They can cause disease but are also part of the human microbiota and live on our skin, body, and on everyday objects in our environment.

When compared to eukaryotic cells, the structure of bacteria is less complex, due to a lack of nucleus and membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi complexes.

Now bacterial cells are often surrounded by several layers, which are collectively called the cell envelope.

Let's start with the cell wall, which is found on the outer surface of the cell membrane and its most important role is to protect the bacteria.

It consists of peptidoglycan, which usually helps differentiate whether bacteria are gram-positive or gram-negative.

Gram-positive bacteria have a single, thick layer of peptidoglycan above the plasma membrane, which allows them to retain the staining dye.

And gram-negative bacteria have a thinner layer of peptidoglycan sandwiched between the surface membrane and the plasma membrane, so they can't retain the dye.

Additionally, the cell wall helps maintain their shape.

The round-shaped bacteria are called cocci, the rod-shaped ones are called bacilli, spiral-shaped ones are spirilla, and sometimes the same bacteria can have multiple forms, in which case they're called pleomorphic.

Some bacteria are covered by a capsule, which acts as a shield that protects the bacteria against phagocytosis and also helps the bacteria adhere to surfaces.

The capsule is considered an important virulence factor, since the strains that lack a capsule are less virulent.

Underneath the bacterial cell wall, there's the plasma membrane, which is the most important layer because it encloses the cytoplasm, which is a gel-like substance composed mainly of water that also contains cell components, enzymes, and various organic molecules.

If the plasma membrane is removed, the cell's contents spill into the environment, and the cell no longer exists.

Now the plasma membrane is responsible for most of the cell's relationship with the and eliminating waste, and also maintains the interior of the bacteria in a constant, highly organized state.

Usually, all plasma membranes are selectively permeable barriers, which allow certain ions and molecules to pass in and out of the cell while preventing the movement of others.

However, in bacteria, the plasma membrane has other important roles, such as respiration, photosynthesis, and the synthesis of lipids and cell wall components.

Now in the bacterial cytoplasm, there are scattered cell components such as ribosomes, which are the sites of protein synthesis.

These ribosomes can be found spread all over the cytoplasm, where they produce proteins that are destined to remain inside the cell, or they can be attached to the plasma membrane, and they are called plasma membrane-associated ribosomes, and in this case, they make proteins that will reside in the cell envelope, or get transported outside the cell.

Another structure found in the cytoplasm is the nucleoid, which is an irregularly shaped region that contains the cell's chromosome.

The chromosomes of most bacteria are a single circle of double-stranded DNA, but some bacteria can have a linear chromosome, while others can have more than one chromosome, and these chromosomes contain the genes needed to provide instructions for protein synthesis in the cell.

Additionally to the genetic material contained within the nucleoid, many bacteria contain extra-chromosomal DNA molecules called plasmids.

These plasmids are small, double-stranded DNA molecules that can exist independently of the chromosome, and they can be circular or linear.

Plasmids have relatively few genes, and their genetic information is not essential to the bacterium, and cells that lack them usually function normally.

However, plasmids often contain genes that confer a selective advantage to the bacterium, such as resistance to antibiotics.

Finally, the bacteria can move or attach to other cells and surfaces with the help of structures that can be found on their external surface.

So they have pili, which are hair-like structures that allow them to attach to other cells, and when these are shorter, they're called fimbriae and help the bacteria attach to surfaces.

They can also have flagella, which are long, whip-like protrusions that allow the bacteria to move.

Alright, as a quick recap, the bacteria are prokaryotic cells that play an important role in human disease and health, and can be found almost anywhere in the environment.

Bacterial cells are often surrounded by several layers, which are collectively called the cell envelope.

They have a cell wall which protects the bacteria, determines whether they are gram-positive or gram-negative bacteria, and also helps maintain their shape.

Some bacteria can also have another extra layer on top of the cell wall, called a capsule, which is a major virulence factor.

Underneath the cell wall, there's the plasma membrane, which is the most important layer because it encloses the cytoplasm and defines the cell, having important roles such as respiration, photosynthesis, and the synthesis of lipids and cell wall constituents.

The cytoplasm contains ribosomes, which are the site of protein synthesis, the nucleoid, which contains the genetic material, and plasmids, which contain genes that confer a selective advantage to the bacterium.