Molecular virology: summary
GENERAL INTRODUCTION PROF. DELPUTTE 22/02
WHY DO WE STUDY VIRUSES?
1. VIRUSES ARE VERYWHERE
There are more viruses than stars in the universe. +- 1031 individual viruses exist on our planet—enough
to assign 1 to every star in the universe 100 million times over. Viruses infiltrate every aspect of our
natural world, in seawater, drifting through the atmosphere, and lurking in miniscule motes of soil.
The number of phages (bacterial viruses) in the biosphere has also been estimated to be around 1031
- Weighs about one femtogram (10-15 grams) with a length of about 125 nm
- Phage biomass on earth: 1016 grams = 1013 kg = 1010 tonnes
o (> 1000-fold the biomass of elephants)
- The length, head to tail of all phages is > 100 million light years
VIRUSES IN THE WATER
Marine caliciviruses in oceans infect zooplanktons and whales and other marine organisms.
- Infect to very high numbers
- Origin of human viruses like hepatitis E virus
- Whales excrete >106 caliciviruses per gram of faeces -> 1013 caliciviruses daily
- Marine caliciviruses remain viable more than 14 days in 15°C seawater
MOST VIRUSES JUST PASS THROUGH US
All plants are infected with specific types of viruses which are not infectious for humans. For example
cabbage has 108 virus particles. Collect waste-water → look for for example pepper mild mottle virus.
If these viruses are present, there is faecal matter present in the waste-water.
2. VIRUSES CAN CAUSE HUMAN DISEASE
- Lower respiratory tract infections
- 20% of human cancers are caused by viruses: e.g. HPV
- Dementia: seems that some viruses are involved in
neurodegenerative diseases, e.g. herpes
In children <5 the greatest cause of death is lower respiratory
tract infections. Also diarrhoeal diseases are in the top 5: immature immune-systems. Measles used to
be a lethal killer, but we have vaccinations.
The majority of viruses that infect us have little or no impact on
our health or well-being because we have an elaborate immune
defence system to fight microbial infection. If compromised, even
the most common infection can be lethal. There are diseases our
immune-system does not work against: yellow fever, AIDS,…
1
, VIRUSES INFECT ALL LIVING THINGS
Besides humans and plants, all living things are affected. No living organism has been found that
does not carry viruses. Also bacteria, parasites, vectors,.. have viruses.
3. VIRUSES CAN ALSO BE BENEFICIAL
Viruses form a sort of gene pool, and are also essential in the
environment for biochemical cycling. The seawater has a limited
biomass of viruses, but when you look at the abundance (number of
genomes), it is huge. This is essential and mostly consists of phages.
These phages continuously kill bacteria. This is important because all the infected bacteria get lysed
and release huge amounts of nutrients. Without phages, it’s difficult for other organisms to survive.
Also, they have potential to avoid antibiotic treatment.
Pathogens can also influence one another, and infection by one virus can have an ameliorating effect
on the pathogenesis of a second virus or even bacteria: co-infection. For example HIV-infected AIDS
patients show a substantial decrease in disease progressions if they are persistently infected with
hepatitis G virus.
4. VIRUSES CAN CROSS SPECIES BOUNDARIES
Viruses generally have a limited host range, yet they can and do spread across species barriers. As
the world’s human population continues to expand and impinge on the wilderness, cross-species
(zoonotic) infections of humans are occurring with increasing frequency.
The AIDS pandemic, the highly fatal Ebola haemorrhagic fever and the SARS and SARS-CoV-2 are recent
examples of viral diseases to emerge from zoonotic infections. Current pandemic of influenza virus
H5N1 in avian -> frightening possibility of transmission to humans of a highly pathogenic strain.
5. VIRUSES DRIVE HUMAN EVOLUTION
Some pre-hominid species have survived and others have extinct. It is probable that there were
infections in the past that killed some species, and didn’t kill others. Some virus infections in the past
have shaped evolution of humans but also other animals.
Every cell in our body contains viral DNA. Human endogenous retroviruses, and elements thereof,
make up about 5 to 8% of our DNA. Most are inactive, fossil remnants from infections of germ cells
that have occurred over millions of years during our evolution. Some of them are suspected to be
associated with specific diseases, and protein products of other endogenous retroviruses are essential.
RETROTRANSPOSONS CLASS I TRANSPOSABLE ELEMENTS
A type of genetic component that copy and paste themselves into different
genomic locations (transposon) by converting RNA back into DNA through the
process reverse transcription using an RNA transposition intermediate. Our
genome is full of this.
2
GENERAL INTRODUCTION PROF. DELPUTTE 22/02
WHY DO WE STUDY VIRUSES?
1. VIRUSES ARE VERYWHERE
There are more viruses than stars in the universe. +- 1031 individual viruses exist on our planet—enough
to assign 1 to every star in the universe 100 million times over. Viruses infiltrate every aspect of our
natural world, in seawater, drifting through the atmosphere, and lurking in miniscule motes of soil.
The number of phages (bacterial viruses) in the biosphere has also been estimated to be around 1031
- Weighs about one femtogram (10-15 grams) with a length of about 125 nm
- Phage biomass on earth: 1016 grams = 1013 kg = 1010 tonnes
o (> 1000-fold the biomass of elephants)
- The length, head to tail of all phages is > 100 million light years
VIRUSES IN THE WATER
Marine caliciviruses in oceans infect zooplanktons and whales and other marine organisms.
- Infect to very high numbers
- Origin of human viruses like hepatitis E virus
- Whales excrete >106 caliciviruses per gram of faeces -> 1013 caliciviruses daily
- Marine caliciviruses remain viable more than 14 days in 15°C seawater
MOST VIRUSES JUST PASS THROUGH US
All plants are infected with specific types of viruses which are not infectious for humans. For example
cabbage has 108 virus particles. Collect waste-water → look for for example pepper mild mottle virus.
If these viruses are present, there is faecal matter present in the waste-water.
2. VIRUSES CAN CAUSE HUMAN DISEASE
- Lower respiratory tract infections
- 20% of human cancers are caused by viruses: e.g. HPV
- Dementia: seems that some viruses are involved in
neurodegenerative diseases, e.g. herpes
In children <5 the greatest cause of death is lower respiratory
tract infections. Also diarrhoeal diseases are in the top 5: immature immune-systems. Measles used to
be a lethal killer, but we have vaccinations.
The majority of viruses that infect us have little or no impact on
our health or well-being because we have an elaborate immune
defence system to fight microbial infection. If compromised, even
the most common infection can be lethal. There are diseases our
immune-system does not work against: yellow fever, AIDS,…
1
, VIRUSES INFECT ALL LIVING THINGS
Besides humans and plants, all living things are affected. No living organism has been found that
does not carry viruses. Also bacteria, parasites, vectors,.. have viruses.
3. VIRUSES CAN ALSO BE BENEFICIAL
Viruses form a sort of gene pool, and are also essential in the
environment for biochemical cycling. The seawater has a limited
biomass of viruses, but when you look at the abundance (number of
genomes), it is huge. This is essential and mostly consists of phages.
These phages continuously kill bacteria. This is important because all the infected bacteria get lysed
and release huge amounts of nutrients. Without phages, it’s difficult for other organisms to survive.
Also, they have potential to avoid antibiotic treatment.
Pathogens can also influence one another, and infection by one virus can have an ameliorating effect
on the pathogenesis of a second virus or even bacteria: co-infection. For example HIV-infected AIDS
patients show a substantial decrease in disease progressions if they are persistently infected with
hepatitis G virus.
4. VIRUSES CAN CROSS SPECIES BOUNDARIES
Viruses generally have a limited host range, yet they can and do spread across species barriers. As
the world’s human population continues to expand and impinge on the wilderness, cross-species
(zoonotic) infections of humans are occurring with increasing frequency.
The AIDS pandemic, the highly fatal Ebola haemorrhagic fever and the SARS and SARS-CoV-2 are recent
examples of viral diseases to emerge from zoonotic infections. Current pandemic of influenza virus
H5N1 in avian -> frightening possibility of transmission to humans of a highly pathogenic strain.
5. VIRUSES DRIVE HUMAN EVOLUTION
Some pre-hominid species have survived and others have extinct. It is probable that there were
infections in the past that killed some species, and didn’t kill others. Some virus infections in the past
have shaped evolution of humans but also other animals.
Every cell in our body contains viral DNA. Human endogenous retroviruses, and elements thereof,
make up about 5 to 8% of our DNA. Most are inactive, fossil remnants from infections of germ cells
that have occurred over millions of years during our evolution. Some of them are suspected to be
associated with specific diseases, and protein products of other endogenous retroviruses are essential.
RETROTRANSPOSONS CLASS I TRANSPOSABLE ELEMENTS
A type of genetic component that copy and paste themselves into different
genomic locations (transposon) by converting RNA back into DNA through the
process reverse transcription using an RNA transposition intermediate. Our
genome is full of this.
2
