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TWiM 43 Letters

Jake writes:

Hello Vincent,
I've been listening to your TWiM podcast now for a few weeks as I am a student at SDSU taking a microbial genetics class with stanley maloy.  It be be another few months until I've caught up to the current episodes.  I saw this recent article on phage use in cleaning up achne and thought you'd like to hear about it.



Franco writes:

The article by Frits R. Mooi et al. linked on the episode “The sound of whooping cough” is contradicted by a study published on the current issue of EID: Schmidtke AJ, Boney KO, Martin SW, Skoff TH, Tondella ML, Tatti KM. Population diversity among Bordetella pertussis isolates, United States, 1935–2009. Emerg Infect Dis  Volume 18, Number 8—August 2012 http://dx.doi.org/10.3201/eid1808.120082.

I would greatly appreciate a comment on this point.

Best regards
Dr. Franco Giovanetti

John writes:

Dear TWIMmers,

I just finished listening to TWIM #39 (yes, I'm behind) and I had a question about the paper Michael reviewed.  Let me spell out how I understood the discussion, and then maybe you can tell me if I've misunderstood something, or if my question even makes sense.

Michael was talking about the question of how mutations arise and are selected for in nature.  The idea supported by Darwin (though long before anyone knew about DNA) was that mutations were in some sense driven by stress, right?  And then, there was this famous experiment which showed that mutants were present in the population before selection pressure was applied--a bunch of microbes were put on a plate with some antibiotic, and a small fraction of them were mutants that had resistance genes.  In this case, there was no pressure on the microbes to adapt the the antibiotic until they were put in a situation where they died unless they had the resistance genes.  Is this right so far?

Then, there was a second experiment, which used less powerful selection--the microbes were put on a plate where the only source of carbon was lactose, which the normal population of these microbes couldn't use for carbon.  And at first, these didn't grow much, but over time, even in an environment where the normal microbes couldn't grow Buy Viagra at all and so selection shouldn't have been able to bring anything new about, some microbes arose that could grow under these conditions.  Again, have I understood the basic idea?

Now, this is where I got a little confused.  From Michael's explanation, what I got was the idea that a bacterial cell can mutate in place even when it's not dividing.  That is, even in these bacteria that were sitting there unable to divide because they couldn't get any carbon from their environment, mutations could arise over time, and this led to a population of microbes that started out without the ability to grow with no carbon sources other than lactose, and then a few mutations arose that allowed some growth.  Is this right?  Is this kind of mutation important in eukaryotic cells, say in cancer or cellular senescence?

An alternative seems to be that the bacteria started out with a huge diversity of mutations already present, many of which gave them some small ability to get carbon from the lactose.  Then, over time, it seems like those would do all the dividing, and you'd get something like classic selection, with the bacteria that could reproduce faster leaving more copies of themselves which themselves could mutate to get better at reproducing.  Was there something observable in the experiment that ruled this out?

Anyway, thanks for answering my amateur questions.  I'm a computer scientist with no biology background, so I apologize if I'm making some dumb mistakes somewhere.

I wanted to also point out a really fun quote I ran across awhile back.  It's in a very old book (_Mutual Aid: A Factor in Evolution_ by Petr Kropotkin)--I think it was published in 1902.  It was a kind of argument against social Darwinism from a Russian naturalist and anarchist, combining observations on history and nature both.

> Mutual aid is met with even amidst the lowest animals, and we must be prepared to learn some day, from the students of microscopical pond-life, facts of unconscious mutual support, even from the life of micro-organisms.

Reading this, the words "biofilms" and "quorum sensing" popped immediately into my head.  Along with all the other kinds of mutualism between microbes.

Thanks again for your wonderful podcasts,


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