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When E. Coli divide faster, they tend to have larger volumes. If the same cells are grown in a poorer medium, they'll have longer replication cycles and shrink. What is the explanation of this phenomenon?

Refs.

http://book.bionumbers.org/how-many-ribosomes-are-in-a-cell/ http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0023126

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  • $\begingroup$ @Charles I don't think the answer is so simple. How do you justify the implication more nutrients --> larger matured cell? Faster growth might just mean that the cell divides faster, so the average volume need not change. There are other examples with mammalian cells, I'll post them here. $\endgroup$ – becko Sep 15 '17 at 8:55
  • $\begingroup$ @Charles You are correct. I meant cell division rate, not growth rate. $\endgroup$ – becko Sep 15 '17 at 12:22
  • $\begingroup$ @becko If you could, please specify the kind of cell(s) you're talking about. So far, this has only been shown to apply to E. coli, given the article you provided. $\endgroup$ – Charles Sep 15 '17 at 12:52
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Rapidly dividing bacteria contain more than one bacterial chromosome, and often more than two. The chromosomes take large amount of space in the cytoplasm and also attach to the cytoplasmic membrane. More chromosomal DNA requires larger cytoplasmic surface area; larger surface, larger volume.

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    $\begingroup$ This is interesting. Is there published research pursuing this line of thought? $\endgroup$ – becko Nov 22 '17 at 19:10
  • $\begingroup$ Cooper & Helmstetter (1968) J Mol Biol 31(3): 519-540. ". . . multiple replication forks during rapid growth [of Escherichia coli] . . ." ncbi.nlm.nih.gov/pubmed/4866337 And many papers since then; for example, recently: Khan et al. (2016) Genetics 202(3): 945-960. ncbi.nlm.nih.gov/pubmed/26801182 $\endgroup$ – user37894 Nov 22 '17 at 20:51
  • $\begingroup$ THanks, but that paper does not mention volume explicitly. I had read some stuff about the multiple forks of the plasmids in bacteria, but did not relate it to volume. Do you happen to know any paper where this connection is pursued? $\endgroup$ – becko Nov 22 '17 at 20:57
  • $\begingroup$ "The average dimensions of cells of Escherichia coli and other bacteria change markedly with changes in growth rate (20), but it has been noted that two parameters appear to remain relatively constant throughout: the ratio of cell mass to number of copies of the chromosome origin at the time of initiation of DNA replication" -- Donachie & Begg (1989) J Bacteriol 171(9): 4633-4639. ncbi.nlm.nih.gov/pubmed/2670889 $\endgroup$ – user37894 Nov 22 '17 at 21:10
  • $\begingroup$ Thanks again! That looks more directly related. I'll read this one and hopefully find some keywords that will allow me to find more recent stuff. $\endgroup$ – becko Nov 22 '17 at 21:22
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Here's one way to think about this. The key question is "how does the cell decide when to divide"?

You can imagine there are two major mechanisms:

  1. The cell needs to accomplish some task that takes more or less a constant amount of time (DNA replication may be a close example). After that's done it can divide

  2. The cell needs to accumulate a certain amount of biomass to grow to a certain size (protein synthesis is closer to this). When it gets there, it can divide.

(1) is often called the "timer" hypothesis and (2) the "sizer" hypothesis.

If (1) was true, then cells would grow at the same rate regardless of the medium. If (2) was true, then all cells would always have the same size regardless of the medium. Neither statement is true, so the truth is some combination of both effects.

However, under the timer hypothesis, larger cell size with faster growth rate comes out naturally, since the cell size will be determined by the amount of biomass that can accumulate during the constant time. So as long as there is some aspect of "waiting for something that takes a certain amount of time", you will get a positive correlation between size and growth rate.

Now this is an extremely naive explanation. In fact, not all bacteria show this correlation. There are also models other than "timer" and "sizer" -- e.g. here's an "adder" model that also explains the size effect.

Google "cell size control in bacteria" for a ton more papers.

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  • $\begingroup$ The timer model explains a correlation between biomass production per unit time (units of mass / time) and cell size. It does not explain the correlation between doubling time (units 1 / time) and cell size. $\endgroup$ – becko Sep 23 '17 at 16:08
  • $\begingroup$ I think this is a misconception. Biomass production rate is also in units of 1/time (exponential growth). More biomass produces biomass faster. In your model (linear biomass, exponential cell number), cells would get smaller over time in constant conditions. $\endgroup$ – Victor Chubukov Sep 23 '17 at 16:19

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