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What is Bacteria? - Definition, Characteristics & Examples

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  • 0:05 The Tree of Life
  • 1:49 Defining Bacteria
  • 3:47 Bacterial Growth and…
  • 5:53 Bacteria in Health and Disease
  • 7:35 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

This lesson will introduce the Bacteria, a large and diverse group of microorganisms. We will examine several key characteristics, as well as a few commonly known examples of bacterial species.

The Tree of Life

This is an example of a phylogenetic Tree of Life. This simplified drawing represents the origin of life on Earth. The black line, or the tree trunk towards the bottom, is the universal ancestor of all organisms. Moving towards the top, time moves forward and new species appear with each new branch. The major branch, also called a domain, to the right, is the Eukarya. This domain includes pretty much every organism you can see with the naked eye: fungi, plants, and animals. The red branch in the center, the Archaea domain, is an ancient group that thrives in extreme environments and uses a variety of strange chemicals to make energy. This group is microscopic, unicellular and prokaryotic, which means they lack a nucleus.

As we continue to move left, we encounter the Bacteria, another domain of microscopic, unicellular prokaryotes. Some of those names might be familiar to the novice microbiologists in the group, like the Gram-positives and spirochetes. Others are probably new. You can see that the Archaea and Eukarya are closer together than the Bacteria and Archaea. Even though the Archaea and Bacteria are both unicellular prokaryotes, genetic evidence has determined that the Archaea are actually more closely related to the Eukarya. The Bacteria are a far more distant relative to both.

In this lesson, we will not get bogged down with learning about the individual groups of Bacteria, but we will learn the major characteristics that define the domain as a whole. In fact, that sounds like a great place to start.

Defining Bacteria

Let's define what constitutes a member of the domain Bacteria. A Bacteria is any unicellular prokaryote that is not a member of the domain Archaea. So you can say that all bacteria exist as a single cell with no nucleus and be correct across the board. The only other blanket statement that applies to all bacteria is that they are very diverse and variable. Exceptions seem to exist with every attempt to generalize in this domain.

For the most part, bacteria are small, typically around one micron in length. For comparison, eukaryotic cells, like those of plants and animals, are usually between 10 and 100 microns. In fact, most organelles inside the eukaryotic cells are the same size as the entire bacterial cell. This small size doesn't leave a lot of room for the bacteria to have their own organelles, so bacterial cells don't contain membrane-bound organelles like the mitochondria and nuclei of eukaryotes.

Most bacteria have rigid cell walls, of which there are several types. These walls are important for preventing the bacterial cell from bursting. The cell wall can also dictate the cell shape, usually one of three basic shapes: round, rod and spiral. Streptococcus, the bacteria that causes strep throat, is round, which is also called cocci. E. coli, the famous research bacterium and common cause of food poisoning, is rod-shaped. Lyme disease, caused by the bite of an infected tick, is caused by the spiral-shaped bacteria Borrelia.

Estimating the number of bacterial species on Earth is very difficult. Some methods project 10 million. Others put the number closer to a billion. Either way, far less than one percent of those species have been identified, and fewer still have been studied and characterized.

Bacterial Growth and Metabolism

Bacteria grow and reproduce by binary fission. An individual cell will copy its genome and then split the cell in half, giving each half a copy of the DNA. This results in two new cells. Some species grow very slowly, taking months to years for a single cell to divide to form two new cells. Other species grow very quickly, with each cell dividing into two new cells every few minutes. E. coli, when provided with oxygen and lots of nutrients, is one of those fast-growing bugs.

The inside of the bacterial cell, called the cytoplasm, contains all the enzymes and their associated metabolic processes needed for survival. Bacteria have very diverse metabolic capabilities, which just means they can eat or breathe a wide range of chemicals. Some bacteria require oxygen, just like you. Others die immediately in the presence of oxygen but thrive in habitats without it, like in your intestine. These species can breathe, or respire, chemicals like iron, sulfate and nitrate. Some organisms eat simple sugars, while others can break down complex hydrocarbons or environmental pollutants. Cyanobacteria perform photosynthesis, obtaining their energy from sunlight. Basically, if there is a potential energy source in nature, you can bet a bacterium has evolved to exploit it.

The diverse metabolic capabilities of bacteria have allowed them to colonize a wide range of environments. It would probably be faster to list the places where bacteria have not been found. Different species can live in air, water and in soil. Bacteria have been found alive and thriving in ice, hot springs, deep oceans and the fringes of the upper atmosphere. These bacteria play important roles in all the global chemical cycles on the planet. But bacteria are not just important in the environment. Recently, scientists have begun to explore the bacteria associated with the human body.

Bacteria in Health and Disease

One of the most fascinating findings from studying the bacteria associated with the human body is that there are ten times more bacterial cells both inside of you and colonizing your body surfaces than the number of human cells in your body! You might think this is a bad thing, but current research has shown that these bacteria are in a beneficial symbiotic relationship with our bodies. They play an important role in boosting our immune system, helping us break down food and in synthesizing important vitamins for our bodies to use.

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