Characters of Different Groups of Prokaryotes

In this article we will discuss about the characters of different groups of prokaryotes.

Domain: Archaea:

The important characters of Domain Archaea (Gr. archaios, ancient) are:

Important Characters of Archaea:

1. Members of the Domain Archaea i.e., Archebacteria can live in different habitats like.

i. Extreme acidic and hot (above 100°C) environment,

ii. Extreme saline condition, and

iii. In anaerobic condition.

2. They may be spherical, rod-shaped, spiral, lobed, plate-shaped, and irregularly shaped or pleomorphic.

3. Some members are unicellular, but others form filaments or aggregations.

4. Cells are 0.1 μm to more than 1.5 μm in diameter and some filaments are about 200 μm in length.

5. Cell wall composed of pseudomurine, polysaccharide or glycoproteins and other proteins (none have muramic acid and D- amino acids, characteristic of bacterial peptidiglycan).

6. They are either Gram-positive or Gram- negative in nature.

7. The membrane lipids have branched chain hydrocarbons connected to glycerol by ether links.

8. They have a covalently linked closed circu­lar genome and its size is generally smaller than other prokaryotes. The size of genome ranges between 0.8-1.1 x 10⁹ daltons.

9. G+C content of DNA varies between 21-68 moles %.

10. Ribosomes are of 70S type.

11. Reproduction takes place by binary fission, budding, fragmentation, etc.

12. Nutritionally, they range from chemolitho- autotrophs to organotrophs.

They share the characters of both Prokary­otes and Eukaryotes:

A. The Prokaryotic characters are:

i) About 1 µm in diameter.

ii) Lack of membrane-bound organelles.

iii) Nucleic acid is not bounded by nuclear membrane.

iv) Ribosome 70s type.

B. The Eukaryotic characters:

i) Presence of cell wall but devoid of peptidoglycan.

ii) Mechanism of protein synthesis and structure of protein.

iii) Genes have introns (present occasio­nally).

C. Unique character i.e., character of their own:

i) Members contain branched chain lipids with ether, these enable them to tolerate extreme pH and heat.

The Domain Archaea is divided into two Phyla — Crenarchaeota and Euryarchaeota, based on rRNA sequences:

1. Crenarchaeota. The group includes thermophilic and hyperthermophilic, generally sulphur metabolising organisms.

2. Euryarchaeota. The group includes pri­marily methanogenic, halophilic, thermophilic and also sulphur reducing organisms.

This group of bacteria lives in such condi­tions that existed as far back as Pre-Cambrian Era i.e., before 3-4 million years. Thus, they are called ancient group.

Domain: Bacteria:

All prokaryotes, except the members of Archaea, are included under the Domain Bacteria. Bacteria of this group are also called eubacteria. The group has been classified on the basis of 16S rRNA homology.

The important characters of this Domain are:

Important Characters of Bacteria:

1. They are available in all possible habitat.

2. Mode of nutrition is mostly heterotrophic, where some may be parasitic, saprophytic or symbiotic; and others are autotrophic due to the presence of photosynthetic pigment, the bacteriochlorophyli.

3. They are unicellular, having cell wall com­posed of three main constituents (i) N-acetyl glucosamine, (ii) N-acetyl muramic acid, and (iii) a peptide chain

4. Inside the wall, a semipermeable cytoplas­mic membrane is present, composed of double layer of phospholipid molecules.

5. The mesosome, which forms by the loca­lised infolding of cytoplasmic membrane, functions as mitochondria of higher plant.

6. Bacteriochlorophyli — if present — is loca­ted in the involuted cytoplasmic membrane.

7. The DNA reticulum is not surrounded by membrane.

8. Profuse number of ribosomes are present with low rate of sedimentation than eukaryotes.

9. They also contain the extrachromosomal hereditary material, the plasmid.

10. The absence of endoplasmic reticulum and golgi apparatus.

11. The common method of multiplication is binary fission.

12. In the true sense, sexual reproduction is absent, but genetic recombination takes place by conjugation, transformation and transduction.

Some bacteria possess one or more flagella at one or more points, composed of several chains of flagellin (a protein molecule).

Gram-negative bacteria bear very small cytoplasmic appendages known as Pilli, composed of pilin, a protein.

Phylum. Proteobacteria:

Important characters:

1. They are the largest and diverse group of bacteria.

2. Members of this group are sometimes called purple bacteria, because of the inclusion of purple photosynthetic bacteria in several of its sub-groups.

3. Members show variation in morphology, metabolism and reproduction.

4. They range from cocci, rod to others with prosthecae, buds and even fruit bodies.

5. Bacteria are of Gram-negative type.

6. Physiologically they may be photoau- totrophs, chemolithotrophs, or chemo- heterotrophs, etc.

Based on 16S rRNA sequence, the phylum is divided into the following 5 classes:

(a) Alphaproteobacteria (contains 6 orders and 18 families), e.g., Rhizobium, Nitrococcus, etc.

(b) Betaproteobacteria (contains 6 orders and 12 families), e.g., Nitrosomonas, Thiobacillus, etc.

(c) Gammaproteobacteria (contains 12 orders and 20 families), e.g., Escherichia, Salmonella, etc.

(d) Deltaproteobacteria (contains 7 orders and 17 families), e.g., Desulphovibrio, Desulfuromonas, etc.

(e) Epsilonproteobacteria (contains 1 orders and 2 families), e.g., Campylobacter, Helicobacter, etc.

Low G+C Gram-Positive Bacteria:

Phylum. Firmicutes:

This group includes the Gram-positive bac­teria with low G+C content in their DNA. Most of the members are heterotrophic and Gram- positive. Because of low G+C content, the mycoplasmas are also included in this group though they lack cell wall and are Gram-negative in nature. Many are endospore forming. They show variation in morphology such as cocci, rods and mycoplasmas are pleomorphic.

Class 1. Clostridia:

The class contains three orders and 11 fami­lies. The members of this group are anaerobic in nature.

Important characters of this group are given below:

Important characters:

1. The members show much variation in the morphology and size.

2. Members are anaerobic in nature.

3. Some genera produce endospore (Clostri­dium, Sporohalobacter etc.), but many others are non-sporing type.

Clostridium is one of the largest bacterial genera.

Class 2. Mollicutes:

The members of the class are commonly known as mycoplasmas. The important charac­ters of the class are given below:

Important characters:

1. Shape of the members of this group varies from cocci, rods, ring-shaped to filamentous or without any particular shape.

2. Diameter of the cells varies from 0.15 to 0.3µm, thereby considered as smallest free- living bacteria.

3. Cells are surrounded by a triple-layered dou­ble membrane containing sterols.

4. Bacteria of this group cannot synthesize peptidoglycan precursors, the main compo­nent of bacterial cell wall and thus lack cell wall.

5. Due to absence of cell wall, they are highly plastic and pleomorphic.

6. They are Gram-positive in nature.

7. Members have genome size ranging from 400 to 1,000 megadaltons. 400 megadaltons is the smallest genome found in Mycoplasma genitalium, a human pathogen.

8. G+C content of DNA varies between 23 and 40 moles %.

9. Cells multiply by binary fission like other bacteria, but genomic replication is not synchronised with cytoplasmic division which lags behind.

10. Cells are devoid of flagella and they are generally non-motile. Some show gliding movement (Mycoplasma gallisepticum, M. pneumoniae) or flexous motility (Spiroplasma).

e.g., Mycoplasma, Spiroplasma, Anaero- plasma, etc.

Class 3. Bacilli:

It is a large class consisting of a wide variety of Gram-positive aerobes and facultative anae­robes.

Important characters are given below:

Important characters:

1. Members are Gram-positive in nature.

2. G+C content is 50 moles % or less.

3. The class includes cocci, endospore forming rod and non-sporing rods, and also other forms.

The class has been divided into orders, Bacillales (includes 9 families) and Lactobacillales (includes 6 families).

Order: Bacillales:

Important characters:

1. Members include different shapes like cocci, rods, mycelial form etc.

2. Both endospore forming and non-sporing rods are present.

3. They may be non-motile or motile. When motile they are with peritrichous flagella.

4. They are catalase positive.

5. They are aerobic to facultative anaerobic.

6. They are commonly saprophytic, but a few are pathogenic.

e.g., Bacillus, Staphylococcus, etc.

Order: Lactobacillales

Important characters:

1. Members include cocci and non-sporing rods.

2. Cells are non-motile.

3. They are catalase negative.

4. Members are anaerobes and facultative anaerobes.

e.g., Lactobacillus, Lactococcus, Strepto­coccus, Leuconostoc etc.

High G+C Gram-Positive Bacteria:

Phylum. Actinobacteria:

This group includes the members with diverse morphological assemblage having pathogenic and economically important mem­bers.

The important characters of the phylum are given below:

Important characters:

1. Morphologically they are of various types ranging from cocci, rods, rods with rudi­mentary branching to elaborate mycelial forms.

2. They are of Gram-positive type.

3. G+C content of DNA is 50 moles % or more, designated as high G+C group.

4. The Phylum contains one class (also called by the same name as Actinobacteria) and six orders.

The phylum includes both useful and harm­ful bacteria.

Useful members:

i. Many species of Streptomyces are use­ful in the commercial production of antibiotics.

ii. Propiniobacterium and Brevibacterium are useful in dairy industry.

iii. Frankia spp. are able to infect many non-leguminous plants like Casuarina, Myrica, Alnus, Coriaria etc., and pro­duce nitrogen-fixing root nodules.

Harmful members:

i. Mycobacterium leprae causes leprosy.

ii. Corynebacterium diphtheriae causes diphtheria in humans.

iii. Actinomyces israelii causes abscesses of connective tissues in man.

Gram-Negative Bacteria:

Phylum. Planctomycetes:

The Phylum Planctomycetes includes only one class, one order and four genera.

Important characters:

1. They are spherical or oval, budding Gram- negative bacteria.

2. Cell walls have distinctive crateriform struc­ture or pits, devoid of peptidoglycan.

3. Nuclear body is membrane bounded (Gemmata obscuriglobus and Pirullela).

4. The genus Planctomyces is attached to sur­face through stalk and hold fast, which is absent in other genera.

e.g., Planctomyces, Gemmata, Pirullela etc.

Phylum. Chlamydiae:

Earlier the Chlamydiae and rickettsias were placed together based on the fact that both are intercellular obligate parasite and Gram-negative in nature. Based on 16S rRNA data, rickettsias have been separated out and placed in the Phylum α-Proteobacteria.

The important charac­ters of the phylum Chlamydiae are given below:

Important characters:

1. Cells of Chlamydiae are very small, coccoid in shape (0.2 to 1.5μ), non-motile and Gram-negative in nature.

2. Cell wall is devoid of muramic acid and peptidoglycan.

3. The size of genome is 4 to 6x 10⁸ daltons, which is very small and thus they have limi­ted metabolic capacity.

4. G+C content of cells ranges between 41 to 44 moles %.

5. They are unable to synthesise ATP from car­bohydrate or other substances and thus they depend absolutely on host cell for energy, called energy-parasite.

6. They have an unique membrane-located translocase to receive ATP from the host cell.

7. They are obligate parasite and not able to grow in culture medium.

8. Infection to host cell takes place by an ele­mentary body (EB), a small spherical struc­ture of about 0.3 to 0.4µm in size.

9. They are obligatory intracellular parasites causing diseases of mammals including human beings, birds, spiders etc. e.g., Chlamydia.

Three species of Chlamydia are important pathogens of human beings and other animals.

These are:

(a) C. psittaci causes psittacosis in humans. It also infects many other animals like parrot, cats, cattle, sheep etc. and invades respiratory, intestinal, genital tracts and also other parts.

(b) C. trachomatis causes trachoma, non­gonococcal urethritis and other diseases of humans.

(c) C. pneumoniae causes pneumonia of humans.

Phylum. Spirochaetes:

The Spirochaetes (Gr. spira — a coil and chaete — hair) are distinguished from others by their flexible spiral cells and characteristic moti­lity. The important characters of this phylum are given below:

Important characters:

1. Cells of Spirochaetes are spiral, long and flexible, they give the characteristic flexuous movement. They also exhibit creeping or crawling movement in contact with solid surface.

2. Cells are thin and measure 0.1 to 3.0µm x 5.0 to 250µm.

3. The characteristic movement of the cells is due to the presence of an unique type of flagellar complex, the axial filament, con­sists of 2-more than 100 prokaryotic flagella extending from the poles and lying inside the outer membrane.

4. They are Gram-negative and chemo- heterotrophic in nature.

5. G+C content of cells ranges between 25-65 moles %.

6. Most of the members grow in aquatic (both fresh water and marine) habitat (Spirochaeta, Leptospira etc.) and a few cause disease in human beings (Treponema pallidum causes syphilis, Borrelia burgdorferi causes lyme disease). e.g., Treponema, Leptospira, Borrelia, etc.

Phylum. Bacteroidetes:

The Phylum is divided into three classes: Bacteroides, Flavobacteria and Sphingobacteria. Some important and well-known genera are Flexibacter (gliding bacteria), Flavobacterium, Bacteroides and Cytophaga.