I've nothing very definite in mind, but such inconsistency, if not due to genetic heterogeneity in the inoculum, suggests the possibility that some heterogeneity is arising during the PO4/As-limited growth period, some cultures stochastically acquiring the ability to pass the barriers by some standard or non-standard (mutational) process. Perhaps you need a way to cell-sort that could separate fast-growing from slow-growing cells in a culture that is in the process of escaping the growth limitation.I'm pretty sure the discordant results aren't due to mutation, for two reasons.
First, the #4 cultures that grew quickly appeared to do so with no lag and no period of slow growth during which mutations could arise. The #4 cultures became turbid just as fast as the control #3 cultures without arsenic, so I think that all of the cells in the inoculum contributed to the growth.
Second, one of the experiments where #4 didn't grow (expt. 2) was begun with cells taken from the fast-growing #4 culture of the previous experiment (expt. 1), rather than from the frozen stock of phosphorus-depleted cells used for the other three experiments. If the growth in expt. 1 was due to a mutant clone, the #4 culture in expt. 2 should also have grown fast, but it didn't.
I started to comment on the original post, but then saw this. I was wondering about a phase variation phenomenon. If some sub-population varies early, there are sufficient nutrients to support rapid growth, whereas if variation occurs late, there would be insufficient nutrients. Have you noticed any differences in the colony morphology or growth properties on your plates for cfus?
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