UNIT II D- CULTURE MEDIA OF MICROORGANISMS

Question 1

Culture media

Answer 1

• specific mixtures of nutrients and other
  substances that support the growth of
  microorganisms
  Principle: In preparing a culture medium for any
  microorganism, the primary goal is to provide a
  balanced mixture of the required nutrients, at
  concentrations that will permit good growth. No
  ingredient should be given in excess because
  many nutrients become growth inhibitory or toxic
  as the concentration is raised.

Question 2

Types of Media: Basal

Answer 2

• those that may be used for growth (culture)
  of bacteria that do not need enrichment of
  the media
  Examples: Nutrient broth, nutrient agar and
  peptone water
• Staphylococcus and Enterobacteriaceae
  grow in these media

Question 3

Examples of Basal Media: Nutrient broth

Answer 3

• A liquid medium used for the cultivation of
  a wide variety of organisms from clinical
  specimens and other materials
• Can be enriched with other ingredients
  such as blood, serum, sugar, etc for
  special purposes

Question 4

Examples of Basal Media: Nutrient agar

Answer 4

- A basic culture medium used to 
subculture organisms for maintenance 
purposes or to check the purity of 
subcultures from isolation plates prior to 
biochemical or serological tests
- Semi-solid form as agar slopes or agar 
butts
- used to maintain control organisms

Question 5

Examples of Basal Media: Peptone water

Answer 5

- a microbial growth medium composed of
peptic digests of animal tissue and 
sodium chloride
- The pH of the medium is 7.2±0.2 at 25 °C 
and is rich in tryptophan
- a non-selective broth medium which can 
be used as a primary enrichment medium 
for the growth of bacteria

Question 6

Types of Media: Enriched

Answer 6

• enriched usually by adding blood, serum or
  egg
  Examples: blood agar and Lowenstein-Jensen
  media
• Streptococci grow in blood agar media

Question 7

Examples of Enriched Media: Blood agar

Answer 7

- Enriched medium used to culture those 
bacteria or microbes that do not grow 
easily
- Used to grow wide range of pathogens 
particularly those that are more difficult to 
grow such as Haemophilus influenzae, 
Streptococcus pneumoniae, and 
Neisseria spp.

Question 8

Examples of Enriched Media: Lowerstein-Jensen medium

Answer 8

• Used for the isolation and cultivation of
  Mycobacteria and acid-bases for
  selective, differential, and enriched media
  for Mycobacterium
• Medium, tubed as deeps, is used for the
  semi-quantitative catalase test

Question 9

Types of Media: Selective

Answer 9

• favour the growth of a particular bacterium
  by inhibiting the growth of undesired
  bacteria and allowing growth of desirable
  bacteria
  Examples: MacConkey agar, LowensteinJensen media, tellurite media (Tellurite inhibits
  the growth of most of the throat organisms
  except diphtheria bacilli).
• Antibiotic may be added

Question 10

Examples of Enriched Media: MacConkey Agar

Answer 10

- A selective and differential culture 
medium for bacteria
- Designed to selectively isolateGramnegative and enteric (normally found in 
the intestinal tract) bacteria and 
differentiate them based on lactose 
fermentation
- Lactose fermenters turn red or pink on 
McConkey agar, and nonfermenters do 
not change color

Question 11

Examples of Enriched Media: Lowerstein-Jensen medium

Answer 11

- a growth medium specially used for 
culture of Mycobacterium species, 
notably Mycobacterium tuberculosis. 
- When grown on LJ medium, M. 
tuberculosis appears as brown, granular 
colonies (sometimes called "buff, rough 
and tough").

Question 12

Examples of Enriched Media: Tellurite media

Answer 12

• a selective medium used for isolation and
  cultivation of Corynebacterium species
• selective due to the presence of inhibitors
  and differential by means of ability of
  organism to reduce potassium tellurite
• Biopeptone provides nitrogenous
  compounds

Question 13

Types of Media: Indicator (Differential) media

Answer 13

• indicator is included in the medium,
  particular organism causes change in the
  indicator, e.g. blood, neutral red, tellurite
  Examples: Blood agar and MacConkey agar

Question 14

Examples of Indicator Media: Blood Agar

Answer 14

• a differential medium that distinguishes
  bacterial species by their ability to break
  down the red blood cells included in the
  media
• often used to distinguish between the
  different species of pathogenic
  Streptococcus bacteria

Question 15

Examples of Indicator Media: MacConkey Agar

Answer 15

• selects for Gram-negative organisms by
  inhibiting Gram-positive organisms and
  yeast and differentiates the Gramnegative organisms by lactose
  fermentation
• Lactose ferments will stain pink while the
  nonlactose ferments will be clear
  colonies.
• The media also has the added advantage
  of inhibiting the swarming of Proteus.

Question 16

Types of Media: Transport media

Answer 16

• used when specimen cannot be cultured
  soon after collection
  Examples: Cary-Blair medium, Amies medium,
  Stuart medium

Question 17

Examples of Transport media: Cary-Blair Transport Medium

Answer 17

• a simple semi-solid non-nutritive medium
  used for the collection and preservation of
  microbiological specimens.
• The minimal nutrients in the medium
  facilitate the survival of organisms without
  multiplication.
• The semisolid consistency provides ease
  of transport, and the prepared medium
  can be stored for up to 1 year after
  preparation at room temperature.
• a modification of Stuart’s Medium which is
  comprised of an improved buffering
  system by replacing sodium
  glycerophosphate with inorganic
  phosphates.
• This improved formulation prevents
  overgrowth of Enterobacteriaceae and
  contributes to the effective preservation of
  Salmonella and Shigella for long periods.

Question 18

Examples of Transport media: Stuart Medium

Answer 18

- a nonnutritional medium which maintains 
the viability of organisms without 
significant multiplication
- The small agar content provides a 
semisolid consistency which prevents 
oxidation and drying during transport
- Sodium thioglycolate is added to 
produce a reduced environment so as to 
improve the recovery of anaerobes
- Methylene blue is the oxidationreduction indicator
- colorless in the reduced state
- blue in the oxidized state
- Calcium chloride maintains permeability 
of bacterial cells
- Sodium glycerophosphate maintains 
the pH of the medium

Question 19

Examples of Transport media: Amies Transport Medium

Answer 19

- used for collection, transport and 
preservation of microbiological 
specimens especially throat, vaginal, and 
wound swab samples
- routinely used for the transport of 
specimens from collection centres (or 
health centres) to the microbiology 
laboratory

Question 20

Types of Media: . Storage media

Answer 20

• used for storing the bacteria for a long
  period of time
  Examples: Egg saline medium, chalk cooked
  meat broth

Question 21

Examples of Storage media: Egg Saline medium

Answer 21

• used in preserving cultures of Gramnegative Bacilli

Question 22

Examples of Storage media: Cooked Meat broth

Answer 22

- Example: Robertson’s Cooked Meat 
(RCM) medium
- used for the cultivation of aerobic, 
microaerophilic, and anaerobic 
microorganisms, especially Clostridium
species
- known as cooked meat broth (CMB) as 
it contains pieces of fat-free minced 
cooked meat of ox heart and nutrient 
broth