Viable Cell Number Of Commercial Active Dried Yeast Biology Essay

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This experiment aims to identify the merit of using pour plate method and spread method. The growth of the bacteria will determine the whether the experiment is successful. The validity range will be 30-300 colony forming unit with only one type of colony. The unsuccessful result is when there is no colony form; less or more than the validity range or more than one type of colony formed (cfu).

Dilution is critical as the unknown sample may have more than one cell. By serial diluting an organism in an agar plate can be reached where only one cell remains in the medium. In the end, a pure culture is obtained. Also, from an initially high concentration, the cell concentration is decreased. Cell concentration in a sample can be in a range of thousand and millions and even billions. Therefore, it makes good sense to dilute the sample. One effective method without using too much diluent is by serial dilution. A process of diluting a sample is by performing a series of repeated dilutions. The serial dilution can be in the form of 2-fold, 5-fold, 10-fold or even 1000-fold dilution, depending on the cell concentration of the sample.

10ml valve with 9ml of saline or sterile water. Shake the undiluted active dried yeast. Then inoculate 1ml of active dried yeast and put in the 9ml of saline or sterile water. Shake the solution and label 10-1. Shake the 10-1 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-2. Shake the 10-2 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-3 . Shake the 10-3 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-4 . Shake the 10-4 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-5. Shake the 10-5 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-6 . Shake the 10-6 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-7 . Shake the 10-7 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-8 . Shake the 10-8 and inoculate 1ml of active dried yeast and put in another the 9ml of saline or sterile water. Shake the solution and label 10-9 .

4.2 Plating using Spread Plate

Using 0.1ml of 10-6, 10-7 and 10-8 dilution spread onto duplicate plates of malt agar using a "hockey stick". When feeling frictional force acting, stop the spreading. Incubate the plate for inverted at room temperature (25ËšC) for 2 days. After incubation, choose only plate that contain between 30-300 colonies per plate and work out the number of yeast cell as cfu/g

4.3 Plating using pour plate

Pipette 1ml of 10-7, 10-8 and10-9 dilution into duplicate Petri dish and add 15ml of molten malt agar ( app 45ËšC) and mix gently and allow the agar to set. Incubate the plate for inverted at room temperature (25ËšC) for 2 days. After incubation, choose only plate that contain between 30-300 colonies per plate and work out the number of yeast cell as cfu/g

Result and Calculation

Method

Dilution

Number of colonies/plate*

cfu/g

Plate 1

Plate 2

Average

Spread plate

10-6

61

50

55.5

5.55X106

10-7

TNTC

TNTC

TNTC

TNTC

10-8

TNTC

TNTC

TNTC

TNTC

Pour plate

10-7

80

TFTC

Nil

Nil

10-8

TFTC

TFTC

TFTC

TFTC

10-9

TFTC

TFTC

TFTC

TFTC

Note *TNTC- too numerous to count, exceeding 250 colonies/plate

*TFTC- too few to count, below 25 colonies/plate

**Report cfu/g to 1 decimal place.

Cell per ml= cell count X dilution factor X volume

Spread plate=55.5 colonies X 106 X (1/0.1)

=5.55X108 yeast cells

Discussion

Only spread plate of dilution of 10-6 is in range. The rest of the plate of both spread plate and pour plate is invalid. One of the problems is the dilution prepared by me and my partner. The contamination by not changing the pipette tips lead to the over growth of the yeast cells.

6.1 Problems of pour plate method

Limitation of the microorganism

Aerobic microorganism may not be suitable as the growth is largely in the agar rather on the surface of the agar.

Longer incubation period

Colonies in the agar may not be visible until later. The colonies in the agar are in partially anaerobic condition. Facultative anaerobes will therefore grow slowly and hence, will not be very obvious after a day of incubation.

Under-estimation of the cell numbers

Some yeast colonies maybe overlapping one another and may be seen as a single CFU. Furthermore the cells are exposed to warm molten agar and this can kill some heat-sensitive yeast and give a lower CFU plate count.

Problem of spread plate

Some yeast colonies maybe overlapping one another and may be seen as a single CFU.

Conclusion

This is a successful experiment although the dilution is cross- contaminated. This is because both the spread and pour plates use the diluted sample which the spread plate shows a valid colony formed. In the future when handling with yeast sample, I will plate the yeast using spread plate method. This is mainly because that yeast is heat- sensitive organism and aerobic fungus.

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