How Do Bacteria Reproduce?

If a single bacterial cell isn't visible to the naked eye, how can we know so much about it?

Scientists have developed powerful microscopes to magnify bacteria — usually ranging from one to a handful of microns (one millionth of a meter) in length, giving us a glimpse into their inner workings and how they compare to other forms of life such as plants, animals, viruses and fungi.

As you might already know, cells act as the building blocks of life, whether they compose our own tissues or a tree branch outside of your window. Humans, animals and plants have cells with genetic information contained in a type of membrane called a nucleus. These types of cells, called eukaryotic cells, have specialized organelles, each with a unique job to keep the cell working and healthy.

Bacteria, however, lack a nucleus, and their DNA floats freely within the cell. These microscopic cells don't have organelles and possess different methods to reproduce and swap genetic material. Bacteria are classified as prokaryotic cells.

Basic categories aside, scientists also place bacteria in different camps based on:

Microbiology — the study of all different types of microbes including bacteria, archaea, fungi, viruses and protozoa — has leveraged a growing wealth of knowledge to further distinguish bacteria from their microbial brethren.

Similar prokaryotic organisms now classified in the domain archaea were previously clumped together with bacteria, but after researchers learned more about them, they gave microbes in archaea their own category for their similarities to eukaryotic cells.

Like humans, plants and animals, bacteria need food to survive.

Some bacteria are autotrophs, meaning they use basic inputs such as sunlight, water and chemicals from the environment to create food (think cyanobacteria, which have been turning sunlight into oxygen for roughly 2.5 million years) [source: Konhauser et al.]. Other bacteria are what researchers call heterotrophs because they draw energy from existing organic matter for food (think dead leaves on the forest floor).

The truth is, what might be appetizing to bacteria is probably repulsive to us. They've evolved to thrive on all sorts of things, from oil spills and nuclear byproducts to human waste and decaying matter.

But bacteria's penchant for a particular food source can benefit society. For instance, art experts in Italy turned to bacteria to chow down on excess layers of salt and glue threatening the longevity of priceless artwork. Bacteria's knack for recycling organic matter also comes in handy, especially considering their tremendous role as recyclers of the Earth's surface — both in soil and water.

On a daily basis, you might be all too familiar with the odor-inducing effects of hungry bacteria from that funky smell in your trashcan while they take on your leftovers, breaking them down and releasing their own gaseous by-products. It doesn't stop there, though. You also have bacteria to blame for those nature-calling moments (yes, flatulence), when bacteria in your gut release foul-smelling methane during digestion.

How do bacteria create colonies in the first place?

Bacteria, like other forms of life on Earth, need to make copies of themselves to survive. While humans and other organisms do this through sexual reproduction, it works a bit differently with bacteria.

But first, we'll discuss why diversity is a good thing.

Life undergoes natural selection, or selective forces in a given environment that allow one type to thrive and reproduce more. As you might recall from biology class, genes are the units that instruct a cell what to do — whether to make your hair color brown or blonde or your eyes green or blue. You get genes from both of your parents that provide a good mix. In addition, sexual reproduction gives rise to mutations, or random changes in the DNA, which creates diversity. The more genetic diversity from which to draw, the more likely an organism can adapt to constraints in an environment.

For bacteria, reproduction isn't a matter of meeting the right microbe and settling down; it's a process of asexual reproduction that involves DNA replication and cell division to create two identical cells. This process, called binary fission, occurs when a single bacterium cleaves itself into two after it replicates its DNA and moves the genetic material into opposite ends of the cell.

Because the resulting two daughter cells are genetically identical to the "mother cell" that they came from, this method of reproduction isn't exactly the best way to create a diverse gene pool.

So how do bacteria get new genes?

It turns out bacteria employ tricks to get the job done, with the end result being horizontal gene transfer, or exchanging genetic material without reproducing. There are a few ways bacteria can do this. One method involves picking up genetic material from the environment outside the cell, relying on other microbes and bacteria (through molecules called plasmids). Another results from viruses that use bacteria as hosts. Upon infecting new bacteria, the viruses incidentally leave genetic material of previous bacteria in the new one [source: Marraffini].

Swapping genetic material gives bacteria the flexibility to adapt — and some do, once they sense stressful changes in the environment such as food shortages or chemical changes.

Developing a better understanding of how bacteria adapt is tremendously important for understanding and combatting bacteria's resistance to antibiotics in medicine. Bacteria can exchange genetic material so frequently that treatments that previously worked likely won't next time.

Now that we've taken a close look at how bacteria work at a microscopic level, let's take a step back to see where you find them in the big picture.

The question isn't, "Where are bacteria?" but rather, "Where aren't bacteria?"

They're found virtually everywhere on Earth. It's impossible to have a full grasp of the number of bacteria on the planet at one time, but some estimates point to some 5 octillion microbes such as bacteria and archaea – that's 10 to the 28th power! [source: Whitman et al.]

Pinning a number on how many species, or classifiable types, of bacteria there are also remains difficult. One estimate points to approximately 30,000 formally identified species, but scientists are constantly learning and adding to their knowledge base and think we haven't scratched the surface for the true number of species out there [source: Dykhuizen].

The truth is that most bacteria have been around for a long, long time. In fact, they gave rise to some of the earliest known fossils, dating back 3.5 billion years [source: University of California Museum of Paleontology]. Scientific evidence suggests cyanobacteria began creating oxygen between 2.5 and 2.3 billion years ago in the world's oceans, which gave rise to the Earth's atmosphere and abundant oxygen for us to breathe [source: Konhauser et al.].

Bacteria can survive in air, water, soil, ice and extreme heat; on plants; and even in our intestines, on our skin and on the skin of other animals.

Some bacteria are extremophiles, which means they can withstand extreme environments that are either very hot or cold or that lack the nutrients and chemicals we typically associate with life. Researchers have uncovered bacteria in the Mariana Trench, the deepest spot on Earth under the Pacific Ocean, in addition to underwater heated vents and in ice.

But the fun extends beyond the researchers in the field. Tourists are fascinated with bacteria's more beautiful natural beauty in places like Opalescent Pool at Yellowstone National Park, which features brightly colored thermophilic (heat-loving) bacteria coloring the landmark.

Although bacteria are valiant contributors to the health of humans and the planet, they also have a dark side. Certain bacteria have the potential to be pathogenic, meaning they can cause illness and disease.

Throughout human history, some bacteria have (understandably) gotten a bad rap, causing public anxiety and hysteria. Take the plague, for example. The bacteria causing plague, Yersinia pestis, has not only killed more than 100 million people, but it's also suggested to have shaped history, even contributing to the collapse of the Roman Empire [source: Centers for Disease Control and Prevention]. Before the advent of antibiotics, or medications capable of treating bacterial infections, infections were difficult to stop.

Even today, these pathogenic bacteria still weigh heavily on our minds. Bacteria causing a range of illnesses — from anthrax, pneumonia, meningitis, cholera, salmonella and strep throat to E. coli and staph infections — can defy our treatments thanks to antibiotic resistance.

This is especially true for Staphylococcus aureus, the bacterium responsible for staph infections. The "superbug" has posed tremendous problems for hospitals and health care clinics, where patients are more likely to be exposed to it during medical implants and catheter insertion.

In a previous section, we talked about natural selection and how some bacteria have more diverse genes to help deal with what their environment throws at them. If you have an infection, and some of the bacteria in your body are different from others, antibiotics might take care of the majority of the bacterial population. But this also gives the microbes not affected by your antibiotics the room to reproduce and take hold. This is why doctors recommend staying away from antibiotics unless you really need them.

Biological weapons are another frightening aspect of this conversation. Bacteria can be used as weapons in some cases, including being deployed in anthrax scares and embedded in aerosol sprays.

And it's not just humans taking a hit from bacteria. Indeed, bacteria even have an appetite for the sunken ocean liner Titanic, too [source: Kaufman]. The species, named Halomonas titanicae, eats away at the metal of the historic ship.

We've learned how bacteria can be harmful. In the next section, we'll look at how they can help us out.

Let's take a moment to examine bacteria's good side. After all, these are the microbes that bring us tasty foods such as cheese, beer, sourdough and other fermented items. They're also the unsung heroes behind medicine and advancing human health.

We also have bacteria to thank for shaping human evolution. Science is gleaning more information from the microbiota, or the microorganisms residing in our own bodies, particularly in our digestive system and gut. Research suggests that bacteria, and the diversity and new genetic materials they bring to our bodies, have allowed humans to adapt to and exploit new food sources they previously could not use [source: Backhed et al.].

Look at it this way: Lining the surfaces of your stomach and intestine, bacteria are "working" for you. When you eat, bacteria and other microbes help you break down and draw nutrients from foods, especially carbohydrates such as corn, potatoes, bread and rice. The more diverse bacteria we consume and are exposed to contribute to this diverse community in our bodies.

Although our knowledge of our own microbial communities is nascent at best, there's evidence to suggest that a lack of certain microbes and bacteria in the body can tie into a person's health, metabolism, and susceptibility to allergens and disease. Preliminary studies in mice suggest that metabolic diseases such as obesity are linked to diversity and health of the microbiome rather than our predominant view of the "calories in, calories out" approach [source: Cox].

Also in early stages, fecal transplants involving the sharing of fecal microorganisms from a healthy individual to another person show early promise. Yeah, this might not be the best mental image, but the science and options to treat certain gastrointestinal illness looks promising. Research is being done on probiotics, microbes thought to have added health benefits, but general recommendations on their use haven't been established as of November 2014.

In addition, bacteria have been game-changers in the development of scientific thinking and human medicine. Bacteria played a lead role in the 1884 development of Koch's postulates, a series of considerations that tie a given microbe to disease.

Among other contributions, researchers studying bacteria serendipitously discovered penicillin — an antibiotic that has saved countless lives — and, more recently, an easier way to edit organisms' genomes that could revolutionize medicine [source: Marraffini]. Researchers have modified bacteria to benefit human health in many ways, including producing insulin for the treatment of diabetes.

We've only begun to understand all we can learn from our bacterial friends.