Distribution, Disinfection and Sterilization of Bacteria

Microbiology Lab report

Experiment 3:

Distribution, disinfection and sterilization of bacteria

Abstract

The purpose of this experiment is to understand that the distribution of microorganism is universal in nature including air, soil, water, animals, and human body. Most microbes do not cause diseases and are not poisonous to human.

1. Microorganisms in air

Material                Agar plate

Methods

1. Mark the bottom of one clean plate with name, date and 'air sample'.
2. Remove the lid of the agar plate and allow the plate to remain uncovered for the entire lab period.
3. Replace the lid and incubate at 37ºC overnight. Observe the growing patterns of microorganisms.

Result

Bacterial colonies grow on the plate.

[pic 1]

1. Microorganisms on human skin

Material                Agar plate

Method

1. Keep one's finger on the agar surface of a clean plate for a short time.
2. Incubate at 37ºC overnight. Observe the growing patterns of microorganism.

Result

Bacterial colonies grow on the plate.

[pic 2]

1. Boiling and autoclaving

Principle

Heat is the simplest mean of sterilizing materials, provided the material is itself resistant to heat damage. Heat acts by denaturing cell proteins and nucleic acids and by disturbing cell membranes. A temperature of 100ºC will kill all vegetative forms but not spore forms of bacteria within 2-3 minutes in laboratory scale cultures. A temperature of 121ºC for 15 min is used to kill spores. There are two kinds of heat sterilization methods: dry heart and moist heat (steam). At same temperature, steam is more effective because steam provides a means for distribute heat to all parts of the sterilization vessel.

Materials

Broth culture of Bacillus subtilis and E.coli; broth tubes (12), pipette, autoclave, metal pot, induction cooker.

Methods

1. Take 12 broth tubes, 2 for each bacteria, divide into three groups, mark the tubes with group number and bacteria names (Bacillus subtilis or E.coli).
2. Using pipette, transfer two drops of bacteria (either Bacillus subtilis or E.coli) into broth tubes according to labeling.
3. Treat the bacteria by boiling (group A) or autoclaving (group B). Leave one group as untreated control (group C).
4. Incubate the broth tubes at 37ºC overnight. Observe the growing patterns of microorganisms and record the result in the table below.

Results

[pic 3]

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[pic 6]                [pic 7]

1. Ultraviolet radiation

Principle

The germicidal/ microbicidal effect of sunlight is due in large part to the action of ultraviolet light. Microbicidal activity of ultraviolet (UV) light depends on the length of exposure which is 20 mins ans the wavelength of UV which is ranged between 200nm-270 nm. Mechanism of the microbicidal activity of ultraviolet (UV) light on microbes concerns the certain photochemical effects on DNA. UV treatment leads to the forming of thymine-thymine dimmers within the one DNA strand that interferes with base pairing and DNA replication.