Problem with Lab result

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• #16106
Layd33foxx
Participant

So I need help on how to do this graph correctly:
ðŸ™„
The experiment is Enumeration of Bacteria:The standard plate count

My results for the Quantitative plating method is:
Dilution Bottle — ml Plated — Dilution — Dilution Factor — Number of colonies
b(1:10,000) — 1.0 — 1:100,000 — 10^4 — TMC
b(1:10,000) — 0.1 — 1:1,000,000 — 10^5 — TMC
c(1:1,000,000) — 1.0 — 1:1,000,000 — 10^6 — 267
c(1:1,000,000) — 0.1 — 1:10,000,000 — 10^7 — 31

b) how many cells per ml were in the undiluted culture?___I didn’t get this answer ðŸ˜³

Second Part:
Optical density determination

Dilution — Optical Density
1:1 — 30% T; 0.56 A
1:2 — 50% T ; 0.33 A
1:4 — 72% T; 0.16 A
1:8 — 85% T; 0.09 A
1:16 — 91% T; 0.05 A

The graph x-axis is Dilution & the y is Optical Density. What number or calculation am I suppose to do to get the optical density because if I use A in optical density my graph goes down negative I dont think it’s correct. Please help. Thank you.

• #109719
JackBean
Participant

a) If you get 267 colonies with 10^6 dilution, the original number is 267.10^6; if you get 31 colonies with dilution 10^7, the original number was 31.10^7. Now just make average of these two numbers.

Was that so hard?

b) the more cells, the more light is dispersed and thus less light is passed through. Thus when you dilute your sample, you have less bacteria per volume and thus the optical density decreases.

• #109729
Layd33foxx
Participant

Thank you for clearing it up ðŸ™‚