Microbial Growth Flashcards
What are most medias based of and why?
2
Most growth medias are based on extracts or digests of animal tissues or plant tissues
These are used because they offer the required nutrients to support the growth of bacteria
Describe the range in different requirements needed for different types of microorganisms.
Some organisms may only need a carbon source to grow while others might need factors such as vitamins, purines, pyrimidines, specific amino acids etc
What are fastidious organisms?
Organisms that require a wide range of factors to grow
What is the name given to organisms that need a wide range of factors to grow?
Fastidious organisms
Give an example of some factors fastidious organisms might need to grow.
Vitamins
Specific amino acids
Purines
Pyrimidines
What are the main ingredients of general-purpose media?
5
Water
Protein derivatives
Yeast extract
Gelling agent
Buffers
Give some examples of what might be part of the growth derivatives used in general-purpose media.
(4)
Amino acids
Salts
Minerals
Growth factors
What is found in yeast extract?
A wide range of amino acids, carbohydrates, minerals and growth factors
What is the gelling agent found in all medias?
Agar
Why is agar used in medias?
It creates a solid media and supports the growth of single colonies
List the different types of media.
Enrichment media
Selective media
Differential media
Selective and Differential Media (combined)
What are enrichment medias?
Medias used to encourage the growth of a desired microbe in a mixed culture
Give two examples of enrichment agars.
Charcoal agars
Blood agars
Why is blood agar used often?
It improves the growth of a range of fastidious organisms and allows some bacteria to demonstrate haemolysis
What are selective medias?
This is where selective antibiotics or chemical inhibitors are added to general purpose agar to inhibit the growth of undesirable microbes while still allowing the growth of the desired microbe
What are differential medias?
Medias which are used to show differences among the microbes that grow on it
What are selective and differential medias?
These are differential medias which incorporate selective agents and
What is Mannitol Salt agar selective for?
It is selective for members of the genus staphylococcus
How is mannitol salt agar selective for members of the genus staphylococcus?
This is due to the high concentration of SALT (7.5%), only staphylococcus can tolerate high saline levels like these
What is mannitol salt agar differential for?
It is differential for staphylococci spps
How is mannitol salt agar differential for staphylococci spps?
(2)
This is due to the sugar mannitol and the pH indicator phenol red
If an organism can ferment manitol than an acid by-product is formed which will cause the phenol red in the agar to turn yellow
Give an example of a staphylococci that will ferment manitol.
Staphylococcus aureus
What is the general rule of thumb when it comes to staphylococci and mannitol?
Most pathogenic staphylococci will ferment mannitol and most non-pathogenic staphylococci will not ferment mannitol
What is MacConkey Agar selective and differential for?
Gram negative bacilli
How does MacConkey Agar isolate and differentiate gram negative bacilli>
This is done based on thier ability to ferment lactose
What is the selective agents in MacConkey Agar?
Bile salts and crystal violet
Explain how MacConkey Agar is selective for gram negative bacilli.
The bile salts and crystal violet suppress the growth of most gram positive organisms
What is the differential agent in MacConkey agar?
Lactose and neutral red indicator
How does MacConkey differentiate bacteria?
3
The lactose present provides a source of fermentable carbohydrate and the neutral red indicator changes colour based on pH
Lactose fermenters produce an acidic environment, the neutral red indicator turns red due to the low pH, red colonies and culture result
Non-lactose fermenters will remain colourless as well as the media
What indicator is used in Mannitol salt agar?
Phenol red
What indicator is used in MacConkey agar?
Neutral red
