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Chapter 1

Biological Sciences 55-237 : Microorganism and Microbiology

Biological Sciences
Course Code
BIOL 2070

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Chapter 1: Microorganisms and Microbiology
Introduction and Major Themes of Microbiology
1.1 What is Microbiology About and Why is it Important?
Microbiology revolves around two interconnected themes:
o Understanding the nature and functioning of the microbial world
o Applying our understanding of the microbial world for the benefit of
humankind and Earth
Use microbial cells to probe fundamental processes of life
o We have developed a sophisticated understanding of chemical and physical
basis of life
o Have learned that all cells share much in common
Microbiology is at the forefront of many important breakthroughs in medicine,
agriculture, industry
Microorganisms can be beneficial or detrimental
Microorganisms collectively constitute the bulk of biomass on Earth and carry out
many necessary chemical reactions for higher organisms
Humans, plants, and animals are intimately dependent on microbial activities for the
recycling of key nutrients and for degrading organic matter
Microorganisms support and maintain life on Earth
1.2 Structure and Activities of Microbial Cells
Microbial cells interact with their environment and with other cells in dynamic ways
Elements of Microbial Structure
All cells have a cytoplasmic membrane
o Permeability barrier
o Separates cytoplasm from outside of the cell
Cytoplasm is an aqueous mixture of macromolecules, small organic molecules
(mainly precursors of macromolecules), various inorganic ions, and ribosomes
Cell wall
o Lends structural strength
o Relatively permeable
o Stronger than the membrane
Eukaryotes are typically much larger than prokaryotes and contain an assortment of
Eukaryotic microorganisms include algae, protozoa and other protists, and fungi
Bacteria and Archaea are not closely related in an evolutionary sense
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Genes, Genomes, Nucleus, and Nucleoid
Life processes are controlled by the genome
A gene is a segment of DNA that encodes protein or an RNA molecule
In eukaryotes, DNA is present as linear molecules within the nucleus
The genome of prokaryotes is a closed circular chromosome
o A few prokaryotes have linear chromosomes
Most prokaryotes have only a single chromosome, but many also contain one or
more circles of DNA called plasmids
o Plasmids typically contain genes that confer a special property on the cell
Most genes on the chromosome are needed for basic survival
Eukaryotic cells typically have much larger genomes than do prokaryotes
Activities of Microbial Cells
In nature, microbial cells typically live in groups called microbial communities
All cells show some form of metabolism by taking up nutrients from the
environment and transforming them into new cell materials and waste products
o During these transformations, energy is conserved that can be used by the
cell to support synthesis of new structures
Growth increase in cell number as a result of cell division
Catalytic and genetic events in a microbial cell are coordinated and highly regulated
to ensure that new cell materials are made in the proper order and concentrations
and that the cell remains optimally tuned to its surroundings
Many microbial cells are capable of motility, typically by self-propulsion
o Allows cells to move away from unfavourable conditions and to exploit new
resources or growth opportunities
Some microbial cells undergo differentiation, which may result in the formation of
modified cells specialized for growth, dispersal, or survival
Cells respond to chemical signals in their environment, including those produced by
other cells of either the same or different species
o These signals often trigger new cellular activities
Microbial cells exhibit intercellular communication
o They are aware of their neighbours and can respond accordingly
Many prokaryotes can transfer genes to or accept genes from neighbouring cells,
either of the same species or of a different species, in the process of genetic
Evolution is the process of descent with modification in which genetic variants
(mutants) are selected based on their reproductive fitness
Evolution in microbial cells can be very rapid when selective pressure is strong
Genetic exchange between prokaryotic cells, which is independent of evolution, can
also significantly accelerate the adaptation of cells to new habitats or to rapidly
changing conditions
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1.3 Evolution and Diversity of Microbial Cells
The First Cells and the Beginnings of Evolution
All cells descended from LUCA
After the first cells arose from nonliving materials, a process that occurred over
hundreds of millions of years, their subsequent growth formed cell populations and
these began to interact with other cell populations and these began to interact with
other cell populations to form microbial communities
Evolution and genetic exchange served up variants that could be selected for
improvements that made their success and survival more probable
Life on Earth Through the Ages
Microbial cells first appeared 3.8-3.9 BYA
During Earth’s first 2 billion years, its atmosphere was anoxic (O2 was absent), and
only nitrogen (N2), carbon dioxide (CO2), and a few other gases were present
Only microorganisms capable of anaerobic metabolism could survive under these
Evolution of phototrophic microorganisms occurred within 1 billion years of the
formation of Earth
The first phototrophs were relatively simple ones
o Purple or green bacteria and other anoxygenic (non-oxygen-evolving)
Cyanobacteria (oxygen-evolving phototrophs) evolved from anoxygenic
phototrophs a billion years later
o Began the slow process of oxygenating Earth’s atmosphere
Triggered by increases in O2 in the atmosphere, multicellular life forms evolved and
continued to increase in complexity
Plants and animals have only existed for half a billion years
o 80% of life’s history was exclusively microbial
Three domains
o Bacteria
o Archaea
o Eukarya
Microbial Diversity
No fossils to guide construction of a microbial evolutionary tree
Each cell contains a record of its evolutionary history embedded in its genes
Genes that encode rRNA have emerged as excellent barometers of microbial
Thousands of species of Bacteria and Archaea exist as well as hundreds of species of
microbial Eukarya
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