Lecture 1
Intro
6-9-2021
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Lecture 2
Heads up in bacteriology
6-9-2021
Classificiation
Biological definition of life: independent metabolism & replication.
Ribosomal RNA is used for classification of bacteria. rRNA is very different from a yeast for example.
The tree of life has been distinguished from using rRNA. There was one common ancestor cell and it
grew out into 3 division: Bacteria (eubacteria), Archaea (archaebacteria) and eucaryotes.
Prokaryotes → single-celled organism; no (distinct) nucleus or cell organelles → archaea & bacteria.
Eukaryotes → organisms whose cell have a nucleus and other organelles enclosed by a plasma
membrane. Organelles are responsible for a variety of functions, like energy production
(mitochondria have their own ribosomes) and protein synthesis.
Gram stain morphology of bacteria
When you gram stain a bacteria, the color at the end shows whether the bacterium is gram-positive
or gram-negative.
- Gram-positive → stay purple.
o 1 membrane layer with a thick (protective) peptidoglycan layer.
o Sporulation can occur. A part of the gram-positive bacterium divides asymmetrically
(spore can have 2 layers). Endospores are always gram-positive!
- Gram-negative → become colorless when decolorized in alcohol/acetone → become red
after treatment with safranin red.
o 2 membrane layers: 1st layer with peptidoglycan – outer membrane layer that
contains LPS (=lipopolysaccharide).
Defining the bacteria morphology, like gram staining, can good to know their behavior, so e.g. the
right antibiotic can be chosen.
Peptidoglycan forms a mesh-like layer around the cell. It is cross-linked by peptide bonds. The
peptidoglycan can be degraded by lysozyme. It is an enzyme in human tears and mucus. It cleaves
the glycan backbone of the peptidoglycan.
Morphology:
- Coccus
- Bacillus
- Coccobacillus
, - Fusiform bacillus
- Vibrio
- Spirillum
- Spirochete.
Mechanisms of genetic transfer between cells
➔ Transformation – active uptake and incorporation of exogenous of foreign DNA. The bacteria
incorporate them into their genomes.
➔ Conjugation – mating / quasi-sexual exchange of genetic information from one bacterium
(the donor) to another bacterium (the recipient). It is done through a sex pilus.
➔ Transduction – transfer of genetic information from one bacterium to another by a
bacteriophage (=bacterial viruses). The bacteriophages pick up fragments of DNA en package
them into bacteriophage particles. The DNA is delivered to infected cells and becomes
incorporated into the bacterial genomes.
➔ Transposition – a transposon can jump between different DNA molecules (once inside a cell).
E.g. plasmid to plasmid or plasmid to chromosome.
Resistance development and evolution
It can be the case that via transformation or
conjugation a bacterium like Staphylococcus aureus
(see MRSA) can receive vancomycin-resistance from
another bacterium, like enterococcus (VRE). In the
case of transformation, the enterococcus cell can
lyse and release its DNA. This can be taken up by
MRSA. It is more likely that it happens via
conjugation. The transposon containing the
vancomycin resistance gene jumped out and
inserted into the multiple antibiotic resistance
plasmid of the MRSA. The new plasmid is readily
spread to other S. aureus bacteria via conjugation.
Bacterial metabolism
Aerobic glucose metabolism → 38 ATP obtained from 1 glucose molecule.
,Fermentation of pyruvate by different microorganisms results in different end products. The clinical
laboratory uses these pathways and end products as a means of distinguishing different bacteria.
Glycolysis input: glucose + 2 ATP.
Glycolysis output: 2 molecules of pyruvate (when there is oxygen) or lactate (absence of oxygen).
Lecture 3
Chapter 14 p142-150
8-9-2021
Bacterial pathogenesis
=interactions between prokaryotes and eukaryotic cells.
Pathogens = bacteria that cause disease when entering the body. Pathogens that infect a cell are
only beneficial for the microbe itself, not for the host. E.g. Salmonella typhimurium.
Pathobionts = commensal bacteria that can cause disease when given the opportunity. They do not
necessarily harm the host, but it can be possible when they go to another site of the body for
example. E.g. Staph. epidermis, E. coli.
Entry into the human body: ingestion, inhalation, trauma needlestick arthropod bite, sexual
transmission. A good barrier is the skin, because most microbes cannot survive at low humidity and
the skin is rather dry. Also mucus has some defense mechanisms. Think of tears that contains
lysozymes that can degrade peptidoglycan. When a gram-positive comes in contact with lysozyme, its
protecting peptidoglycan layer will degrade and the bacterium can be more easily attacked by the
host. Besides lysozyme, ciliated epithelium that lines the upper respiratory tract, acid and bile in the
GI tract and other antibacterial secretions in tears and mucus work as protection.
Opportunistic bacteria take advantage of preexisting conditions, such as immunosuppression, to
grow and cause serious disease (e.g. skin of burn victims; lungs of patients with cystic fibrosis).
Transmission of microorganisms
Via:
- Saliva
- Aerosols
- Blood
- Skin contact
- Genital secretions
- Fecal-oral route
- Vectors such as mosquitos
Human barriers (summed up)
Skin:
, - Dry and acidic
- Keratin (difficult to degrade)
- Erosion (shedding of bacteria)
- Toxic fatty acid
- Microbiota (competition)
- Antimicrobial peptides (defensins)
Mucosa:
- Lysozyme
- Lactoferrin (Fe-limitation)
- Secreted IgA
- Antimicrobial peptides (defensins)
Skin and mucosa (together with epithelium, gastric acid, gal, digestive enzymes and competition
microbiota) is a natural barrier. Besides the natural barrier there are also the innate immune
responses, which is a fast antigen-non-specific local response, and antigen-specific immune
responses.
Bacterial infection mechanisms
Virulence = the ability to cause disease.
Virulence factors = mechanisms bacteria use to maintain their niche. They enhance to bacteria to
remain in the body and harm the body to cause disease. Mechanisms:
- Adherence
- Motility
- Invasion
- Toxic byproducts of growth (gas, acid).
- Exotoxins: degradative enzymes, cytotoxic proteins.
- Induction of inflammation: superantigens (special group of toxins), endotoxin (LPS; only
gram-negative bacteria).
- Immune evasion: capsule, extracellular proteins (protein A), intracellular growth.
Some bacteria have adhesins that bind to specific receptors on the tissue surface and keep the
organisms from being washed away. Many of these adhesin proteins are present at the tips of
fimbriae/pili and bind tightly to specific sugars on the target tissue. There are different types of pili,
for different interactions (different binding/anchoring needed for a certain tissue).
Flagella are there to direct and let the bacterium move. After entering the eukaryotic cells, the
energy inside the cell is used for mobility (so not pili anymore).
Toxins are bacterial products that directly harm the tissue or trigger destructive biological activities.
Toxins and toxin-like activities include degradative enzymes that cause lysis of cells or specific
receptor-binding proteins that initiate toxic reactions in a specific target tissue.
- Endotoxins promote excessive or inappropriate stimulation of (innate) immune responses.
Endotoxins is a component in LPS (lipid A portion). This why they are only found in gram-
negative bacteria.
- Exotoxins are proteins that can be produced by gram-positive or gram-negative bacteria and
include cytolytic enzymes and receptor-binding proteins that alter a function or kill the cell.
In many cases, the toxin gene is encoded on a plasmid.
Haemolysis by toxins of bacteria = rupturing (lysis) of red blood cells. This can lead to cell death.
➔ Alpha hemolysis = incomplete destruction of red blood cells in the blood (leaves a greenish
color behind).
➔ Beta hemolysis = process of complete destruction of red blood cells (clear zone around the
bacterial growth).
➔ Gamma hemolysis = no involvement of any breakdown of red blood cells.
In general, a bacterium cannot benefit if its toxins make a pore in the cell, causing it to die. A more
effective way is to interfere in another way with toxins, e.g. release of ions.
