Bio Calculation HELP

Answer 1

Hi Sarah B.,

It might help you to concentrate on the relationship between volume of a solution, concentration of the solution, and amount of solute. You can ALWAYS check that you have it right by "dimensional analysis", that is, checking that your units cancel out! So: amount (usually, g, but here, CFU's) = concentration (g, etc./mL) * volume (mL). Do you see, g = (g/mL)*mL , the units match when canceled appropriately?

OK, now. What was it for the plating out from C:? 0.1mL, unknown concentration, amount = 47 CFU.

So set up the equation above: 47 CFU = conc * 0.1 mL

Solves to conc (in C) = 470 CFU/mL, right?

Then you just need your dilutions A->B->C Each step there had 0.1mL ->10 mL, that's a 100x dilution. So A was 100 X 100 X concentration of C, or 4700000 CFU/mL (4.7 * 10^6 CFU/mL).

**********************now, that's what your teacher is expecting. But ....******************

Or was it? As a proper student of science, you already know that data has variability. Can you give the 95% probability limits on the concentration of A (assuming exactly correct dilutions, etc.)? Because, just because you found 47 CFU, doesn't mean that that is really what there really is, based on a larger sampling. This will require you to do a little reading on statistics. One way of approaching the answer to "how much is there really", is to look at what sort of range you might expect on a single test, such as you did, if you in fact had a true level of 47. For this, you want a Poisson Distribution Calculator (such as from stattrek) for λ (average rate of success ) = 47 , and Poisson random variable = the values to be tested for likelyhood, with limits of 0.05 to 0.95 as cumulative distribution. That's 36 - 58 (as vs. 47), approximately. That is to say, about 5% of the time, you would get <36 count or >58 count, just randomly.

This is not the exactly correct calculation for "how much is there really", but it does give you a general idea of the rather wide range of uncertainty entailed.

As I said, your coursework likely only entails the stuff above the ******. This latter stuff is the nitty gritty of "significance of data", which you would get into (only) if you take statistics, or enter science as a major.

-- Cheers, -- Mr. d.