The culture media

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Information about The culture media

Published on August 18, 2013

Author: twitchangel



Discussion on the types of culture media. Definitions and terminologies that should be noted in the discussion of the topic. Notes provided by the microbiology professor.


 CULTURE - the growth or crop of microorganisms obtained in a culture medium after its incubation period.  CULTURE MEDIUM - any material where microorganisms may thrive for their nourishment and reproduction.  INCUBATION PERIOD – is the time needed to let previously inoculated culture media to show a distinct colony or colonies within a desired temperature. It is also the time needed for the microorganisms to adapt, grow and multiply in its new environment.

 COLONY - a group of microorganisms growing together characteristically in a culture medium.  INOCULUM – the fished-out colony hanging on the wire loop, cotton swabs, etc. that is ready for transfer to another culture medium for cultivation and/or for further isolation.

TYPES OF CULTURE:  Contaminated culture – a culture that accidentally contains one or more group of microorganisms which should not be growing there at all.  Mixed- culture – means that there are two or more desired species of microorganism living in the culture medium.  Pure culture – a culture that contains only one group of microorganisms which is usually obtained when microorganisms in a culture medium are all of the same species.

TYPES OF CULTURE MEDIA – according to PHYSICAL STATE  Solid media/ Semi-solid media- characterized by the sol-gel reversibility; those media wherein agar is added as the solidifying agent.  Ex. Nutrient Agar, BAP, EMB  Liquid Media – prepared without substances like gelatin or agar.  Ex. Nutrient broth

TYPES OF CULTURE MEDIA – according to USE  SIMPLE MEDIA – can facilitate different types of microorganisms.  ENRICHED MEDIA – are broth or semi-solid medium containing a rich supply of special nutrients that promote the growth of a particular fastidious organism while not promoting the growth of other microorganisms that might be present. Thus, the desired species grows best and predominates in the culture, and a pure culture could easily be obtained from a mixed specimen.  SELECTIVE MEDIA – have added inhibitors that discourage the growth of certain organisms without inhibiting the growth of the one sought.

 Differential Media- are used to grow several species of bacteria, but each bacteria grown has a distinctive appearance.  Transport Media – have been devised to protect pathogens present in clinical specimens that might not otherwise survive or that might be overgrown by non-pathogens during the transport of the specimen from the patient to the laboratory. Such a lapse of time may involve either minutes, hours, or days.

1. Nutrient Agar (NA) – consists of beef extract, peptone, agar, and water. It is often used as a base in the making of more complex media. 2. Nutrient Broth (NB) –forms the basis of most media employed in the study of medical pathogenic bacteria.

Three types of nutrient broth:  Meat infusion broths – consists of watery extract of meat made by extracting lean meat for 24 hours at 2°C. Any remaining juice is squeezed from the meat. The extract is then simmered for 15 minutes and filtered. Protein is removed in this process and must, therefore be replaced by adding 1% peptone. Sodium Chloride is added last. These broths generally contain a high percentage of fermentable sugar, which makes them unsuitable for many purposes,  e.g., toxin production.

Three types of nutrient broth:  Meat extract broths – are prepared from peptone and commercial meat extracts. They can be easily prepared and are widely used. However, they are less nutritious than infusion and digest broths. They are most suitable for preservation of stock cultures.  Ex: Brain- Heart Infusion (BHI) – most useful because it allows many fastidious organisms to grow.

Three types of nutrient broth:  Digest broths – consists of watery extract of lean meat digested with a proteolytic enzyme. The proteolytic enzyme employed varies, though generally it is trypsin in the form of a pancreatic extract.  Ex: Hartley’s broth – ideal for growth of exacting microorganisms.

1. Blood Agar Plate (BAP) – used to encourage the growth of bacteria which are unable to grow on nutrient agar and to detect hemolytic streptococci. 2. Chocolate Agar Plate (CAP) – Used particularly as a supply of factors essential for the growth of Haemophilus sp., and when incubated with CO2 it is ideal for the isolation and maintenance of Neisseria meningitidis and N. gonorrhoeae. 3. Selenite F broth – an enriched medium for salmonellae and some shigellae. 4. Milk agar- protein diet

1. Mannitol Salt Agar (MSA) – encourages the growth of Staphylococci but inhibits the growth of other gram (+) bacteria. It is also a differential agar because colonies or various species of Staphylococci have different appearance on MSA. 2. Bordet-Gengou Medium – used in the isolation of Bordetella pertussis. 3. Cetrimide Agar – used in the isolation of Pseudomonas aeruginosa. 4. Thayer-Martin Agar – used in the isolation of genus Neisserai.

5. Lowenstein- Jensen Medium (LJ medium) – used in the isolation of genus Mycobacteria. 6. Saboroud’s Dextrose Agar (SDA) – used in the isolation of fungi. 7. Mueller- Hinton Agar (MHA) – for the primary isolation of Neisseria gonorrhoeae and N. meningitides It can also be used for the determination of antimicrobial sensitivity and, in particular, with the Kirby- Bauer technique.

1. Eosin Methylene Blue (EMB) – frequently used to separate and identify enteric gram (-) organisms in fecal specimen. It can be used in the isolation and identification of Candida albicans.  Lactose fermenters have blue- black center (LF organisms produce acid that acts on the two dyes, Eosin and Methylene blue).  Non-lactose fermenters are colorless.

2. MacConkey Agar (MAC) – both differential and selective media, used for primary planting and subculturing of enteric organisms.  Lactose Fermenters appear pink to red (as a result of the action of acid formed during the fermentation of lactose on the indicator, neutral red).  Non- lactose fermenters remain colorless.

3. Salmonella – Shigella Agar (SSA)- a differential and selective media designed specifically for the isolation of pathogenic enteric bacilli belonging to the Salmonella and Shigella genera.  Lactose Fermenters yield red colonies.  Non-lactose fermenters are colorless.  H2S producers are colonies having black centers. (Ferric citrate being the indicator). 

 Stuart’s Transport Medium is ideal for swabs, eg., maintaining viability of gonococci on cervical swabs.

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