Bacteria are single-celled organisms that do an enormous range of things, from digesting food in your gut to recycling nutrients in soil to causing infections. Your body alone contains roughly 38 trillion bacterial cells, slightly outnumbering your own 30 trillion human cells, with a combined weight of about 0.2 kg. That old claim that bacteria outnumber human cells 10 to 1 has been revised; the actual ratio is closer to 1.3 to 1. Most of these bacteria are not harmful. The vast majority play roles that keep you healthy, keep ecosystems functioning, and even make the foods you eat possible.
How Bacteria Help You Digest Food
Your large intestine hosts the densest bacterial community in your body, and these microbes handle jobs your own cells cannot. Human digestive enzymes break down simple sugars, proteins, and fats effectively, but they cannot touch certain complex carbohydrates like dietary fiber and resistant starch. Gut bacteria can. They ferment these otherwise indigestible compounds and produce short-chain fatty acids, particularly one called butyrate, which serves as a primary energy source for the cells lining your colon.
This process works through a kind of relay system. One group of bacteria breaks down resistant starch and produces simpler compounds like acetate and lactate. Other bacterial species then convert those intermediates into butyrate. There’s even evidence of bacteria acting cooperatively at a cost to themselves: one species has been shown to break down a type of plant fiber externally, making the fragments available to neighboring species that return the favor in other ways. In the mucus layer of your gut, specialized bacteria chop up mucin (the protein in mucus) into smaller sugar fragments that feed bacteria lacking the tools to do this on their own.
Training Your Immune System
Bacteria in your gut do more than digest food. They play a direct role in shaping how your immune system responds to threats. Different bacterial species trigger different types of immune cells. Some, like certain Clostridium species, promote the development of regulatory T cells, which are the immune cells responsible for calming inflammation and preventing your body from overreacting to harmless substances like food proteins. Others, like segmented filamentous bacteria, stimulate a more aggressive type of immune cell that helps fight off actual infections.
This balance matters. The regulatory T cells that commensal bacteria help generate produce an immunosuppressive signal that keeps your immune system from attacking the bacteria themselves, or from mounting inflammatory responses to things that aren’t dangerous. When this system breaks down, the result can be chronic intestinal inflammation. In essence, your immune system learns what to tolerate and what to attack partly based on the bacteria it encounters early in life and continues to interact with daily. The population of regulatory immune cells even varies by location in the body, shaped by whichever bacteria are most abundant in that area.
Bacteria in Soil and the Atmosphere
Outside the body, bacteria are essential to the cycling of carbon and nitrogen, two elements that all life depends on.
When plants and animals die, soil bacteria are the primary decomposers. They break down the carbon-based molecules in dead organic matter, incorporating some carbon into their own bodies and releasing the rest as carbon dioxide back into the atmosphere. In most soils, which contain oxygen, CO2 is the main gas released through this microbial activity. The speed of decomposition depends on factors like temperature, moisture, soil acidity, and the availability of nitrogen and phosphorus. Without this bacterial recycling, dead material would accumulate and the carbon locked inside it would be unavailable to living organisms.
Nitrogen cycling is equally critical. The atmosphere is about 78% nitrogen gas, but plants cannot use nitrogen in that form. Certain bacteria, most famously Rhizobium species, live inside root nodules on legume plants (beans, peas, clover) and convert atmospheric nitrogen gas into ammonia compounds that plants can absorb and use to build proteins. This process, called nitrogen fixation, is the reason farmers rotate crops with legumes: the bacteria naturally fertilize the soil.
How Bacteria Cause Disease
A small fraction of bacterial species are pathogenic, meaning they cause illness. They do this primarily through toxins, and these toxins work in distinct ways.
- Membrane-damaging toxins punch holes in your cells or use enzymes to dissolve cell membranes. One well-known example is produced by Staphylococcus aureus and specifically destroys white blood cells, often causing recurrent skin infections and abscesses.
- Superantigens hijack the immune system. Normally, immune cells activate in small, targeted numbers. A superantigen bypasses the usual checks and can activate up to 40% of circulating immune cells at once, flooding the body with inflammatory signals. This is the mechanism behind toxic shock syndrome.
- A-B toxins are two-part molecules. One part attaches to a host cell, and the other slips inside and interferes with the cell’s internal signaling. Tetanus toxin, for instance, travels along nerve fibers to the spinal cord, where it blocks the release of signals that normally inhibit muscle contraction, causing the characteristic muscle spasms.
- Endotoxins are components of the outer membrane of certain bacteria. When these bacteria die and break apart, the released fragments trigger immune cells to produce intense inflammatory signals. In large quantities, this can lead to dangerously low blood pressure and organ damage.
Bacteria in Food Production
Fermentation is one of the oldest uses of bacteria in human life. Lactic acid bacteria convert sugars into lactic acid and acetic acid, which quickly lower the pH of food. That acidic environment inhibits the growth of spoilage organisms and pathogens, effectively preserving the food while creating the tangy flavors of yogurt, sauerkraut, kimchi, sourdough bread, and pickles.
In sourdough, different bacterial species divide the labor. One type preferentially consumes maltose (a sugar released when enzymes break down starch in flour), while a yeast partner grows on glucose. During a 20-hour fermentation, the available sugars in the dough drop from about 3.3 milligrams per gram of dry ingredients to 0.8 milligrams, as the microbes convert them into acids and gas. The gas produces the rise, the acids produce the flavor, and the low pH keeps the bread safe to eat.
Bacteria in Medicine and Biotechnology
One of the most significant medical applications of bacteria is the production of human insulin. Before recombinant DNA technology, insulin for people with diabetes came from pig or cow pancreases, which sometimes caused allergic reactions and had inconsistent quality. Today, the gene for human insulin is inserted into E. coli bacteria, which then produce the hormone through fermentation. This recombinant insulin is purer, more consistent, and available in large quantities at lower cost, supplying the global demand for the hormone.
The same basic approach, inserting a human gene into bacteria and using them as tiny manufacturing plants, now produces a range of medications and vaccines. Bacteria are appealing for this work because they reproduce quickly and can be grown in large fermentation tanks.
How Bacteria Share Survival Traits
Bacteria evolve fast, and one reason is their ability to share genes directly with each other, not just pass them to their offspring. This horizontal gene transfer happens through three main routes. In conjugation, two bacteria physically connect through a tiny tube-like structure and one passes genetic material to the other. In transduction, a virus that infects bacteria accidentally packages bacterial DNA and delivers it to a new host. In transformation, a bacterium picks up free-floating DNA from its environment, often released by dead bacteria nearby.
These mechanisms are the primary way antibiotic resistance spreads. A single bacterium that develops or acquires a gene conferring resistance to an antibiotic can share that gene with entirely different species. Only about 1% of known bacterial species are naturally capable of transformation, but conjugation and transduction are widespread enough that resistance genes can move through bacterial communities rapidly, which is why antibiotic resistance has become one of the most pressing challenges in modern medicine.