Chapter 1 (Paragraphs 1-6 & 9-14)
The cytoplasmic membrane (permeable barrier) separates the inside of the cell (the
cytoplasm) form the outside. The cytoplasm is a mixture of macromolecules
(proteins, lipids, nucleic acids and polysaccharides), small organic molecules, various
inorganic ions and ribosomes. Some cells have a cell wall (relative permeable),
located outside the cell membrane. Most microorganisms have a cell wall.
There are two structural classes of cells: prokaryotic cells and eukaryotic cells.
Eukaryotic cells
Plants, animals, algae, protozoa and fungi
Contain organelles
DNA in the nucleus
Genomes are big and wide
Prokaryotic cells
Bacteria and Archaea
Lack a nucleus and typically lack organelles
DNA as a nucleoid
Most cells have plasmids
Genomes are small and compact
Microbial cells exhibit intercellular communication: they are aware of their
neighbors and can respond accordingly.
Many prokaryotic cells can also exchange genes with neighboring cells, either of the
same species or of a different species, in the process of horizontal gene transfer.
Microbes are abundant in habitats that are much too harsh for other forms of life.
Such microorganisms are called extremophiles.
Robert Hooke wrote Micrographia, the first book devoted to microscopic
observations, he illustrated many microscopic images. This was the first known
description of microorganisms.
Antoni van Leeuwenhoek was the first person to see bacteria and he drew them
down (wee animalcules). Van Leeuwenhoek constructed extremely simple
microscopes containing a single lens to examine various natural substances for
microorganisms. These microscopes were light microscopes, where the sample is
illuminated with visible light.
Bacteria can be divided into two groups: Gram-positive and Gram-negative. After
Gram staining, Gram-pos bacteria appear purple/violet and Gram-neg bacteria
appear pink. The color difference arises because of differences in the cell wall
structure of the bacteria.
After dyeing the cells, the cells are treated with ethanol, which decolorizes Gram-neg
cells but not Gram-pos cells. Finally, cells are counterstained with a different-colored
stain. As a result, Gram-pos and Gram-neg cells can be distinguished
microscopically by their different colors.
Disadvantages from stain:
It can kill cells
It distorts their features
, Two forms of light microscopy (that can improve image contrast of unstained (live)
cells):
Phase-contrast microscopy: based on the principle that cells differ in
refractive index from their surroundings. This results in a dark image on a light
background.
Dark-field microscopy: light does not pass through the specimen. Instead,
light is directed from the sides of the specimen and only light that is scattered
when it hits the specimen can reach the lens. The specimen appears light on a
dark background.
The resolution of the dark-field microscope is often better than the resolution of the
light microscope.
The fluorescence microscope visualizes specimens that fluoresce. Cells are made
to fluoresce by illuminating them from above with light of a single color. Cells appear
to glow in a black background. The cells fluoresce either because they contain
naturally florescent substances or because they have been stained with a fluorescent
dye.
Pasteur studied crystals formed during the production of alcohol. He observed two
types of crystals that had mirror-image structures. He discovered that chemically
identical substances can have optical isomers, which have different molecular
structures that can influence their properties. Pasteur went on to discover that
microorganisms could discriminate between optical isomers.
Robert Koch found evidence for the germ theory of infectious diseases. Koch
formulated a set of rigorous criteria, known as Koch’s postulates, for definitively
linking cause and effect in an infectious disease. Koch’s postulates stressed the
importance of laboratory culture of the putative infectious agent followed by
introduction of the suspected agent into virgin animals and recovery of the pathogen
from diseased or dead animals.
Koch discovered the cause of tuberculosis and he was awarded with the Nobel Prize
for Physiology or Medicine.
Sergei Winogradsky proposed the important concept of chemolithotrophy, the
oxidation of inorganic compounds to yield energy. He was also the first to
demonstrate the process of nitrogen fixation.
The universal tree of life based on rRNA genes sequences is a genealogy of all life
on Earth. It is a true phylogenetic tree: a diagram that depicts the evolutionary
history (the phylogeny) of all cells and clearly reveals the three domains. The root of
this tree represents a point in time when all extant life on Earth shared a common
ancestor, called LUCA.
Microorganisms can be divided into four groups: Bacteria, Archaea, Eukarya and
viruses.
Bacteria (prokaryotic cell structure)
Among this group, 30 major phylogenetic lineages (phyla) have been described.
Almost all bacteria in cultivation belong to one of the four phyla: Actinobacteria,
Firmicutes, Proteobacteria and Bacteroidetes. There are only 30 phyla described, but
there is strong evidence that almost 80 phyla exist.
