izzanom wrote:Hey guys can anyone explain what it means when the rules say ¨measuring bacterial growth¨.
Direct counting: Hemocytometer, counting chamber, plate counts
Indirect counting: Turbidity/Optical Density/Beer's Law, dry weight
If you look up those terms you'll probably learn most of what you need to know about measuring bacterial growth. Also, know the difference between viable and nonviable cells.
izzanom wrote:Additionally, whats a way to tell how many cells would be present after x amount of time.
The most common way to describe this is the doubling time of the population. For example, if it doubles every 10 hours and starts at 2 million, then:
- after 10 hours it will have doubled once -> 2 million times 2 = 4 million
- after 20 hours it will have doubled twice -> 2 million times 2 times 2 = 8 million
- after 30 hours it will have double twice -> 2 million times 2 times 2 times 2 = 16 million
etc. You can replace the "times 2" with 2^1, the "times 2 times 2" with 2^2, the "times 2 times 2 times 2" with 2^3, and so on, and get:
population = 2 million times 2^D where D is the number of times it doubled. Since it doubles once every 10 hours, then D=t/10 where t is the number of hours since the start.
So: P(t) = 2 million times 2^(t/10)
If you want to generalize this function, you can just replace "2 million" with "whatever you started with," which I'll call S. And you can replace 10 with the doubling time d (the time required for the population to double).
So the population as a function of time is
 = S * 2^t^/^d)
, where P is the population (in number of cells), t is the time (in hours in this case, but it can be any unit of time as long as all your units stay consistent), S is your starting population (this is often given the variable P-naught or P-initial, which look like P with a tiny o next to it or P with a tiny i next to it, respectively; but you can use whatever variable you want), and d is the doubling time.
Sometimes they give you a few data points and ask you to find out the population at a certain future time. The best thing to do is find the doubling time based on the points given and then go from there.
All of these calculations assume that the cells have infinite resources and room to grow; otherwise their growth would slow as resources/nutrients are depleted (think the microbial growth curve).