very basic question - (Sep/22/2009 )

Dear all,

Please clarify as to how many cells are present in a single medium sized colony of E.coli after an incubation of 24 hours on an LB agar plate?

supposing one cell to start with and optimal conditions Ecoli can double every 30min. So you can calculate: 1 - 2 -4 =1h; 8 -16 =2h, 32 - 64 =3h........app. 1.4 x 10^14 =24h

Ratna on Sep 22 2009, 12:24 AM said:

I needed to know this one time, so did it for my organism of interest. It's not E. coli, and it replicates a little slower than E. coli, and I think I incubated for 16 hrs or so. I ended up with 2 x 10^8 or something like that, I think. It was much lower than I expected it to be.

How I did it was to take a plug of agar that contained the entire colony, resuspended it in saline, serially diluted it down to 10^-8, and spread 100 ul of the -5, -6, -7, and -8 dilutions onto media and counted the colonies. It was a pretty simple thing to do if you need to do it yourself. You can estimate by calculating growth rate and such, but that may vary by strain, antibiotics present, plasmids in the strain, and a bunch of other things. It also assumes everything in that colony is still alive, and that growth in the middle of the colony or at the bottom of the colony is the same as outside or on top.