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So obviously, viruses are nonliving. But when my teacher was teaching viruses in the video (we're doing "flip" learning this semester), the way he described it, it seemed like the viruses responded to their environment in that they moved around until they found a cell of the right type, and then they latched on and hijacked it.

I had always thought of it more like that they were just kind of floating around, carried by the bloodstream (when in an animal/plant), until they "bumped into" the right kind of cells and both sets of membrane proteins "docked". But my theory/idea doesn't really make sense because it doesn't account for how viruses would be able to infect bacteria.

But, the idea that viruses propel themselves doesn't make much sense either, because viruses are nonliving, and one of the characteristics of life that they do not meet is that living things acquire and use energy.

So I guess my question is, how are viruses propelled? Do they move themselves, or are they moved by external forces?



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All of your reasoning is correct - viruses are not motile (i.e. not self-propelled).

I don't understand why you think this would cause a difficulty in the case of bacteria.

Edit in response to comment @Remi.b

Some cursory research on estimating probabilities of collisions between particles engaged in random walks has revealed some very challenging maths. So I decided to simply look at some data.

Remarkably there is a fairly recent paper describing investigations of the kinetics of bacteriophage adsorption.

Moldovan, et al. (2007) On Kinetics of Phage Adsorption. Biophysical Journal 93:303–315

I didn't realise it was still possible to publish papers like this, but in fact it is very interesting. For our current purposes we only need to consider the data presented in Figure 3, which show that when E. coli cells at a density of 108 cells ml-1 are mixed with bacteriophage λ at a density of 5 x 104 particles ml-1, then 90% of the phage have attached to a bacterial cell within less than 10 min.

Just to relate this to a real life situation, it is estimated that sea water contains bacteria at a density of 106 cells ml-1 and phage at 5 x 107 particles ml-1.

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The OP says "But my theory/idea doesn't really make sense because it doesn't account for how viruses would be able to infect bacteria". I think the OP's issue with the fact that viruses are not motile is that he/she doesn't get how they would manage to "find" bacteria (or host cells). One could maybe improve its answer by talking about probability of "finding a new host" and about the behavior (in short) of the virus when it enters in contact with a potential host bacteria. –  Remi.b Mar 7 at 10:43
What do you mean with "I didn't realise it was still possible to publish papers like this"? –  jarlemag Mar 8 at 13:32
It was a bit of a throwaway comment - it seems to me to hark back to the days of the phage group (Delbruck, Luria, Hershey et al), and it is particularly interesting that the authors are physicists (as was Delbruck). As I said, it is an interesting paper, and I certainly didn't mean any disrespect to the authors. –  Alan Boyd Mar 8 at 17:24
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You are right, viruses are neither alive, nor are they moving by themself. They are moved by passive movements (e.g. the bloodstream or movements of the air) until they meat their target cells. This can be either a cell in the human body (for flu viruses these are cells of the respiratory tract for example) or bacteria (for bacteriophages).

For bacteria both can happen, viruses floating around meet a bacteria they can infect or the other way that moving bacteria meet the virus.

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