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Bacterial death in a growing population

  1. Jan 9, 2008 #1
    Hi,

    I'm writing to you as a graduate applied math student who has taken a recent interest in biology and the modeling of all kinds of microbiological systems, something I plan on continuing with in my PhD. As such, I'm modeling biology without knowing as much about it as I should :P My question is this:

    Assume a bacterial population where all the cells are identical except that some of them are over-expressing a little needed gene, or some of them are unable to make use of a certain sugar in their environment. Now, assuming a plentiful growth medium, by what mechanism will these bacteria with reduced fitness die out?

    I imagine that they will clone themselves more slowly than the other bacteria, but even if a maximum population is reached, shouldn't they just keep multiplying, albeit more slowly?

    Regards

    ViktigLemma
     
  2. jcsd
  3. Jan 9, 2008 #2

    jim mcnamara

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    One possible answer to the concept of a growth endpoint: an end product of metabolism builds up, and all cells shut off some enzymes in a feedback loop. Eventually all cells stop growing. Example: yeast produce ethanol, when ethanol concentrations are high enough the yeast begin a shutdown. Yeast that have higher tolerance persist longer.

    Another viewpoint is K, the carrying capacity. There is a finite limit to the number cells that can physically grow in one spot, even if there are lots of nutrients. This may be related to access to the medium, or more likely, some form of allelopathy - when too many organisms are physically too close there may be a buildup of a compound that is otherwise not toxic at lower concentrations - not the idea of alcohol above, just density-dependent responses. Kinda like 'good fences make good neighbors'.

    See if one of those will work for you.
     
  4. Jan 9, 2008 #3
    What I see is that I definitely need to know more about bacteria and bacterial growth in general.

    The things you mention should definitely be part of my model. I've no idea how to implement it though.

    If I understand this correctly, once a population has reached it's maximum size as allowed by its physical compartment and availability of nutrients, death of cells occur because of external stress, that may well be caused by the cells themselves. Cells with a lower fitness will be less able to fend off this stress, and so they will die with a higher probability and after a while these mutants are lost?
     
  5. Jan 11, 2008 #4

    jim mcnamara

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    You have bad assumptions, IMO. Bacteria, when the going gets rough, may form resting bodies called spores. Spores are amazingly resistant to adverse conditions - this is why an autoclave (steam sterilizer) operates at temperatures above the boiling point of water.

    Spore formation means the individual cell did not die, it went into hibernation to pass the time when more favorable conditions arise again. "hibernation" is not the best term, but you get the idea.

    One more point - many bacteria can swap DNA segments, even across species. This is called transformation. So, if you start a population in media with two genetically different cells, the genetic makeup of cells you end up with at the endtime may be a hybrid of those two cells.
     
  6. Jan 12, 2008 #5

    iansmith

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    Biofilm formation would be the alternative for some bacteria that do not form spore and some other go into a type of hibernation state we called "Viable but non-culturable" (VNBC)
     
  7. Jan 14, 2008 #6
    Excellent. Since I'm going to continue working on this model for a while I would like to figure how just how bad my assumptions are. In the final report I'd like to include criticism of the model assumptions. I have searched the internet, but I think my lack of knowledge makes it hard to find the right sources.

    Preferably I'd like to take courses in micro biology but I think I'm stuck with just reading the material. May I ask, what typical sources - books, papers - should I investigate to learn more about this matter?
     
  8. Jan 14, 2008 #7

    Moonbear

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    I agree that a course in microbiology would be the best approach, especially if you intend to pursue this work further for a Ph.D. project.

    Until then, certainly getting your hands on a good microbiology textbook would be the most efficient starting point so you at least understand the basics and all the terminology/definitions so you can then start delving more properly into the literature. I'll leave it to iansmith to recommend a good textbook for such a need, since he's the resident microbiologist and is likely to be more familiar with the range of texts and which are most up-to-date in content.
     
  9. Jan 14, 2008 #8

    iansmith

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    The textbook I own for general microbiology is Microbiology, 4ed by Prescott, Harley and Klein. It is a fairly good book to introduce a lot of the concept and basic knowledge of microbiology and covers a wide range of sub-fields ranging from medical, environment and industrial sub-fields. The most recent edition (6ed) might cover newer topic.

    There some part available online
    http://highered.mcgraw-hill.com/sites/0072556781/student_view0/
     
  10. Jan 16, 2008 #9

    Bystander

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    Mathematical Biology, J. D. Murray might be useful.
     
  11. Jan 18, 2008 #10
    Thanks for your suggestions. It seems that I will be allowed to take microbiology courses in my PhD so I might actually get my facts straight this way, but until then I'll look into these books.
     
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