6: Viable plate count

So we can’t poke a cell to see if it’s alive — how else can we decide if a bacterial cell is alive?? What do live cells do that dead cells don’t? Well, one big thing is, live cells grow and reproduce. And they do it fairly quickly (remember, every half hour or so in the case of Vibrio bacteria in the lab). So if we were to take a batch of cells and wait a day or so, we would soon know if they were alive or not.

Let’s refine this idea a little, by taking a look at a … A day in the life of a microbe

Out in the real world (like in an oyster, in water  or your bloodstream), bacteria are limited by temperature and nutrients. In the lab, we’re going to put the  bacteria on a medium, which will do two things:

  1. take away limiting factors by providing the nutrients and growth conditions that promote growth, and
  2. fix each individual cell in one place, so it doesn’t get moved around.

So, let’s watch a single  bacterial cell, conveniently named Minnie, sitting in the middle of a batch of medium. The medium has every creature comfort that Minnie could want, and she pulls in nutrients as fast as she can. Soon she’s doubled in size and she divides in two, creating little Minnies 2 and 3.  These two also pull in nutrients as fast as they can, and a half hour later, they too are ready to divide, and they do.  Minnies 4, 5, 6 and 7 keep following the same pattern.

time: 0 min
1 rocky
time: 50 min
1 rocky
time: 1 hr 40 min
1 rocky
time: 2 hrs 40 min
1 rocky


Luckily, none of the Minnies move around much. So, they’re all stuck sitting in a little pile, but they don’t mind, as long as there are plenty of nutrients to go around. And pretty soon we have a pile consisting of Minnies 8 through 15, all descendents of Minnie the First. And so on.

plateEventually (after about 24 hours) this pile will be big enough for one of us human beings to see without even using a microscope. This tiny but visible pile is called a colony, and the original Minnie was the original colony-forming unit, or CFU.

Of course Minnie was probably not the only CFU around.  Over on the other side of the agar, Ginnie was sitting around, minding her own business, enjoying the warmth and nutrients, and growing at the same rate.  So while a few million descendants of Minnie formed a colony on one side of the plate, a few million descendants of Ginnie form a colony of the other side.  And so on, one colony for each original colony forming a unit (i.e. bacterial cell or clump of cells).

photo credits: plate