Bacteria

Bacteria has being around for billions of years.

Many of the bacteria are beneficial to us.

Some are harmful to us.

Let us look at the phylogeny of bacteria.

The earliest living organism that existed, is called 'last universal common ancestor' or LUCA.

The major domains of life, bacteria, archaea and eukarya evolved from LUCA.

Bacteria and archaea are single celled prokaryotes.

We human being, along with animals and plants belong to the multicellular domain eukarya.

Archaea is an ancient form of life, which thrive in extreme environments.

Bacteria diverged from other domains billions of years ago.

There was some horizontal gene transfer between domains.

For example, chloroplasts in plants, and mitochondria in animals evolved from bacteria.

Now they are integral part of eukaryotes.

Apart from these gene transfers, bacteria is a distinct domain by itself.

We will discuss bacteria in this module.

Estimates of the number of types of bacteria vary widely.

It varies from 10 million to 1 billion.

Less than 1% of the bacteria present in the planet,

has been identified.

There is probably no place in earth, where bacteria does not exist.

It exists in air, water, and soil.

Bacteria has been found in ice, hot water springs, and in the upper atmosphere.

Bacteria play an important role in the chemical cycle on earth.

The human body itself is host to bacteria.

There are probably 10 times more bacterial cells, than human cells, in our body.

Most of them exist in a symbolic relationship with our body.

They help to boost our immune system.

They help us break down food, ansynthesis vitamins.

There are some types of bacteria which are harmful to us.

They are called pathogens.

Salmonella or e.coli can cause food poisoning.

Bacteria in the teeth results in plaque which causes cavities.

There are many types of bacterial infections which are harmful to us.

Bacterial cells are very small in size.

A typical bacterial cell will be about one micron in size.

They are ten times smaller than eukaryotic cells.

Eukaryotic cells are between 10 to 100 microns in size.

Bacterial cells do not have membrane bound organelles.

Our DNA is organised in chromosomes which are linear.

The chromosomes are enclosed in a nucleus.

Bacteria does not have a nucleus.

All their genetic material are inside the cell in a region called Nucleoid region.

They have extra bits of DNA in small structures called plasmids.

The genetic material of bacteria is wound in a loop.

They do not have Introns.

Every bit of the DNA is part of a gene, which will have a function.

They have plasmids which are little extra bits of DNA.

The DNA floats in the cytoplasm of the bacterial cell.

They also have ribosomes.

The cytoplasm is enclosed in a plasma membrane.

The plasma membrane comprises of a phospholipid bilayer.

Surrounding the plasma membrane is the cell wall.

The cell wall could be made up of a chemical called peptidoglycan.

Peptidoglycan is a polysaccharide.

The cell wall has multiple cross links between it.

This makes it a stable structure.

It prevents the bacteria from bursting.

The cell wall also determines the shape of the bacteria.

This is different from a cell wall of plant and fungi.

Animals like us do not have a cell wall.

Antibiotics target the cell wall.

The antibiotic penicillin, for example punches holes in the cell wall,

which causes the cell wall to rupture.

The antibiotics do not impact human cells, because our cells do not have a cell wall.

The cell wall is enclosed in a capsule.

The capsule is jelly like, and is a polysaccharide.

The bacteria has tiny protrusions called pilli.

It has a tail like structure called flagella.

The flagella and pilli help the bacteria to move around.

The pilli surrounding the bacteria allows them to grab material.

The capsule helps it to form a biofilm.

Antibodies attach themselves to the pilli.

Antibodies are made for a specific type of bacteria.

If the pilli changes, the antibodies cannot attach itself,

and we will not be immune to that bacteria.

Bacteria comes in different shapes.

Many types of bacteria have a spherical shape.

They are called cocci.

Many of them will be rod shaped.

They are called bacilli.

E.coli bacteria widely used in research, has a bacilli shape.

Streptococci is a string of cocci.

Staphylococci is another type of spherical shaped bacteria.

Some bacteria are spiral shaped.

These are some of the common shapes of bacteria.

There are many other shapes of bacteria.

One way to classify bacteria, is according to their shape.

Another way to classify them, is according to what we call as a gram stain.

Gram stain uses chemicals to stain the bacteria.

There are two types of bacteria.

Gram positive and gram negative bacteria.

Gram negative bacteria will have a plasma membrane.

Then they will have a cell wall.

Then they are going to have another membrane outside it.

This extra plasma membrane differentiates it from gram positive bacteria.

Gram negative bacteria do not stain as brightly as gram positive bacteria.

Gram negative bacteria are more difficult to treat.

The extra membrane that surrounds the cell wall,

makes it difficult for antibiotics to reach the cell wall.

Gram positive have only one lipid by layer membrane, and a cell wall.

With this kind of classification, a bacteria could be called gram positive staphylococci.

There is another way to classify bacteria, according to their way of life.

Autotrophs are those organisms which produce their own food.

Heterotrophs are those which depend on other sources for their food.

Bacteria uses different sources of energy, for their survival.

Phototrophs use sunlight as a source of energy.

Phototrophs which use organic compounds as a source of carbon are called photoheterotrophs.

Phototrophs which use carbon fixation as a source of carbon, are called photoautotrophs.

Blue, green algae are photoautotrophs.

Lithotrophs use inorganic compounds as a source of energy.

Depending on their source of carbon, they are called lithoheterotrophs of lithoautotrophs.

Organotrophs use organic compounds as a source of energy.

Depending upon the source of carbon they are called chemoheterotrophs or chemoautotrophs.

Most bacteria are chemoautotrophs, example, e.coli.

They eat our food, do cellular respiration and get the carbon from that food.

Interestingly, humans are chemoheterotrophs.

We get our energy from eating organic compounds.

Our source of carbon is also organic compounds.

Plants are an example of photoautotrophs.

Of course, plants and humans are not bacteria, and are made up of eukaryotic cells.

Bacteria reproduce in a process called binary fission.

One cell of bacteria copies the DNA in the nucleoid region, and then split into two.

The two splits into four and so on.

This is the way the bacteria multiplies.

Bacteria can reproduce in a relatively short time.

Some type of bacteria take only 20 minutes to reproduce.

Other may take months and years.

e.coli is one of the fast reproducing type of bacteria.

As long as there is no mutation, the reproduced bacteria are identical to each other.

They have another interesting feature.

Bacteria have plasmids, which are bits of DNA.

These plasmids can copy their DNA and share it with another cell.

The pilus of one bacteria attaches to the pilus of another bacteria, and the DNA is transferred.

This is called conjugation.

A virus can infect a bacteria.

This virus can take the DNA from one bacteria and bring it to another.

This is called transduction.

These are some of the ways that bacteria can evolve.

This evolution helps it to adapt to changing environments.

We take an antibiotic to treat a bacterial infection.

In case, we do not take the full course to eliminate all the bacteria,

it is possible that some of the remaining bacteria develop resistance to the antibiotic.

A lot of this resistance is developed in the DNA of the plasmids.

The bacteria that is left behind can transfer the DNA in the plasmids to other bacteria.

These bacteria can multiply and develop into a new antibiotic resistant variant of the bacteria.

There is one type of bacteria which glows only in a community.

This means that bacteria can talk to each other.

Scientists are just beginning to understand how bacteria communicate with each other.

They communicate through a process called quorum sensing.

The bacteria gives off chemicals called autoinducers.

Some of them will be picked up by bacteria of the same species.

Some of them will be picked up by all nearby bacteria.

If the bacteria is isolated, nobody is going to hear the chemical message.

When there is more bacteria, and more bacteria, eventually there are going to be many autoinducers.

This is going to trigger genes to be released.

In the case of the glowing bacteria, this would be a gene which causes it to glow.

This is why this type of bacteria glows as a community.

This is just one example of bacteria behaviour by communicating.

It is possible that they can communicate to increase virulence, to promote a disease.

These aspects of bacteria are still an area of ongoing research.

Right now we treat bacterial infections with antibiotics.

It is possible in future that we may have drugs that interfere with bacteria communicating with each other.

Bacteria are microscopic organisms.

However they are capable of fairly complex behaviour,

which needs more research to understand them better.