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Elements of Microbial Structure
Elements of Microbial Structure
All cells have much in common and contain many of the same components. For example, all cells have a permeability barrier called the cytoplasmic membrane that separates the inside of the cell, the cytoplasm, from the outside . The cytoplasm is an aqueous mixture of macromolecules—proteins, lipids, nucleic acids, and polysaccharides—small organic molecules (mainly precursors of macromolecules), various inorganic ions, and ribosomes, the cell’s protein-synthesizing structures.
Ribosomes interact with cytoplasmic proteins and messenger and transfer RNAs in the key process of protein synthesis (translation). The cell wall lends structural strength to a cell. The cell wall is relatively permeable and located outside the membrane; it is a much stronger layer than the membrane itself. Plant cells and most microorganisms have cell walls, whereas animal cells, with rare exceptions, do not.
Prokaryotic and Eukaryotic Cells
Prokaryotic and Eukaryotic Cells
Examination of the internal structure of cells reveals two distinct patterns: prokaryote and eukaryote. Eukaryotes house their DNA in a membrane-enclosed nucleus and are typically much larger and structurally more complex than prokaryotic cells. In eukaryotic cells the key processes of DNA replication, transcription, and translation are partitioned; replication and transcription (RNA synthesis) occur in the nucleus while translation (protein synthesis) occurs in the cytoplasm. Eukaryotic microorganisms include algae , protozoa and slime molds, collectively called protists, and the fungi. The cells of plants and animals are also eukaryotic cells. A major property of eukaryotic cells is the presence of membrane-enclosed structures in the cytoplasm called organelles. These include, first and foremost, the nucleus, but also mitochondria and chloroplasts. As mentioned, the nucleus houses the cell’s genome and is also the site of RNA synthesis in eukaryotic cells. Mitochondria and chloroplasts are dedicated to energy conservation and carry out respiration and photosynthesis, respectively.
In contrast to eukaryotic cells, prokaryotic cells have a simpler internal structure in which organelles are absent However, prokaryotes differ from eukaryotes in many other ways as well. For example, prokaryotes can couple transcription directly to translation because their DNA resides in the cytoplasm and is not enclosed within a nucleus as in eukaryotes. Moreover, in contrast to eukaryotes, most prokaryotes employ their cytoplasmic membrane in energy-conservation reactions and have small, compact genomes consisting of circular DNA. In terms of cell size, a typical rod-shaped prokaryote is 1–5 µm long and about 1 µm wide, but considerable variation is possible. The range of sizes in eukaryotic cells is quite large. Eukaryotic cells are known with diameters as small as 0.8 µm or as large as several hundred micrometers. Despite the many clear-cut structural differences between prokaryotes and eukaryotes, it is very important that the word “prokaryote” not be given an evolutionary connotation. The prokaryotic world consists of two evolutionarily distinct groups, the Bacteria and the Archaea. Moreover, the word “prokaryote” should not be considered synonymous with “primitive,” as all cells living today - whether prokaryotes or eukaryotes - are highly evolved and closely adapted to their habitat.
Viruses are a major class of microorganisms, but they are not cells. Viruses are much smaller than cells and lack many of the attributes of cells. Viruses vary in size, with the smallest known viruses being only about 10 nm in diameter. Instead of being a dynamic open system, a virus particle is static and stable, unable to change or replace its parts by itself. Only when a virus infects a cell does it acquire the key attribute of a living system—replication. Unlike cells, viruses have no metabolic capabilities of their own. Although they contain their own genomes, viruses lack ribosomes. So to synthesize proteins, viruses depend on the biosynthetic machinery of the cells they have infected. Moreover, unlike cells, viral particles contain only a single form of nucleic acid, either DNA or RNA (this means, of course, that some viruses have RNA genomes).
Group of microorganisms
Group of microorganisms
(Place of microorganism in living world)
Haeckel’s kingdom Protista :
E.H. Haeckel, 1866 proposed this concept. Apart from plant and animal kingdom, the third kingdom “Protista” was developed by him. The protista includes unicellular organisms that are typically neither plant nor animal. These organisms, the protists, include bacteria, algae, fungi and protozoa. Bacteria are referred as lower protists and others are called as higher protists.
Whittaker’s five kingdom concept :
R.H.Whittaker (1969) proposed the concept of five kingdom. He divided the living things in to 5 kingdoms. 1. Planta (photosynthetic) 2. Fungi (Nutrient absorption) 3. Animalia (nutrient ingestion) 4. Protista (Eukaryotic, unicellular) includes microalgae and protozoa 5. Monera (prokaryotic) includes bacteria.
Three domains of life (Present concept):
The evolutionary relationships between organisms are the subject of phylogeny. Phylogenetic relationships between cells can be deduced by comparing the genetic information (nucleotide or amino acid sequences) that exists in their nucleic acids or proteins.
Carl Woese, an American microbiologist, pioneered the use of comparative rRNA sequence analysis as a measure of microbial phylogeny and, in so doing, revolutionized our understanding of cellular evolution. Genes encoding rRNA from two or more organisms are sequenced and the sequences aligned and scored, base-by-base, for sequence differences and identities using a computer; the greater the sequence variation between any two organisms, the greater their evolutionary divergence. Then, using a treeing algorithm, this divergence is depicted in the form of a phylogenetic tree.
From comparative rRNA sequencing, three phylogenetically distinct cellular lineages have been revealed. The lineages, called domains, are the Bacteria and the Archaea (both consisting of prokaryotic cells) and the Eukarya (eukaryotes). The domains are thought to have diverged from a common ancestral organism called last universal common ancestor (LUCA) early in the history of life on Earth.
The phylogenetic tree of life reveals two very important evolutionary facts: (1) As previously stated, all prokaryotes are not phylogenetically closely related; (2) Archaea are actually more closely related to Eukarya than to Bacteria.
Based on the molecular taxonomy, the living organisms were divided into 3 domains. (Domain refers the highest level of classification like kingdom)
Recent work, however, has shown that what were once called "prokaryotes" are far more diverse than anyone had suspected. The Prokaryotae are now divided into two domains, the Bacteria and the Archaea, as different from each other as either is from the Eukaryota, or eukaryotes. No one of these groups is ancestral to the others, and each shares certain features with the others as well as having unique characteristics of its own.
Domain 1. Bacteria – includes prokaryotic organisms
Domain 2.Archaea – includes certain prokaryotes found in enormous numbers, often from unusual habitats such as very hot, very salty, or very anaerobic (no oxygen)
Domain 3. Eukaryota – including eukaryotic organisms
Carl Woese's three domains of life
Lecture about three domains of life
Three domains and archaea
Bergey’s manual of systematic bacteriology :
Bergey’s manual is the international standard for bacterial taxonomy. The entire world of bacteria and archaea were divided into 2 kingdoms. Kingom I is Archaeota and the Kingdom II is bacteria. The manual divide the bacteria into 5 volumes. Volume I include archaea, cyanobacteria and phototrophs Volume II include the proteobacteria Volume III Bacteria with low G+C Gram positives and Volume IV high G+C Gram positives and Volume V. Planctomycetes, spirochaetes, Fibrobacteres, Bacteroides and Fusobacteria.
Group of microorganisms :
Group of microorganisms :
The microorganisms are divided into six distinct groups. There descriptions are as follow :
A. Bacteria
A. Bacteria
are group of phylogenetically related prokaryotic organisms (but not as close previously told) distinct from archaea.
- size ranges from 0.5 – 3.0 mm
- They are unicellular
- presence of cell wall (composed of lipopolysaccharide and peptidoglycon)
- simple internal structures
- able to grow in artificial media
- reproduce asexual simple cell division (binary fission)
Importance:
- some cause diseases
- important role in nutrient cycling in soil
- useful in industrial production of antibiotics, vitamins, dairy products, value added products
B. Archaea
B. Archaea
are group of phylogenetically related group of prokaryotic microorganisms which are primitive and differ with bacteria
- size ranges from 0.5 – 2.5 mm
- They are unicellular
- simple internal structures
- differ with bacteria by absence of peptidoglycon in cell wall
- Most of them live in extreme environment (high salt, methane producing organisms, thermophiles)
Importance:
- Methane (biogas) production;
- Waste water treatment and energy production;
- Temperature resistant enzymes for molecular biology and biochemistry (ex. DNA polymerase from Thermococcus
C. Fungi
C. Fungi
- are the group of eukaryotic microorganisms lack of chlorophyll.
- size ranges from 5 – 10 mm
- consist of both unicellular (yeast) and multicellular (mushroom) organisms
- saprophytic nature (derive food through dead organic matter) (renamed as organotrophs)
- Presence of cell wall (made of Chitin – polymer of N-acetyl glucosamine)
- Presence of asexual and sexual reproduction
Importance:
- Cause plant diseases; few animal and human diseases too
- Decomposition of organic matter in the soil and nutrient cycle
- Food spoilage
- Yeast – industrial importance
- Antibiotic industry
- Used as food (mushroom)
D. Protozoa
D. Protozoa
(protos – first; zoon – animal) are group of eukaryotic microorganisms lack of cell wall.
- size ranges from 2.0 to 200 mm
- Large numbers are survive in soil, fresh water and marine water
- Most are motile
- Reproduction by asexual and sexual processes
Importance :
- They play important role in food chain of aquatic environment
- Some cause diseases
- Importance in the ecological balance of many microbial communities in soil and aquatic environment
- Industrial waste treatment
- Sewage water treatment
E. Algae
E. Algae
are the group of eukaryotic microorganism that contain chlorophyll and carry out the oxygenic photosynthesis
- contain different type of pigments
- size ranges from 1 mm to many meters
- Unicellular and multicellular
- Most occur in aquatic environment
- reproduction by sexual and asexual process
Importance:
- Food producer of aquatic environment (primary producer)
- Used as food supplements
- Used in pharmaceutical industry
- Economically important products – agar, alginic acid, carrageenan
- Some cause disease
F. Viruses
F. Viruses
are non cellular microorganisms.
- Size ranges from 0.015 – 0.2 mm
- Cannot grow in laboratory condition (needs living host)
- Obligate parasites
- Lack metabolic machinery for energy generation and reproductive system for replication
- Depends higher organisms for energy and reproduction
Importance:
- Cause disease to human, animal, microorganism.
- Few used in molecular biology as vector (to transfer the genetic elements to an organism)
A description about different groups of microorganisms
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