“A-choo!”

“Bless you!” -  we would say in response. If only we knew how many virions (virus particles) a person with the flu ejects in just one sneeze. We would step back a few meters, with the first signs that he is about to sneeze. In one sneeze, there are, on average, around 30 thousand drops of saliva per 0.1 microliters, and in one cough, the situation is not much better – up to about 4,000 drops. If we take into consideration that each drop consists of up to about one hundred virions, well, you are welcome to do the math yourselves

A person sneezing  | Photograph: Public Health Image Library, CDC

Before you step back in order not to catch anything, stop for a moment and think: if just one sneezer ejects so many virions, how many virions does he have in his throat? How many virions are there in all the collective sick people’s throats? Scientists estimate that in all of the bodies of water in the world there are about 1031 virions – a number not so far from the estimate for the number of stars in the universe – so how can you even start guessing how many types of viruses are there actually in the world, and how do you divide them into families?

Sounds impossible? Maybe, but you have to start from somewhere. Researchers from a number of research facilities around the world came together for a joint study that set out to assess the number of mammalian virus families. The researchers chose the "Indian flying fox" bat as the mammal representative. This bat is one of the largest bat species in the world, and is also a host for many viruses that are able to jump from it and infect other mammals – for instance, Ebola, rabies, and coronaviruses (which cause diseases such as SARS and pneumonia etc).

The “Indian flying fox” bat  | Photograph: Wikipedia

Using innovative methods of genetic sequencing, the scientists collected information on different types of viruses from nine known families in the body of the bat, and identified 58 different types of viruses lurking on it. You may say this not so much, but hold on just a moment – if this bat has just 58 virus types, and it is just one representative of around 5,500 types of mammals that exist in the world, how many viruses do all the mammals have in total?

Before you do the math yourselves, note this: a simple multiplication like this assumes that the number of viruses lurking on each mammal is identical, or at least that 58 is their average. This assumption, of course, is completely wrong, but for now we do not have a more accurate way to estimate this number. So if we are already into educated guessing, let us take this exercise one step further: according to current assessment, there are around 1,740,330 types of multicellular organisms. How many types of viruses do you think lurk on all multicellular organisms? And what about the viruses that lurk on the massive amount of unicellular organisms? Scary. Good luck to the scientists!

How are viruses characterized?
In order to organize the confusing numbers a bit, the International Committee on Taxonomy of Viruses (yes, at least one international committee is required to solve this kind of problem) has created a characterization method for virions, that goes according to their properties. The method begins with the properties of their genetic material, continues with the proteinous capsid that stores this genetic material inside it and ends with the size and shape of the virion

Let us name some of the interesting properties appearing in the committee’s official method:

The type of genetic material: this property has two categories (1) virions containing RNA as the genetic material and (2) those containing DNA.

The symmetry of the capsid: the capsid is the proteinous envelope of the virion, which is characterized by three types of symmetries: a capsid with icosahedral symmetry (a shape with twenty equally-sized faces), a capsid with helical symmetry and a capsid with complex symmetry.

The existence of a fatty envelope (membrane): some virions bud out of the cell that created them inside a fatty envelope, which they have “stolen” from the cell itself, and which covers the capsid; others leave the cell naked, with no envelope.

The genomic architecture: the virion genome is characterized by a number of properties stemming from the structure of chemical material comprising it either the DNA or RNA. First, as we already must know, DNA in human cells appears as a double helix of two strands turning around each other, and both turning around another axis. Nevertheless, DNA can also exist as a single strand without losing genetic information, since the two strands are complimentary, and the genetic information on one strand is reflected in the other. RNA also has this property, and both can appear in virions as a single strand or as two complimentary strands.

Second, DNA and RNA share an interesting common property: their strands have a directionality that dictates the direction from which the transcription (in DNA) or translation into protein (in RNA) of the molecule will begin. Transcription of the DNA molecule is done according to the information on one of the strands, termed the AntiSense strand, marked with a minus sign (-). The complimentary strand is termed the Sense strand and is marked with a plus sign (+).

The structure of the DNA molecule displays its directionality: on the left strand, the 3’ end is at the bottom, and on the right one it is on the top. The 5’ end is on the opposite sides. | Illustration: Madeleine Price Ball, Wikipedia
If we now combine these two properties together, we will discover there are actually three types of virions: those that contain just the Sense strand, those with just the AntiSense strand, or those that contain both in a double-helix structure. RNA also has directionality, and therefore its strands are also termed AntiSense and Sense. RNA-containing viruses come in three “flavors” as well.

A third property is related to whether the genetic material is stored in a circular, linear or segmented structure. Finally, the fourth property relates to its length, namely the number of bases along the strand.

The size of the virion: the diameter of the virion ranges between 18 nanometers and 14 thousand nanometers (nanometer=1 billionth of a meter).

In sum, scientists these days are attempting to classify viruses into groups and families according to their properties. Only thousands of viruses are documented in the records of the International Committee on Taxonomy of Viruses to date, and it seems there is much more work to be done. ‎

As genetic sequencing techniques become more sophisticated in the future, we will be able to add more criteria and properties to the virus classification system, such as with the appearance of common sequences in their genetic material. Until then, scientists will continue using the best tools at hand in order to learn about these wondrous creatures, which are involved in everything – the viruses.

Dr. Haim Haviv
Davidson Institute of Science Education
Weizmann Institute of Science

Article translated from Hebrew by Elee Shimshoni, PhD student at the Weizmann Institute of Science.

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