What can we learn from growth curves?

Here's the results of the Bioscreen growth curves I ran for Rhodobacter capsulatus strains:

Each dot is the mean OD600 of 15 replicate wells, each containing 300 µl of culture, with ODs read every 20 minutes for 45 hours.  The cultures all grew up at about the same times, but I've shifted the X-axes so the curves don't overlap.  OD values below about 0.015 are not significantly above the backround absorption of the culture medium. The Y-axis is a log scale, so when doubling time is constant the dots will fall in a straight line.

I did these runs 'just-in-case', because I'm going to be working with Rhodobacter capsulatus at Memorial University in Newfoundland for the next few months (on sabbatical leave) and thought they probably wouldn't have a convenient Bioscreen that I could use.

Now I need to figure out what we learn from these, and whether I should do any more experiments before I leave UBC.

The simplest expectation is that once the cells have adjusted to the medium (after 'lag phase') they will grow at a constant rate until they run out of nutrients or experience other bad consequences of high cell density (little oxygen, accumulation of toxic byproducts).  But all of these cultures instead exhibit 'diauxy', a mid-growth shift from one resource to another.  We see  this as a brief slowing or even cessation of growth at about OD=0.05 (orange shaded band), after which growth resumes, often at a different rate.  The pause occurs because the cells need time to adjust their metabolism to a change they've caused in the medium, such as exhaustion of one nutrient or new availability of another. 

I don't know enough about R. capsulatus metabolism to speculate about what the change might be, but it might affect production of Gene Transfer Agent particles.  The pause isn't due to lysis of GTA-producing cells, because it's not changed in the ∆∆ strains, which have deletions of the GTA gene cluster and lysis gene.

SB1003, B10 and YW1 are all 'wildtype' strains, I think.  Strain YW1 grows much slower than the others, although it still speeds up after the growth pause, and it reaches a slightly lower final density.

Strain DE442 carries a mutation that causes over-expression of the GTA genes and over-production of GTA particles.  Growth curves in a 2013 paper found that this strain had a substantial drop in OD once growth ceased, thought to be due to lytic release of GTA particles, but no drop is seen in the Bioscreen culture.  That work used a low-phosphate version of a different medium, RCV.  But an earlier paper found strong lysis with the same complex medium I used (YPS), and low lysis with the high-phosphate (10 mM) standard RCV medium.

The lab upstairs has both low-phosphate and high-phosphate versions of the RCV medium, so I'm going to repeat the time course with both.