27 Questions
What is the minimum temperature to which the medium should be cooled?
50 °C
Why is it necessary to flame the lip of the bottle whenever the lid is replaced?
To prevent contamination
What is the purpose of incubating the plates at 37 °C for 48 hours?
To check for sterility and dry out excess moisture
What should be done if there is growth of microbes on the surface of any of the agar plates?
Discard the particular agar plate
How much of the plate surface should be covered with the medium?
One-half
What is the purpose of using disinfectants or sterilization methods in laboratories?
To kill or inactivate microorganisms
What is the primary consideration when preparing a culture medium for the cultivation of a microorganism?
The nutritional requirements of the microorganism
What is the difference between a synthetic medium and a defined synthetic medium?
A synthetic medium has unknown chemical composition, while a defined synthetic medium has known specific kinds and amounts of chemical substances
What is the term for a culture medium that is used to isolate and purify microorganisms from a mixed culture?
Selective medium
What is the term for a liquid culture medium?
Broth
What is the effect of high concentration of sugar or salt on cells?
They lose water
What is the primary function of soaps and detergents in chemical methods?
To remove microbes, oils, and dirt
Which of the following is NOT a characteristic of a disinfectant?
It should kill all forms of life
What is the primary function of alkali in soap?
To destroy microorganisms
What is the primary use of chlorine?
To kill pathogens in water
What is the primary use of phenol derivatives?
To disinfect skin, instruments, and furniture
Why is it necessary to heat the entire metal inoculating loop in the flame?
To kill any bacteria that may be present on the loop
What is the purpose of streaking the agar plate in quadrants?
To ensure that the bacteria are evenly distributed across the agar plate
What is the purpose of an enriched growth medium?
To promote the growth of a particular organism by providing essential nutrients
Why should the agar plate be opened only minimally when streaking the bacteria?
To prevent contamination from other sources
Why are agar plates incubated upside down?
To reduce bacterial contamination and prevent water condensation from accumulating on the lid
What is the purpose of heating the red blood cells to 56 °C in chocolate agar?
To lyse the red blood cells and provide growth factors for fastidious bacteria
What is the purpose of autoclaving the agar preparation?
To sterilize the agar preparation
What is the purpose of cooling the inoculating loop before picking the inoculum?
To prevent the loop from burning the agar
What type of bacteria can be grown on chocolate agar?
Fastidious respiratory bacteria
What is the purpose of adding distilled water to the dehydrated medium?
To dissolve the agar powder
What is the temperature and pressure used for autoclaving?
121 °C, 15 psi
Study Notes
Enrichment Media
- Enrichment media are used to promote the growth of a particular organism by providing essential nutrients and minimizing the growth of normal competitors
- Examples include chocolate agar, which is a variant of blood agar plate, containing red blood cells that have been lysed by heating to 56 °C
- Used for growing fastidious (fussy) respiratory bacteria, such as Haemophilus influenza, which require growth factors like NAD and hematin found inside erythrocytes
Agar Preparation
- Weight the dehydrated medium (containing agar in powder form) and add distilled water, stirring to break up clumps of powder
- Pour into a glass bottle with a funnel, cap loosely, and label the bottle
- Autoclave for sterilization (121 °C, 15 psi, 15-20 min) and then remove from autoclave
Media Preparation
- Culture media are nutrient materials used in laboratories to grow microorganisms
- A culture is a microorganism that grows and multiplies in or on a culture medium
- Successful cultivation of a microorganism requires understanding its nutritional requirements and supplying essential nutrients in the proper form and proportion
Types of Media
- Synthetic media: prepared in the laboratory from materials of precise or reasonably well-defined composition
- Complex media: contain unknown or varying concentrations of nutrients
- Selective media: allow growth of specific microorganisms and inhibit others
- Differential media: distinguish between different microorganisms based on their growth characteristics
- Enrichment media: promote growth of a particular organism
Aseptic Technique
- Use sterile (aseptic) technique to ensure a new medium contains organisms of only a single species
- Heat metal inoculating loops in a flame until red hot before picking the inoculum (broth or agar)
- Cool the loops before use
- Streak the agar plate with bacteria, flaming the loop between each quadrant
- Label the agar plate, keeping the lid minimally open to prevent contamination
Incubation
- Incubate agar plates upside down to reduce bacterial contamination and water condensation
- Keep samples in an incubator at 37 °C for 48 hours to check for sterility
Chemical Methods
- Disinfectants reduce the infectivity of a material, but may not kill all forms of life present
- Chemical agents include soaps and detergents, acid, alkali, heavy metals, halogens, alcohols, and phenol derivatives
- These agents aid in the removal of microbes, oils, and dirt, and help destroy microorganisms
Learn about the role of osmotic pressure in preserving food and the use of chemical agents in disinfection. Discover how these methods prevent the growth of microorganisms and reduce infectivity.
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