organ and complete its journey to maturity. A recent studyshowed that the bacteria actually changes the gene expression in other organs of the host squid, too. These learnings led scientists to a profound discovery—that microbes can sculpt animal bodies.There are an estimated 400,000 unique species of plant life on earth, and all of them depend on mineral nutrients for survival. The most vital of these is nitrogen, which serves as a primary component of both chlorophyll and amino acids. Chlorophyll, as you probably know, is the compound plants use to produce essential sugars in partnership with water and sunlight, through a process called photosynthesis. Amino acids are the building blocks of proteins, which fuel the structural integrity and metabolic function of plants, allowing them to grow big and strong.You might assume the world’s plants get the majority of their nitrogen from the air. After all, it’s by far the most abundant element in earth’s atmosphere. The truth is that the gaseous nitrogen in our atmosphere (N) is useless to plants, because it is (mostly) inert. Have you ever heard the term bioavailability? It’s defined as ‘the degree and rate at which a substance is absorbed into a living system.’ For plants, nitrogen is only bioavailable in solid forms like ammonium (NH) and nitrate (NO). That’s where a special class of microorganisms called nitrogen-fixing bacteriacome into play. They’re able to transform the nitrogen gases in our atmosphere into solid nitrogen compounds that plants are able to use for their own ends. No Miracle Grow necessary.Bacteria are some of the most resilient organisms on the planet. They’re found just about everywhere, from hydrothermal vents on the ocean floor to permafrost deep in the arctic. Some species, toughened by billions of years of evolution, can even survive without oxygen and sunlight. It’s no surprise, then, that bacteria are also found inside of us. What is surprising is the sheer scale of our personal microbiomes—38 trillion microbes, mostly bacteria, living primarily in our gastrointestinal tracts. Fortunately, this diverse community of microorganisms is harmless. In fact, many of our resident bacterial strains and species are actually essential to our wellbeing.Consider dietary fiber. We know it’s good for us, but for a long time, we didn’t really know why. The digestive enzymes that our bodies produce aren’t capable of breaking it down and absorbing it for nutritional value, so there must be some other mechanism at work. Turns out, the mutualistic bacteria in your colon are perfectly capable of metabolizing this tough plant compound. When they do, they produce a critically important byproduct: short-chain fatty acids (SCFAs). And our bodies can use those. In fact, SCFAs are the primary source of energy for the cells lining your colon. That’s just the beginning, too. Bacteria are also responsible for the synthesis of key vitamins, the identification of foreign pathogens, and the strengthening of our gut barriers.Inside almost every single cell in your body live powerhouses called ‘mitochondria’. Through cellular respiration, they take in nutrients, break them down, and unlock the energy our cells need to function.⁣ Beyond this critical role, mitochondria also influence cell growth, division, and death, help determine a cell’s lineage, and support in the production of critical compounds like iron and hormones like cortisol and estrogen. Just about every cellular process is linked to mitochondria in some way. Without them, complex life would have never evolved on earth. So where did these mitochondria come from?In the 1960s, biologist Lynn Margulis found evidencethat mitochondria (and chloroplasts, the plant equivalent) are descendants of bacteria. Today, this is widely known and accepted as endosymbiotic theory. It hypothesizes that some 2.5 billion years ago, a small, autotrophic bacterium was engulfed by a larger primitive archaean cell. A symbiotic relationship was formed, where the engulfing cell provided protection and nutrients, while the bacterium provided additional energy through cellular respiration. This offered a considerable evolutionary advantage. Over the course of a billion years, these engulfed individual bacteria evolved into mitochondria. That means bacteria don’t just live within us—they are part of us too.