Exercises 5: Post-Laboratory

Discussion for Sterilization

v

Set-up B:

Set-up A:

Set-up C:Set-up D:

Non-sterile medium in a Sterile Petri dish

Sterile medium in a Sterile Petri dish

Non-sterile medium in a Non-sterile Petri dish

Sterile medium in a Non-sterile Petri dish

Results

After 24 hours of incubatio

n

Non-sterile medium in a Sterile Petri dish

Sterile medium in a Sterile Petri dish

Non-sterile medium in a Non-sterile Petri dish

Sterile medium in a Non-sterile Petri dish

After 48 After 48 hours of hours of incubatioincubatio

nn

Non-sterile medium in a Sterile Petri dish

Sterile medium in a Sterile Petri dish

Non-sterile medium in a Non-sterile Petri dish

Sterile medium in a Non-sterile Petri dish

DiscussiDiscussionon

For Non-sterile Media:The microorganisms form mostly at the center of the medium.

For Non-sterile Petri dishes:At first, the microorganisms

form mostly in between the wall and the dish. Then, they spread at

the surface of the medium.

Death Rate α [Microorganisms]

Thermal Death Time (TDT)-time required to kill a known population of

microorganisms in a specific suspension at a particular temperature.

↑Temperature, ↓TDT and ↓Temperature,

↑TDT

Significance of this Significance of this experiment:experiment:

In order to have a successful experiment, the medium and the Petri dish must be properly sterilized.

Answers to Guide Answers to Guide QuestionsQuestions

1. How does moist heat and dry heat sterilization eliminate microbial growth?

Dry HeatMoist HeatKills by protein

coagulation/denaturation of enzymes and

essential proteins.There is a breakage of H-bonds that holds protein 3-D structure

Kills by destructive oxidation of the essential cell

constituents rather than protein coagulation

2. Explain the difference when performing sterilization in a microbiology laboratory in UP Baguio as compared to UP Manila. What necessary adjustments may be done?

There is no difference. Why?We are just concern with the system inside a pressure cooker or an autoclave which is not affected by the pressure and temperature of

the outside environment. We can still manage to sterilize culture media and

materials in an autoclave (121oC, 15 psi, 15 mins.) without making any adjustments.

3. Account for the differences in the microbial growth (absent or present) in the four set-ups.

Set-up Surface Inner Colony(groups)

nsM-sP Present Present Present

sM-sP Absent Absent Absent

nsM-nsP Present Present Absent

sM-nsP Present Absent Present

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