Infectious diseases
Lecture 1: Introduction
Chapters: 4, 5, 7
Malaria, TB and Cholera are the main infectious diseases that currently cause death.
Infectious diseases cause about a quarter of all deaths worldwide.
Host-pathogen interactions are central for this course. The
human body contains 10x more microorganisms compared
to human cells. These are the human microbiome or
commensals.
There are five classes of pathogens:
- Prions → proteins that aggregate in our cells (mad
cow disease)
- Bacteria
- Viruses
- Fungi
- Parasites → protozoa (unicellular), worms
(multicellular)
Protozoa, fungi, bacteria and viruses are considered
microorganisms. Microorganisms are organisms you can’t
see by the naked eye. Viruses are a ‘grey area’ because
they can’t reproduce by themselves, they need a host cell.
Worms, protozoa and fungi are eukaryote and bacteria are
prokaryote.
1
,Prokaryotic cells:
- No nucleus
- DNA is one circular chromosome
- Additional DNA as plasmids
- Transcription &translation simultaneously in
cytoplasm
- Rigid cell wall
Eukaryotic cells:
- DNA in nucleus as multiple chromosome
- Nuclear membrane, organelles
- Transcription in nucleus
- Translation in cytoplasm
- Cell wall only in fungi and plants
Both viruses and bacteria come in different shapes and
forms. For viruses there are tabacco mosaic virus, adenovirus, influenza virus and
bacteriophages. For bacteria there are cocci, rod-shaped and spiral. Bacteria can also cluster
in different ways: diplococci, streptococci and staphylococci.
Another way of determining what kind of bacteria you are working with is gram-staining.
Dark or purple stained bacteria are considered gram-positive and pinkish bacteria are gram-
negative. This is based on the composition of the cell envelope. Gram-negative bacteria have
a thin peptidoglycan layer with an additional outer membrane.
Infection → colonization and growth of a microorganism within a host. Does not always
cause disease.
Disease → damage to host, which interferes with normal functions of the host.
2
,Pathogenicity → the potential of a microorganism to inflict damage on its host (often
referring to genetic components of the pathogen, host independent)
Virulence → the extent of damage that a pathogen can inflict on its host (host-pathogen
interactions, host dependent)
Robert Koch defined the particular steps that a scientist/should take to determine which
pathogen is infecting a patient: Koch’s postulates.
1. The same pathogen must be present in every case of the disease, but not in healthy
individuals
2. The pathogen must be isolated from the sick host and grown in a pure culture
3. The pure pathogen must cause the same disease when given to uninfected hosts
4. The pathogen must be re-isolated from the newly infected host, and shown to be the
same organisms as isolated initially
The postulates do not apply to certain cases:
1. No animal model available (chlamydia)
2. Bacteria do not grow in the lab (leprosy)
3. Viruses: need host cells to grow them in the lab
4. Presence of pathogens in healthy individuals (opportunistic pathogens)
Opportunistic pathogens → cause disease in weakened individuals or when in an unusual
location; often belong to the commensal microflora.
Primary pathogens → causes disease in healthy individuals.
Virulence factors → molecules of pathogenic microorganisms that contribute to their ability
to cause disease.
Intracellular have certain advantages: hiding from the immune system.
Acute infection → multiply as fast as possible and spread rapidly (more virulent, a lot of
damage)
Chronic infection → staying in the host for a long time (less virulent, causing little damage)
3
, In a typical acute infection, the adaptive system determines whether patients recover or die.
The innate system is present from the start but unable to kill the pathogen in all patients. The
adaptive system comes in later and is not able to eliminate infection in weakened individuals.
Chronic infections often have one or more acute phases:
The major advantage of infecting chronically is there is more chance for transmission (host
usually don’t feel as sick). The disadvantage is that the pathogen must deal with both the
innate and adaptive immune system to stay in the host for a long time.
Lecture 2: Requirements for infections; Bacterial anatomy
Chapters: 5, 9
Literature: Merrel & Falkow
4
Lecture 1: Introduction
Chapters: 4, 5, 7
Malaria, TB and Cholera are the main infectious diseases that currently cause death.
Infectious diseases cause about a quarter of all deaths worldwide.
Host-pathogen interactions are central for this course. The
human body contains 10x more microorganisms compared
to human cells. These are the human microbiome or
commensals.
There are five classes of pathogens:
- Prions → proteins that aggregate in our cells (mad
cow disease)
- Bacteria
- Viruses
- Fungi
- Parasites → protozoa (unicellular), worms
(multicellular)
Protozoa, fungi, bacteria and viruses are considered
microorganisms. Microorganisms are organisms you can’t
see by the naked eye. Viruses are a ‘grey area’ because
they can’t reproduce by themselves, they need a host cell.
Worms, protozoa and fungi are eukaryote and bacteria are
prokaryote.
1
,Prokaryotic cells:
- No nucleus
- DNA is one circular chromosome
- Additional DNA as plasmids
- Transcription &translation simultaneously in
cytoplasm
- Rigid cell wall
Eukaryotic cells:
- DNA in nucleus as multiple chromosome
- Nuclear membrane, organelles
- Transcription in nucleus
- Translation in cytoplasm
- Cell wall only in fungi and plants
Both viruses and bacteria come in different shapes and
forms. For viruses there are tabacco mosaic virus, adenovirus, influenza virus and
bacteriophages. For bacteria there are cocci, rod-shaped and spiral. Bacteria can also cluster
in different ways: diplococci, streptococci and staphylococci.
Another way of determining what kind of bacteria you are working with is gram-staining.
Dark or purple stained bacteria are considered gram-positive and pinkish bacteria are gram-
negative. This is based on the composition of the cell envelope. Gram-negative bacteria have
a thin peptidoglycan layer with an additional outer membrane.
Infection → colonization and growth of a microorganism within a host. Does not always
cause disease.
Disease → damage to host, which interferes with normal functions of the host.
2
,Pathogenicity → the potential of a microorganism to inflict damage on its host (often
referring to genetic components of the pathogen, host independent)
Virulence → the extent of damage that a pathogen can inflict on its host (host-pathogen
interactions, host dependent)
Robert Koch defined the particular steps that a scientist/should take to determine which
pathogen is infecting a patient: Koch’s postulates.
1. The same pathogen must be present in every case of the disease, but not in healthy
individuals
2. The pathogen must be isolated from the sick host and grown in a pure culture
3. The pure pathogen must cause the same disease when given to uninfected hosts
4. The pathogen must be re-isolated from the newly infected host, and shown to be the
same organisms as isolated initially
The postulates do not apply to certain cases:
1. No animal model available (chlamydia)
2. Bacteria do not grow in the lab (leprosy)
3. Viruses: need host cells to grow them in the lab
4. Presence of pathogens in healthy individuals (opportunistic pathogens)
Opportunistic pathogens → cause disease in weakened individuals or when in an unusual
location; often belong to the commensal microflora.
Primary pathogens → causes disease in healthy individuals.
Virulence factors → molecules of pathogenic microorganisms that contribute to their ability
to cause disease.
Intracellular have certain advantages: hiding from the immune system.
Acute infection → multiply as fast as possible and spread rapidly (more virulent, a lot of
damage)
Chronic infection → staying in the host for a long time (less virulent, causing little damage)
3
, In a typical acute infection, the adaptive system determines whether patients recover or die.
The innate system is present from the start but unable to kill the pathogen in all patients. The
adaptive system comes in later and is not able to eliminate infection in weakened individuals.
Chronic infections often have one or more acute phases:
The major advantage of infecting chronically is there is more chance for transmission (host
usually don’t feel as sick). The disadvantage is that the pathogen must deal with both the
innate and adaptive immune system to stay in the host for a long time.
Lecture 2: Requirements for infections; Bacterial anatomy
Chapters: 5, 9
Literature: Merrel & Falkow
4
