Bacteria - Culture Media & Types 

Published by MedNotes10 min read


Microbial cultures are used to determine the type of organism, its abundance in the sample being tested, or both. It is one of the primary diagnostic methods of microbiology and used as a tool to determine the cause of infectious disease by letting the agent multiply in a predetermined medium.

A. On Consistency:


1. Solid Media.

Advantages of solid media:

(a) Bacteria may be identified by studying the colony character,

(b) Mixed bacteria can be separated.

Solid media is used for the isolation of bacteria as pure culture. 'Agar' is most commonly used to prepare solid media. Agar is polysaccharide extract obtained from seaweed. Agar is an ideal solidifying agent as it is : (a) Bacteriologically inert, i.e. no influence on bacterial growth, (b) It remains solid at 37°C, and (c) It is transparent.

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2. Liquid Media.

It is used for profuse growth, e.g. blood culture in liquid media. Mixed organisms cannot be separated.

Liquid media provide greater sensitivity for the isolation of small numbers of microorganisms. Examples of liquid media include nutrient broth, sugar media, and enrichment media. Composition and uses of some common liquid media are given in table. Liquid media have the following disadvantages:

o   Identification of mixed cultures growing in liquid media requires subculture onto solid media so that isolated colo-nies can be processed separately for identification.

o   Growth in liquid media also cannot ordinarily be quantitated.

o   Bacteria grown in liquid cultures often form colloidal suspensions

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B. On Chemical Composition :


1. Routine Laboratory Media
2. Synthetic Media - These are chemically defined media prepared from pure chemical substances. It is used in research work.

ROUTINE LABORATORY MEDIA

These are classified into six types:
(1) Basal media,
(2) Enriched media,
(3) Selective
(4) Indicator media,
(5) Transport media, and
(6) Storage media


1. BASAL MEDIA - Basal media are 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.

2. ENRICHED MEDIA - The media are enriched usually by adding blood, serum or egg. Examples: Enriched media are blood agar and Lowenstein-Jensen media. Streptococci grow in blood agar media.

3. SELECTIVE MEDIA - These media favour the growth of a particular bacterium by inhibiting the growth of undesired bacteria and allowing growth of desirable bacteria. Examples: MacConkey agar, Lowenstein-Jensen media, tellurite media (Tellurite inhibits the growth of most of the throat organisms except diphtheria bacilli). Antibiotic may be added to a medium for inhibition.

4. INDICATOR (DIFFERENTIAL) MEDIA - An indicator is included in the medium. A particular organism causes change in the indicator, e.g. blood, neutral red, tellurite. Examples: Blood agar and MacConkey agar are indicator media.

6. TRANSPORT MEDIA -These media are used when specimen cannot be cultured soon after collection. Examples: Cary-Blair medium, Amies medium, Stuart medium.

7. STORAGE MEDIA - Media used for storing the bacteria for a long period of time. Examples: Egg saline medium, chalk cooked meat broth

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IDENTIFICATION OF BACTERIA

Identification of bacteria can be done by:

(i) conventional (culture and identification by biochemical reactions), (ii) automated culture techniques

(iii) molecular methods.

Automated Culture Techniques

Conventional culture methods often yield poor results because of low bacterial load. Therefore, various automated blood culture techniques have been in use since last decade.

Advantages:

The major advantages of automated blood culture techniques are: • Continuous automated monitoring (once in every 15–20 min by the instrument). • Other advantages: More sensitive,↑ yield, rapid, less labor intensive

Disadvantages:

(i) high cost

(ii) inability to observe the colony morphology as liquid medium is used,

(iii) no separate detection in mixed cultures,

(iv) radioactive hazards for BACTEC.

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