Practical 2 - Nutritional & physical requirements of microbes Flashcards
What are the two major categories of media used for routine cultivation of bacteria?
Chemically defined media and Complex media
What is chemically defined media?
Media that is composed of known quantities of chemically pure, specific organic/inorganic compounds.
The use of chemically defined media requires a microbiologist to know an organisms
Specific nutritional needs
What is complex media?
Media where the exact chemical composition is unknown.
What is complex media made of?
Extracts of plant and animal tissues - variable in their chemical composition. Most contain amino acids, sugars, vitamins, minerals but quantities are unknown
What are the three types of special-purpose media?
- Selective media
- Differential/selective media
- Enriched media
How does selective media work?
By selecting a specific group of bacteria. They incorporate chemical substances that inhibit the growth of one type of bacteria while permitting the growth of another.
Give three examples of selective media
- Phenylethyl alcohol agar
- Crystal violet agar
- 7.5% sodium chloride agar
How does phenylethyl alcohol agar work as a selective medium?
Isolates most gram positive bacteria. The phenylethyl alcohol is partially inhibitory to gram negative organisms
How does crystal violet agar work?
This is selective for most gram negative microbes
How does 7.5% sodium chloride agar work?
Inhibitory to most organisms other than halophilic organisms. Useful in detection of members of the genus staphylococcus
What does differential/selective media distinguish between?
These can distinguish among morphologically and biochemically related groups of organisms.
Give three examples of differential/selective media
- Mannitol salt agar MSA
- MacConkey agar MAC
- Eosin Methylene Blue agar EMB
What does mannitol salt agar contain and how does it work at selectivity?
Contains a high salt concentration which in inhibitory to most bacteria other than staphylococci (SELECTIVITY)
Mannitol salt agar contains mannitol as the carbon source, how does this differentiate between staphylococci strains?
Only certain staphylococci can metabolise mannitol
When mannitol is metabolised what happens?
An acid is produced which turns the indicator in the medium from red to yellow
What does MacConkey agar contain and how does it work at selectivity?
MacConkey agar contains crystal violet
Inhibits the growth of gram +ve organisms
Also contains bile salts and these inhibit many bacteria except enteric bacteria - those that live in the human gut
What are enteric bacteria?
Ones that live in the human gut
How does MacConkey agar work at differentiation?
Carbohydrate source is lactose when this is fermented acid is produced which results in yellow colour in medium.
Lactose fermenters will take up neutral red dye from medium to develop a pink colour
Non-lactose - colourless
How does Eosin Methylene Blue agar work at selectivity?
Partially inhibitory to gram positive bacteria
How does Eosin Methylene Blue agar work at differentiating?
Differentiates between enteric lactose fermenters and non-fermenters. E.Coli particularly distinctive - green sheen
What is enriched media?
Media that have been supplemented with highly nutritious materials such as blood, serum or yeast extract.
An example of enriched media is
Blood agar
What does blood agar contain?
Blood and other nutrients
What is a fastidious organism?
A fastidious organism is any organism that has a complex nutritional requirement. In other words, a fastidious organism will only grow when specific nutrients are included in its diet.
What are the three haemolytic activities?
- Gamma haemolysis
- Alpha haemolysis
- Beta haemolysis
What happens in gamma haemolysis?
No lysis of red blood cells - no significant change in appearance surrounding colony
What happens in alpha haemolysis?
Incomplete lysis of RBCs
Reduction of haemoglobin to methomeglobin
Results in greenish halo around the bacterial growth
What happens in beta haemolysis?
Lysis of the RBCs
Clearing zone around the colony
Organisms whose growth range falls between 5ºC and 20ºC with an optimum of 11ºC are called
Psychrophiles
Bacterial species that will grown within 20ºC to 45ºC - many grow at 37ºC are called
Mesophiles
Bacterial species that have an optimum growth temperature of 45ºC to 65ºC are called
Thermophiles
Organisms with an optimum temperature at about 90ºC are called
Extreme thermophiles
What is special about endospore forming organisms in terms of temperature?
Fairly narrow temperature limits in vegetative state
Spores can survive in a dormant stage over a very wide temperature range
Having a pH optima between 6.5 and 7.5
Neutrophiles
Bacteria able to grow below pH6 are
Acidophiles
Bacteria able to grow above 8.5 are
Alkalophiles
