What are Bacteria?

There are various bacteria, but they are usually have one cell (unicellular). Bacteria come in different sizes and shapes. Coccus (1 micrometer) is a round bacteria. Bacillus (2 to 5 micrometers) is the rod-shaped bacteria. Spirillum (10 micrometers) is this wavy sort-of-thing and vibrio, which is this curved rod.

Honestly, the vibrio looks like a banana.

The Difference Between Bacteria and Viruses


Bacteria are prokaryotic and have one cell (unicellular). Most bacteria, unlike our cells, don’t have a nucleus or membrane-bound organelles. They have what’s called a “nucleoid” – where the unbound chromosome sits freely.

Most bacteria have a cell wall, which covers the plasma membrane of the cell and have this special molecule for their cell walls called peptidoglycan (big word that means “protein sugar”) that can help protect them from danger.

Depending on what kind of bacteria, they can live anywhere and are either pathogenic or symbiotic (or helpful to us humans). These guys are microscopic. A number of them are heterotrophic (eat other organisms) or autotrophic (make their sugars by photosynthesis).


Viruses cannot live without a living host.

Bacteriophages (or just “phages”) are viruses that infect bacteria with their DNA or RNA to reproduce, which often cause more problems. However, phages get used in science to help cure diseases!

Viruses are very small compared to bacteria and except by our immune system, there’s no way to get rid of them. That’s why you get vaccinated. When you do, a weak or fragmented viral agent that looks like the bad guy gets attacked by your white blood cells. So, next time you get sick with a look-alike virus, your immune system recognizes it and can fight off the infection.

So why is the common cold so elusive? Because the rhinovirus changes all the time. Besides, it’s a virus. We can’t kill viruses with medicine – how are we going to do kill the rhinovirus?

How Bacteria Move

To get around, bacteria use special appendages such as flagella and cilia. Cilia are hair-like attachments that typically surround the cell. The flagella (longer than the cilia) is in a fixed wavy state, twirls around. The flagella is sometimes at the end, the sides, or the front of the cell.

How They Multiply and Exchange DNA

To multiply, first, bacteria replicate their DNA and cut themselves in half. This is binary fission. In the center of the cell, bacteria have an elongated circular DNA that 2,000 to 5,000 genes. This is the chromosome. Besides the chromosome, there is another circular DNA, which has 5 to 10 genes. This is the plasmid. To exchange their plasmidal DNA, some bacteria have sex pili which connect to other sex pili and exchange plasmid DNA. This is conjugation.

Besides conjugation, bacteria can take genes from destroyed bacteria for themselves. This is transformation.

Different Genes for Different Things, or Should I say “Plasmids”?

Plasmids are the circular genome that has unique properties to the chromosome inside the bacterial cell.

There are different forms of plasmids that do a number of things.

  1. R plasmids – have genes that code for proteins that give antibiotic resistance.
  2. Conjugation plasmids – allows plasmids to transfer DNA cell to cell.
  3. Metabolic plasmids – carries genes that code for proteins that break-down toxic molecules.
  4. Virulence plasmids – have genes that code for toxins.

Their Cell Walls and Plasma Membranes

Each bacteria have a cell wall (not to be confused with the plasma membrane, which is a semi-permeable membrane). A bacterial cell wall, made up of peptidoglycan, is a strong yet porous arrangement outside the plasma membrane. Peptidoglycan (“protein” “sugar”) made from sugars and amino acids. Amino acids make up proteins. Fun fact, penicillin (made from fungus) breaks down the cell wall in bacteria.

Gram Staining

If you ever heard someone call a bacteria Gram-negative or Gram-positive? Here’s why. Hans Christian Gram created a stain used to recognize bacteria with different peptidoglycan. Bacteria with thick peptidoglycan are Gram-positive. Bacteria with thin peptidoglycan are Gram-negative. While, some bacteria that are Gram-negative don’t have peptidoglycan at all. After Gram staining, Gram-positive bacteria show up as purple and Gram-negative show up as red.

Gram-negative bacteria have an outside membrane made out of lipopolysaccharides (LPS). Polysaccharide means “many sugars.” It’s a molecule of sugar. In the case of lipopolysaccharides, these molecules have both lipids and sugars. When these bacteria fall apart, LPS becomes an endotoxin, which gives the person nausea, fever, a change in blood pressure, and shock.

Some bacteria have a lattice of polysaccharides and glycoproteins (proteins that have carbohydrates on them) that covers the cell called a glycocalyx. This helps the bacteria attach to host cells as well as medical implants and other smooth surfaces. It also can help bacteria hide from white blood cells.

Some bacteria even have a slime layer that allows

Tricky buggers.

Mycobacterium have a cell wall that has lipids and mycolic acid. Lipids are biological molecules that are insoluble in water. Mycolic acid is a collection of long fatty acids. Acid-fast staining, used to find these critters as opposed to regular Gram-staining, involves staining bacteria that have lipid cell walls red so that it stands out from a blue background.


Ribosomes make proteins and are made up of ribosomal RNA and connected together in two parts.

Bacteria and eukaryotic cells have different ribosomes. Eukaryotic cell ribosomes are larger than bacterial ribosomes.

Bacterial cells are different from eukaryotic cells, but like eukaryotic cells, bacteria have basic needs such as the many macronutrients: carbon, oxygen, nitrogen, hydrogen, phosphorus, and sulfur.

Plant cells, which are eukaryotic cells, and most bacterial cells share the same kind of organelles such as the gas vacuole, which help the cell stay afloat and the chloroplast (needed for photosynthesis).

The Endospore

What’s amazing about the bacteria in the Bacillus, Clostridium, and Sporosarcina classes is that when they are in a harsh environment they create an organelle called the endospore, which can withstand boiling, freezing, drying, high acid, radiation, and antibiotics. The only way to kill the spores and the bacteria is to autoclave them in 121 °C or submerge them in chlorine bleach. The organelle helps the bacteria survive when their food is scarce.

Different Kinds of Bacteria

They are different types of bacteria. Photoautotrophs such as algae get their energy from the sun and use inorganic carbon dioxide. Photoheterotrophs, which can use the sun for energy, but cannot use inorganic CO2 by itself. They have to eat organisms to get their carbon. Chemoautotrophs get their energy from chemicals and use inorganic carbon dioxide. Chemoheterotrophs get their energy from chemicals, but have to get their carbon from organisms.


Willey, Joanne M. Sherwood, Linda M. Woolverton, Christopher J (2014). Prescott’s Microbiology. (9th Ed.). New York, NY: McGraw Hill