Microbiological Culture Media

Question 1

What is a solid media?

Answer 1

There is liquefiable media- this contain thermoplastic agents such as agar and gelatin.
Non-liquefiable- eg. silica as gelling agent or use of natural products such as grains, potato slices, cooked meat etc.

Question 2

What is the most important thermoplastic gelling agent and why?

Answer 2

Agar is the most important thermoplastic gelling agent because it is inert, stable once gelled and inexpensive.

Question 3

What is liquid media also known as?

Answer 3

It is also known as broth media

Question 4

What is semi-solid media?

Answer 4

it contains a small % of agar (usually less than 0.5 %) and it is used for bacterial motility, biochemical tests, growing microaerophiles.

Question 5

What are the two major classes of media?

Answer 5

1. Chemically defined Media (synthetic)

2. Complex or undefined media (non-synthetic)

Question 6

What is chemically defined media?

Answer 6

constituents and their exact concentrations are known. it can be used for metabolic/physiological/genetic studies or production of metabolites.

Question 7

What is complex media?

Answer 7

The exact chemical composition of these media are not known. it can include plant and animal tissue extracts. it can be used to support the growth of most heterotrophs and are nutrient rich.

Question 8

What are artificial complex media examples?

Answer 8

it can include natural extracts such as malt or meat, peptones or yeast extract.

Question 9

What are natural examples of complex media?

Answer 9

It can be obtained from natural sources and can include fruit juices.
These natural products are used as the media without modifications.

Question 10

What is general-purpose media?

Answer 10

It has multiple functions such as for storage, enumeration, isolation and general culture, an example of this is nutrient agar.

Question 11

What is selective media?

Answer 11

It is used for isolation of specific types/groups of microorganisms.
It contains substances that can inhibit the growth of specific types/groups of bacterium while permitting the growth of other.
an example of this is MacConkey agar is used to isolate Gram negatives contains crystal violet which tends to be inhibitory towards most gram positive bacteria.

Question 12

What is differential media?

Answer 12

It is used to distinguish among morphologically and biochemically related organisms.
it contains compounds that either produce a change in the appearance of the organism cultured or the surrounding medium.

Question 13

What is an example of differential media?

Answer 13

an example is thioglycolate broth and it is used to differentiate oxygen requirement levels of various microbes.
A reaction with sodium glycolate in the medium reduces oxygen availability,
Gradual decrease in oxygen concentration with increased distance from the surface of the media.
The presence of oxygen in the media is indicated by a pink colour change.

Question 14

Aside from thioglycolate broth, what is another differential media?

Answer 14

MacConkey agar

• apart from bile salts contains lactose and pH indicator (neutral red)
• organisms that have the ability to ferment lactose produce acid and therefore appear red at the surface.
• By contract, non-lactose fermenters remain uncoloured since they do not produce acid.

Question 15

What is minimal media?

Answer 15

• This chemically defined medium contains minimal requirements for growth
• it does not support the growth of some or all auxotrophic strains of a given organism.
• only growth of prototrophic strains is permitted
• often used in metabolic studies and in taxonomic descriptions of organisms.

Question 16

What is enriched media?

Answer 16

• These media are supplemented with highly nutritious materials such as blood, serum, yeast, and extract.
• mainly used for the cultivation of fastidious organisms.
• Blood agar which contains blood as an enrichment ingredient is used for the culture of members of the Streptococcus genus.
• Blood agar also allows the classification of streptococci based on their haemolytic activities such as gamma haemolysis, alpha haemolysis and beta haemolysis.

Question 17

Enrichment Media

Answer 17

Broth medium containing substances that allow target group to multiply before plating onto selective or differential agar.

Question 18

Transport media

Answer 18

• maintains specific conditions for stabilizing cells during transport to lab
• Cary-blair semi-solid transport medium

Question 19

Enumeration media

Answer 19

• used for accessing or monitoring the total and/or viable cell count.
• Plate count agar and RZA are two examples.

Question 20

Assays

Answer 20

• used to measure specific biochemical

- Example- resistance to antibiotics

Question 21

What does medium influence?

Answer 21

1. rate of cell growth/proliferation
2. Metabolic pathways utilized, hence metabolites produced.
3. Gene product quantity (regulation of gene expression

Question 22

What is medium optimization?

Answer 22

Medium optimization studies involve the construction of growth curves to monitor cell proliferation rates.

Question 23

why culture preservation is crucial?

Answer 23

• This allows a stock of producer cells to be maintained:
  1. For use over a long period of time
  2. As a backup in the event of errors in the process or losses due to contamination.
  3. As a reference for regulatory bodies/scientists performing checks (traceability & accountability.

Question 24

What do culture preservation techniques need?

Answer 24

They all require GOOD ASEPTIC technique.

Question 25

What is periodic transfer?

Answer 25

• Cells are maintained in culture by periodic transfer to fresh medium.
• Frequency of transfer is usually defined.
• Transfers/passages are performed during the exponential phase of growth.

Question 26

What is a mineral oil slant?

Answer 26

• Stock cultures are grown on a slant and covered with sterile mineral oil.
• slants are refrigerated at 2-8 degrees
• cultures may be preserved for up to 15-20 years.

Question 27

What is minimal medium/distilled water

Answer 27

• cultures are washed and stored in either minimal medium or distilled water
• they are kept refrigerated at 2-8 degrees
• Cultures may be kept in storage for up to between 3-5 months or more.

Question 28

What is drying?

Answer 28

• cultures are dried on suitable sterile surfaces (soil, filter paper disks, gelatin drops).
• these are then stored in a desiccator at refrigeration temperature.
• Alternatively, they may be frozen to improve viability.

Question 29

What is lyophilization?

Answer 29

• This process is also known as freeze-drying
• Water is removed from cultures by sublimation in the presence of a cryoprotective agent (glycerol).
• Cultures are subsequently sealed in sterile ampules and refrigerated or frozen.
• Viability >15 years but up to 30 years has been reported.

Question 30

What is cryo-storage?

Answer 30

• cultures are stored in media/cryopreservant (usually glycerol) and frozen down in a controlled rate freezer.
• They are then stored in liquid nitrogen at a temperature of -195 degrees
• Cultures may be kept viable for >15 years