Lecture - 29 Microbiology, Archaebacteria and Eubacteria
Microbiology:- Microbiology is the science that deals with the study of microorganisms. The term
microbiology derives its name from three Greek words mikros [small] bios [life] and logos
[study].
Microorganisms:- These are tiny and invisible to naked eye. So, they can be looked into and
studied only with the help of microscope.
> Small subcellular or cellular living beings with milli-micron or micron in size and are not
visible to our naked eyes are called micro-organisms.
> Micro-organisms are basically classified under the following 2 groups:
a. Prokaryotic microbes:-
i. Subcellular living entities:- Prions, viroid, viruses
ii. Cellular organisms:- Bacteria, cyanobacteria etc.
b. Eukaryotic microbes:- These include cellular microbe belonging to following groups
i. Algae:- Ex: Chlamydomonas, Diatoms.
ii. Fungi:- Ex: Yeast, Rhizopus.
iii. Protozoans:- Ex: Plasmodium, Amoeba.
History of microbiology:-
i. Robert Hooke (1635 - 1703):- He was a “polymath’ he made many scientific discoveries in
the 17th century, including making one of the first microscopes and also using a copy of
one of Leeuwenhoek’s microscopes to see and draw details of the structure of plant cells
and some microbes.
ii. Antony van Leeuwenhoek (1632-1723):- He made the first useful microscopes in the 19th
century, they were fiendishly difficult to make and use, they were essentially a lens held
in a metal clip, the lens was made from a tiny drop of molten glass, and he used such a
microscope to see the first microscopic cells.
iii. Ilya Metchnikoff (1845-1916):- He was the first to realize that animals such as us had a
defence system against infection, what we now call the immune system.
iv. Paul Ehrlich (1854-1915):- He searched for the “magic bullet” against infectious disease, he
synthesized the first successful (but very toxic) drug against a disease – syphilis, it was an
arsenic derivative he called salvarsan.
v. Gerhardt Domagk (1895-1964):- He developed the first useful drug against a variety of
bacterial infections, the first sulfa drug –prontosil. Ironically, he died of an infection!
vi. Sir Alexander Fleming (1881-1955) and Selman Waksman (1888-1973):- They discovered
the first relatively safe and effective antibiotics (of natural origin) – isolated from
microorganisms. Fleming discovered penicillin, Waksman discovered streptomycin and a
number of other antibiotics.
vii. Louis Pasteur (1822-1895):- He was a chemist, he made many great discoveries, and he
performed a crucial experiment using a swan necked flask that proved that new life did
not just spontaneously arise from substances like rotting meat. For centuries before
Pasteur, many people believed in Spontaneous Generation- the belief that life is
generated spontaneously from dead organic matter.
viii. Robert Koch (1843-1910):- He made many important discoveries in microbiology, Koch initiated the use of the seaweed polysaccharide gel called agar as a stable material for the formation of a gel on which separated and pure (single species) colonies of bacteria and fungi could be grown (actually it was the wife of a colleague of his who suggested it), this was a critical advance, and he also stated and used his Koch’s postulates (discussed later) required to prove that a given organism caused a given disease.
Archaebacteria:-
> Archaebacteria are known to be the oldest living organisms on earth.
> They belong to the kingdom Monera and are classified as bacteria because they resemble bacteria when observed under a microscope.
> They are completely distinct from prokaryotes.
> They share slightly common characteristics with the eukaryotes.
Characteristics of Archaebacteria:-
- Archaebacteria are obligate or facultative anaerobes, i.e., they flourish in the absence of oxygen and that is why only they can undergo methanogenesis.
- The cell membranes of the Archaebacteria are composed of lipids.
- The rigid cell wall provides shape and support to the Archaebacteria. It also protects the cell from bursting under hypotonic conditions.
- The cell wall is composed of Pseudomurein, which prevents archaebacteria from the effects of Lysozyme. Lysozyme is an enzyme released by the immune system of the host, which dissolves the cell wall of pathogenic bacteria.
- These do not possess membrane-bound organelles such as nuclei, endoplasmic reticulum, mitochondria, lysosomes or chloroplast. Its thick cytoplasm contains all the compounds required for nutrition and metabolism.
- They can live in a variety of environments and are hence called extremophiles. They can survive in acidic and alkaline aquatic regions, and also in temperature above boiling point.
- They can withstand a very high pressure of more than 200 atmospheres.
- Archaebacteria are indifferent towards major antibiotics because they contain plasmids which have antibiotic resistance enzymes.
- The mode of reproduction is asexual, known as binary fission.
- They perform unique gene transcription.
- The differences in their ribosomal RNA suggest that they diverged from both prokaryotes and eukaryotes.
Types of Archaebacteria:- Archaebacteria are classified on the basis of their phylogenetic relationship.
i. Crenarchaeota:-
- The Crenarchaeota are Archaea, which exist in a broad range of habitats.
- They are tolerant to extreme heat or high temperatures.
- They have special proteins that help them to function at temperatures as high as 230 degrees Celsius.
- They can be found in deep-sea vents and hot springs, regions with superheated water.
- These include thermophiles, hyperthermophiles, and thermoacidophiles.
ii. Euryarchaeota:-
- These can survive under extremely alkaline conditions and have the ability to produce methane, unlike any other living being on earth.
- These include methanogens and halophiles.
iii. Korarchaeota:-
- They possess the genes common with Crenarchaeota and Euryarchaeota.
- All three are believed to have descended from a common ancestor.
- These are supposed to be the oldest surviving organism on earth.
- These include hyperthermophiles.
iv. Thaumarchaeota:- These include archaea that oxidize ammonia.
v. Nanoarchaeota:- This is an obligate symbiont of archaea belonging to the genus Ignicoccus.
Importance of Archaebacteria:-
- Archaebacteria have compelled the scientists to reconsider the common definition of species.
- Species are a group with gene flow within its members.
- The archaebacteria exhibit gene flow across its species.
- The Archaebacteria are methanogens, i.e., they are capable of producing methane.
- They act on the organic matter and decompose it to release methane which is then used for cooking and lighting.
Examples of Archaebacteria:-
i. Lokiarcheota:-
- It is a thermophilic archaebacterium found in deep-sea vents known as the Loki’s castle.
- It has a unique genome.
- Some of the genes of the genome are involved in phagocytosis.
- They also possess the eukaryotic genes that are used by the eukaryotes to control their shapes.
- It is believed that Lokiarcheota and eukaryotes shared a common ancestor several billion years ago.
ii. Methanobrevibacter smithii:-
- It is a methane-producing bacteria found in the human gut.
- It helps in the breakdown of complex plant sugars and extracts energy from the food consumed by us.
- Some help to protect against colon cancer.
- People suffering from colon cancer and obesity have very high levels of Euryarchaeota bacteria in their gut.
- The Archaebacteria cannot perform photosynthesis and show high levels of gene transfer between lineages. The discovery of Archaebacteria has made scientists believe that life can exist even in extreme environmental conditions.
Archaebacteria versus Eubacteria:-
1. Alternative Names:-
Archaebacteria:- Archaebacteria are called ancient bacteria.
Eubacteria:- Eubacteria are called true bacteria.
2. Size:-
Archaebacteria:- Individual archaebacterium is 0.1-15 μm in diameter.
Eubacteria:- Individual eubacterium is 0.5-5 μm in diameter.
3. Shape:-
Archaebacteria:- Archaebacteria are spheres, rods, plates, spiral, flat or square-shaped.
Eubacteria:- Eubacteria are cocci, bacilli, vibrio, rods, filaments or spirochetes in shape.
4. Complexity:-
Archaebacteria:- Archaebacteria are simple in their organization.
Eubacteria:- Eubacteria are more complex than archaebacteria.
5. Habitat:-
Archaebacteria:- Archaebacteria are found in extreme environments.
Eubacteria:- Eubacteria are found everywhere on earth.
6. Cell Wall:-
Archaebacteria:- Cell wall is composed of pseudo peptidoglycans.
Eubacteria:- Cell wall is composed of peptidoglycans with muramic acid.
7. Membrane Lipids:-
Archaebacteria:- Membrane lipids of archaebacteria is ether-linked, branched, aliphatic chains, containing D-glycerol phosphate.
Eubacteria:- Membrane lipids of eubacteria are ester-linked, straight chains of fatty acids, containing L-glycerol phosphates.
8. RNA Polymerase:-
Archaebacteria:- RNA polymerase of archaebacteria consists of a complex subunit pattern, which is similar to eukaryotic RNA polymerase.
Eubacteria:- RNA polymerase of eubacteria consists of a simple subunit pattern.
9. Transfer RNA:-
Archaebacteria:- No thymine is present in the TψC arm of the tRNA, carrying methionine.
Eubacteria:- Thymine is present in most of the tRNA, carrying N-formyl methionine.
10. Introns:-
Archaebacteria:- Introns are present in archaebacteria.
Eubacteria:- Introns are absent in eubacteria.
11. Growth and Reproduction:-
Archaebacteria:- Asexual reproduction methods like binary fission, budding and fragmentation are used by archaebacteria during their reproduction.
Eubacteria:- Other than binary fission, budding and fragmentation, eubacteria are capable of producing spores in order to remain dormant during unfavorable conditions.
12. Glycolysis/Kreb’s cycle:-
Archaebacteria:- Archaebacteria exhibit neither glycolysis nor Kreb’s cycle.
Eubacteria:- Eubacteria exhibit both glycolysis and Kreb’s cycle.
13. Types:-
Archaebacteria:- Archaebacteria are three types: methanogens, halophiles and thermophiles.
Eubacteria:- Eubacteria are two types: gram positive and gram negative.
14. Examples:-
Archaebacteria:- Halobacterium, Lokiarchaeum, Thermoproteus, Pyrobaculum, Thermoplasma and Ferroplasma are the examples of archaebacteria.
Eubacteria:- Mycobacteria, Bacillus, Sporohalobacter, Clostridium and Anaerobacter are the examples of eubacteria.