Classification of culture media

A.CLASSIFICATION BASED ON NATURE OF ONSTITUENTS/INGREDIENTS

1. Those media that require living cells or tissues which are parasitized by the microorganisms to be cultured, A perfect example is cultivation of Rickettsiae and viruses which are obligate intracellular parasites that require living host cells within which only they can multiply.

2. Those media that doesn’t require living cells or tissues, These media may be again divided into:

a. Synthetic media or Defined Media
b. Non-synthetic media or Complex Media

a. Synthetic media or Defined media:

These are the media in which only pure chemicals in definite concentrations are used. Due to their known chemical compositions, these media are useful for nutritional and metabolic studies.

b. Non-Synthetic Media or Complex Media:

These are the media in which the exact chemical composition of each of the constituents is not known with certainty.

B.CLASSIFICATION BASED ON CONSISTENCY (PHYSICAL SATE) OF MEDIUM

On the basis of consistency media may be divided into four categories as:

a. Liquid media
b. Semisolid media
c. Liquefiable solid media
d. Solid media

a. Liquid media:

These media are used in the liquid form e.g., Nutrient Broth, Brucella Broth, Nitrate Broth, etc.

b. Semi-solid media:

These media contain 0.5% or less of agar imparting custard consistency to the medium. e.g. Cysteine Trypticase Agar medium.

c. Liquefiable solid media:

These media are prepared by adding a suitable amount of gelatin or agar to the liquid medium to remain solid when cool but become liquid when warm or vice-versa. For this reason, these media are also called “solidly reversible to liquid media”, e.g. Salmonella-Shigella Agar, Nutrient Gelatin Medium, Bile-Esculin Agar Medium, etc.

d. Solid media:

These media always remain solid e.g., Nutrient Agar Medium, Bile Salt Agar Medium, etc.

C. CLASSIFICATION BASED ON APPLICATION OR FUNCTION

On the basis of application, bacteriological media may be classified as follows:

a. Cultivation media
b. Storage media
c. Enrichment media
d. Differential media
e. Selective and selective differential media
f. Assay media
g. Maintenance media
h. Transport media
i. Enumeration media
j. Anaerobic media
k. Sugar media

a. Cultivation Media

These are the media which are used for the general cultivation of bacteria e.g., Nutrient broth or nutrient agar is the most common cultivation medium used in the microbiology laboratory.

b. Storage Media

In such media, bacteria are stored as stock cultures for longer periods and hence serve as a source of viable cultures. Bacterial cultures are preserved and stored in such media by lyophilization(freeze-dry method).

C. Enrichment Media

These are the media in which the nutritional environment(may contain blood, serum, hemoglobin, or special growth factors like vitamins and amino acids) is adjusted in such a way that the growth of certain bacterial types is enhanced selectively within a mixed population i.e., the media selectively favors the growth of one group of organisms over another. E.g. GN Broth, Selenite F, Loeffler’s serum slope, etc.

d. Differential or indicator Media

These media differentiate two organisms by their characteristic reactions towards the medium like hemolysis or biochemical changes. For example, Blood agar medium is an ideal differential medium that differentiates hemolytic species from non-hemolytic species e.g., ß-hemolytic (complete hemolysis in which clear/transparent zones are formed around colonies) Streptococcus pyogenes or Staphylococcus aureus can be differentiated from a- hemolytic (incomplete or partial hemolysis in which zones of green discoloration are formed around colonies)) Streptococcus pneumoniae (Pneumococcus). Similarly, on MacConkey Agar or EMB Agar, lactose fermenters (pink-colored colonies) like E.coli can be differentiated from non-lactose fermenters (another color different from pink) like Shigella.

e. Selective and Selective Differential Media:

i. Selective Media

The media allows the growth of selective groups of organisms while inhibiting others by various inhibitors like antibiotics, dyes, bile salts, etc. Important examples are MacConkey agar for isolation of E.coli, Deoxycholate citrate agar for Salmonella and Shigella, Lowenstein-Jensen medium for M.tuberculosis, etc.

ii. Selective Differential

Media Some selective media play a dual role in the selective growth of organisms and the differentiation of various groups of organisms based on their biochemical reactions in a particular media. Such media is selective differential media e.g. Blood Agar medium is a common selective medium used for isolation of most of the bacterial pathogen but due to variation in the hemolytic character of the organisms (S.pyogenes and S. pneumoniae), the medium can serve the dual role of acting as a differential medium as well.

f. Assay Media

These media are also called media for special purpose as these media (with known composition) have profound influence on the bacterial cells with respect to formation of enzymes, toxins, antibiotics and other bioactive product. E.g. Pyridoxine Deficient Growth medium for Streptococcus faecalis yields cells containing large amounts of tyrosine decarboxylase apoenzyme. Such media are used by technologists to test the effectiveness of antimicrobial drugs and to assess the effect of disinfectants, antiseptics, cosmetics and preservatives on the growth of microorganisms.

g. Maintenance Media

These media are used to maintain physiological characteristics and hence the viability of bacterial cultures.

h. Transport Media:

These media inhibit self-destruction and multiplication of microorganisms and hence maintaining the specimen in a “status quo” condition. In other words these media are used to maintain and preserve specimens that are to be held for a longer period of time prior to clinical analysis .These media sustain delicate species like Neisseria species which are delicate and may die rapidly if not held under stable and suitable conditions.

Different types of specimen are transported in different transport media like;

Cary-Blair and Amies Transport Media :

Used for transporting fecal material on the rectal swab, and are effective in preserving Salmonella, Shigella, and Vibrio. Stuart Transport Medium: Used for CSF and cervical specimens. The medium preserves both Neisseria spp. as well as Trichomonas vaginalis for 18-24 hours.

Venkataraman-RamaKrishnan (V.R) medium:

It is used for the transport of stool specimens of a patient suspected to be suffering from cholera. The cholera bacilli do not multiply in this medium but remain viable for several weeks. The medium also prevents overgrowth by other organisms.

Bile Peptone Transport medium

This medium is also used to transport stool specimen in cholera infection. It is useful medium for field works where more delay is expected in processing specimen.

Brucella Broth:

Used to transport biopsy specimens from duodenum ulcer patients (H.pylori infection). The medium preserves the etiological agent for 24-48 hours under refrigeration conditions.

i. Enumeration Media

These media are used primarily to count the number of microorganisms in milk, water, food and other samples and find their broad application in concerned industries.

j. Anaerobic media

Such media are used for the growth of anaerobes e.g. Robertson’s cooked meat medium or Thioglycolate broth for isolation of Clostridium species.

k. Sugar media

In such media, sugar fermentation reactions are carried out which is important for the identification of most of the organisms.

DIFFERENT CULTURE MEDIA & THEIR USES

S/No.	Medium	Uses
1	Nutrient agar	Basic medium for culturing bacteria which have no special nutritional requirement.
2	Blood agar	Used for culture of pathogen and detecting hemolytic bacteria.
3	Chocolate blood agar	Used for Haemophilus influenza, Neisseria species.
4	MacConkey agar	For differentiating lactose fermenting and non-lactose fermenting bacteria.
5	Deoxychoclate citrate agar(DCA)	Selective and differential media for enteric pathogens.
6	Mannitol salt agar	Selective and differential media Staphylococcus aureus.
7	Tellurite agar	Selective media for Corynebacterium diphtheriae
8	Thiosulfate citrate bile salt sucrose (TCBS) agar	Selective and differential media for Vibrio cholera.
9	Tryptone Soya broth	For culturing blood pathogens.
10	Tryptone Soya agar	For wide range of pathogens especially pathogen of blood Selective and differential media for Salmonella and Shigella species.
11	Xylose lysine deoxycholate (XLD) agar	For wide range of pathogens especially pathogens of blood.
12	Eosin-Methylene blue agar	Selective and differential media for the member of enterobacteriaceae
13	Sugar fermentation media	Determination of oxidative fermentative metabolism with added carbohydrate by Gram negative bacilli.
14	Loeffler Serum media	For sub culturing Corynebacterium diphtheria
15	Lowenstein Jensen acid media	For culturing Mycobacterium tuberculosis.
16	Amies transport media	For Neisseria gonorrheae and other pathogens.
17	Cary-Blair transport media	For fecal pathogens like Salmonella & Shigella, Vibrio etc.
18	Peptone water	Basic liquid media, for routine culture. Basal media for sugar fermentation tests.
19	Nutrient broth	For routine cultures.
20	Glucose broth	For blood culture, culture of delicate organisms.
21	Alkaline peptone water	Peptone water with pH 9.0. Medium for Vibrio cholerae.
22	Thyoglycollate broth	For culturing blood anaerobes.
23	Tryptone water	For E.coli.
24	Nitrate broth	In differentiating Mycobacterium species and Gram negative bacteria
25	Litmus milk media	Identification of Enterococci and some Clostridium species.
26	Selenite F broth	Enrichment media for Salmonella species in feces & urine.

DIFFERENT CULTURE MEDIA & THEIR USES

This is all for the classification of culture media under microbiology.