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Samenvatting Microbiology
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Lecture 1 Impact of microorganisms on humansMicroorganisms (microbes) are life forms too small to be seen by the human eye. They diverse in form
and function and inhabit every environment that supports life. Many microorganisms are single-celled,
some form complex structures, some are multicellular. Microorganisms live in microbial communities
(see picture). Microorganisms are:
The oldest form of life
A major fraction of earth’s biomass
Surrounding plants and animals (forming very tight symbioses)
Affecting human life (infectious diseases, food and water, soils,
animal health, fuel)
Streaking on an agar plate gives you single colonies (cultivation). The important thing about these single
colonies is that each colony derives from one cell. This allows a microbiologist to isolate a micro-
organism from the environment and study it in isolation. Because studying microbial communities is
really complex.
The microorganisms can be distinguished in prokaryotic cells and eukaryotic cells. A prokaryotic cell is
recognized by the absence of a nucleus. There is also extra chromosomal DNA, which can be found in
plasmids. The plasmids contain important genetic information and can be exchanged between cells, via
horizontal transfer. For example, antibiotic resistance. The eukaryotic cell is much more complex and
contains more cell organelles.
The genome is a cell’s full set of genes. The eukaryotic DNA is located in linear chromosomes within the
nucleus. Prokaryotic DNA has typically single circular chromosome that aggregates to form the nucleoid
region.
Properties of all cells:
Structure Metabolism Growth Evolution
Properties of some cells:
Differentiation Communication Motility Horizontal gene transfer
,Morphology is the size and shape of a cell. Size range for prokaryotes is 0,2 μm to 600+ μm in diameter.
Most are between 0,5 and 10 μm long. The size range for eukaryotic cells is typically 5 to 100 μm in
length. Size matters, because when bacteria take up compound, there dependent on diffusion in the
cytoplasm. The larger the cell, the smaller the surface-volume-ratio is. This ratio is important, because
microorganisms constantly exchange nutrients and compounds and if they are dependent on diffusion,
it means that the larger they get, the slower they will grow. Major morphologies of prokaryotic cells:
Coccus Rod Spirillum
Spirochete Budding Filamentous
The microbial world is subdivided in 3 domains: bacteria, archaea and eukarya. Viruses obligate
parasites that only replicate within host cell. They are not cells and do not carry out metabolism, but
take over infected cells to replicate. Viruses have small genomes of double-stranded or single-stranded
DNA or RNA. They are classified based on structure, genome composition and host specificity (e.g.,
bacteriophages).
Microorganisms can be both beneficial and harmful to humans: agents of disease, food and agriculture,
valuable human products, energy generation and environmental clean-up. Microorganisms as agents of
disease controlled infectious diseases over the past 120 years. They do this via bacterial and viral
pathogens. Most microorganisms are beneficial, e.g., vaccination and antibiotic therapy. Many aspects
of agriculture depend on microbial activities:
Nitrogen-fixing bacteria
Cellulose-degrading microbes in rumen
Gut microbiome: digests complex carbohydrates in humans
The combination of microorganisms and food can have negative and positive impacts. It can cause food
spoilage and foodborne disease and also harvest, storage, safety, prevention of spoilage influenced by
microbes. However, they also improve food safety by preservation from dairy products and other food
products.
Lecture 2 History of microbiology/microscopy
Microbiology began with the microscope. Robert Hooke (1635-1703) was the first one to describe
microbes (micrographia in 1665). Antoni van Leeuwenhoek (1632-1723) was the first one to see
bacteria. Van Leeuwenhoek’s microscope was a light microscope (illuminating sample with visible light).
, Magnification is the ability to enlarge an image. The resolution is the ability to distinguish two adjacent
objects as distinct and separate. The limit of resolution for a light microscope is about 0,2 μm. Oil
immersion is a drop of oil used between the specimen and the lens (100x magnification). This improves
the image, because the rays of light coming from the sample are less bended in the case of oil, than in
the case of air.
Light microscopes and electron microscopes use lenses to focus light or electrons passed trough a
specimen to produce a magnified image. Both light and electron microscopists utilize stains to increase
the contrast and visibility of the specimen under study as well as to aid in the classification of those
specimens.
In order to enhance the contrast of non-pigmented cells, stains are added. Staining is increasing the
contrast for bright-field microscopy. The dyes are organic compounds that bind to specific cellular
materials. Basic dyes are positively charged and bind strongly to negatively charged cell components
(e.g., nucleic acids, acidic polysaccharides, cell surfaces). Examples are: methylene blue, crystal violet
and safranin. These dyes do not discriminate between bacteria. Staining cells for microscopic
observation goes as follows:
Differential stains also allow to separate bacteria in different groups. It discriminates between gram-
positive and gram-negative bacteria. The gram-positives have a much thicker cell wall and they have a
single cell membrane. While the gram-negatives have 2 of these membranes with a perioplasmic space
in between and a much thinner cell wall. First, color all cells purple. Then, add iodine solution for 1 min.
All cells remain purple. The next step is to decolorize with alcohol for 20 sec. The gram-positive cells are
purple and the gram-negative cells are colorless. At last, counterstain with safranin for 1-2 min: gram-
positive cells are purple and gram-negative cells are pink to red.
Cells can be made visible with different types of light microscopy. Although staining is widely used in
light microscopy, staining often kills cells and can distort their features. Two forms of light microscopy
improve image contrast of unstained (and thus live) cells. These are phase-contrast microscopy and
dark-field microscopy. Phase-contrast microscopy is based on the principle that cells differ in refractive
index (that is, the ability of a material to alter the speed of light) from their surroundings. Light passing
through a cell thus differs in phase from light passing through the surrounding liquid. In the dark-field
microscope, 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 reaches the lens. Thus, the specimen
appears light on a dark background.
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