In this article we will discuss about the characters of different groups of prokaryotes.
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.
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.
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:
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:
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:
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:
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).
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.
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:
This group includes the members with diverse morphological assemblage having pathogenic and economically important members.
The important characters of the phylum are given below:
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.
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.
i. Mycobacterium leprae causes leprosy.
ii. Corynebacterium diphtheriae causes diphtheria in humans.
iii. Actinomyces israelii causes abscesses of connective tissues in man.
The Phylum Planctomycetes includes only one class, one order and four genera.
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.
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:
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.
(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.
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:
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.
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.