General Biology/Classification of Living Things/Eubacteria

Introduction

On this page, the Kingdom Eubacteria will be explored. Bacteria are omnipresent; they are in our bodies and in our foods. Bacteria are also prokaryotes; the great majority live as single cells, but they are able to produce colonies or link up in chains to form filaments.

Classifying bacteria

There are numerous ways to classify bacteria.

Shape

Since bacteria typically live by themselves, it is easy to distinguish them based on their shape. Bacterial cells have three main shapes:

1) cocci: round with bumps

2) bacilli: rod-shaped with lacerations

3) spirilli: spiral-shaped with grooves

A bacterium's shape can give the bacterium some advantages. For example, spiral bacteria can move through fluids more easily than can cocci or bacilli bacteria.

Furthermore, bacteria can be classified by their growth characteristic patterns (Groupings). The prefix diplo- denotes cells arranged in pairs. The prefix staphylo- describes bacteria arranged in clusters like grapes. The prefix strepto- denotes bacteria arranged in a chain.

Cell Wall Structure

1) Gram-positive: Gram-positive bacteria have simple cell walls, composed of only one layer of peptidoglycan before the plasma membrane. When stained with violet and red dye, gram-positive bacteria appear purple. Sometimes blue, depending on the temperature. The large amount of peptidoglycan in these bacteria traps the violet dye in the cytoplasm, masking the red dye.

2) Gram-negative: Gram-negative bacteria have more complex cell walls, consisting of one layer of a lipopolysaccharide membrane and a peptidoglycan layer. Gram-negative bacteria appear red.

Carbon and Energy Sources

Organisms need energy to do work and carry out metabolism and carbon to build organic cellular structures.

1) Photoautotrophs - Energy and carbon are both obtained from photosynthesis.

2) Photoheterotophic - Energy is obtained from photosynthesis, but carbon must be obtained from food.

3) Autotrophic - Energy must be obtained from consuming food, while carbon is obtained from photosynthesis.

4) Heterotrophic - Both energy and carbon must be obtained from food.

Reproduction of bacteria

Asexual reproduction

Typically, bacteria reproduce asexually. They are not able to reproduce by mitosis or meiosis because they lack nuclei. Instead, they undergo binary fission:

1. The bacterium duplicates its original, single chromosome.

2. With two copies of its original, single chromosome, the cell elongates (stretches) to separate the two chromosomes.

3. A septum (divider) begins to form between the two copies of chromosomes.

4. The septum finishes forming and divides the one bacterium in two parts.

5. The bacterium separates into two daughter bacteria.

Sexual reproduction

In unfavourable conditions, bacteria can reproduce sexually through a process of conjugation:

1. A pilus (a bridging structure) forms between two bacteria as a link.

2. One bacterium of the two transfers all or some of its chromosome to the other across the pilus.

3. With the new genetic content, the receiving bacterium then undergoes binary fusion to produce more cells with the same genetic composition.

Spore formation

If the surrounding environment is unfavourable for a bacteria, they may enter a dormant phase so they can protect themselves. In this case, the bacteria develops a tough outer coating surrounding their DNA and a small amount of cytoplasm. This forms small seedlike structures called endospores. The endospores disintegrate when the living environment becomes favourable again.

When bacteria are protected by endospores, they are more difficult to be killed. However, one reliable method is to subject the endospores to an autoclave, a container which contains highly pressurised steam.

Genetic Recombination

The genetic composition of a bacteria can be changed in three ways: transduction, transformation, and conjugation. Not all of bacteria's chromosomes are in the nucleoid of bacteria. Some of the bacteria's genes are contained in plasmids, which are small loops of DNA which are separate from the main chromosome. They contain one to a few genes, and they are often different from those found in the chromosome.

Transduction

Transduction occurs when a bacteriophage infects a bacterium and injects the phage's nucleic acid into the bacteria. The phage's nucleic acid then becomes part of the bacteria's DNA. For more information, please see the lesson on Viruses: General Biology/Classification of Living Things/Viruses.

Transformation

1. A bacterium encounters DNA from another dead or damaged bacterium.

2. The recipient bacterium accepts the nucleic acid.

3. The recipient and donor DNA are combined.

4. The recipient cell is now genetically different.

Conjugation

Please refer to Sexual Reproduction above.

Bacteria & Human Health

As with most things, there are good and bad bacteria. Some examples of good bacteria are nitrifying bacteria and members of the lactobacillus genus. Harmful bacteria can cause pnuemonia, botulism, infections, and many other diseases in the human body.

To stop unwanted bacteria growth in humans, antibiotics affect bacteria in many ways. Some prevent the bacteria from building or repairing cell walls or membranes, from making more RNA or DNA, or target certain organelles. Antibiotics generally kill bacteria directly or inhibit their growth to let the body's immune system finish the job.

To select the most effective, physicians must apply understanding and knowledge of bacteria. For example, cephalosporin antibiotics are more effective on gram-negative bacteria.

Eubacteria Practice Questions

1. Give two similarities between binary fission and mitosis.

2. Give two differences between binary fission and meiosis.

3. Classify the following as either photoautotrophic, photoheterotrophic, chemoautotrophic, or chemoheterophic:

a) sunflower

b) goldfish

c) human being

d) cyanobacteria

e) iron-oxidising bacteria

4. Between cocci, bacilli, and spirilli bacteria, which type would resist drying the best?

5. Bacteria can help other bacteria resist antibiotics by sharing genetic information.

a) In what part of the bacteria would such genetic information be located?

b) Which method of genetic transfer would bacteria most likely use for this purpose?

6. Would it be possible for some bacteria to remain in the endospore stage for hundreds of years? Support your argument.

Answers to Eubacteria Practice Questions