1. Exercise 21: Enumeration Work as two pairs per group (max 5 groups)
Lab 18 Goals and Objectives:
Exercise 21: Enumeration
Work as two pairs per group (max 5 groups)
Both pairs use same Escherichia coli culture
Use pre-poured plates: no molten agar pour plates
Divide work: One pair does Standard Plate Count, other does Absorbance Readings
Plate count pair needs: Absorbance pair needs:
3 – 99ml bottles sterile water 4 nutrient broth tubes
4 pre-poured plates 5 cuvettes
4 tubes of beads ml pipettes
(use beads to spread bacteria on plate)
1ml pipettes
For absorbance readings, make the required dilutions (1:2, 1:4 1:8, and 1:16) in the nutrient broth tubes and then transfer ~4ml of each to a separate cuvette for reading in the machine. Cuvette #1 is for the original undiluted culture (1:1)
*BRING AN ENVIRONMENTAL WATER SAMPLE FOR NEXT LAB: NO SALT WATER*

2. Fig 21.1
*Label the plates with the dilution factor!

3. Fig 21.9
1. Make dilutions in culture tubes
2. Put 4ml of original culture in a cuvette, label it 1:1
3. Transfer ~4ml
each dilution to
a cuvette
labeled with
dilution (1:2,
1:4, 1:8, 1:16)
4. Read absorbance number for all 5 in spec
beginning with most dilute

4. To save time:
1. 4ml culture into 1:2 culture tube with broth, mix with pipette
2. Suck up whole volume (8ml) with new pipette, dispense 4 ml into 1:4 broth culture tube and remaining 4 ml into 1:2 cuvette.
3. New pipette, mix 1:4 culture tube, suck up whole volume, dispense 4ml into 1:8 broth culture tube and remaining into 1:4 cuvette.
etc.

5. Exercise 21: Enumeration Work as two pairs per group (max 5 groups)
Lab 18 Goals and Objectives:
Exercise 21: Enumeration
Work as two pairs per group (max 5 groups)
Both pairs use same Escherichia coli culture
Use pre-poured plates: no molten agar pour plates
Divide work: One pair does Standard Plate Count, other does Absorbance Readings
Plate count pair needs: Absorbance pair needs:
3 – 99ml bottles sterile water 4 nutrient broth tubes
4 pre-poured plates 5 cuvettes
4 tubes of beads ml pipettes
(use beads to spread bacteria on plate)
1ml pipettes
For absorbance readings, make the required dilutions (1:2, 1:4 1:8, and 1:16) in the nutrient broth tubes and then transfer ~4ml of each to a separate cuvette for reading in the machine. Cuvette #1 is for the original undiluted culture (1:1)
*BRING AN ENVIRONMENTAL WATER SAMPLE FOR NEXT LAB: NO SALT WATER*