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Aurora
5. Transfer thus prepared Blood Agar base to a 50oC water bath+
6. When the agar base is cooled to 50oC, add sterile blood agar aseptically and mix well
gently+
7. Avoid formation of air bubbles
8. Store the plates at 2-8 oC, preferably in sealed plastic bags to preventing loss of moisture+
Using the same procedure above with only minor alteration
by lysis of Red blood cells in the above medium gives the
medium a chocolate-brown color, which is called chocolate
agar+
Chocolate agar:
This is a modified blood agar medium+ When blood or hemoglobin is added to the heated basal media, it turns brown
(hence the name)+ This medium supports the growth of most
bacteria, including some that do not grow on blood agar (i+e+,
Haemophilus, some pathogenic Neisseria strains)+
Fig 1.16: Chocolate agar (Foster. D;
Chen.J, 2015)
Selective Media and Differential Media
Selective media are designed for the recovery of specific organisms that may be present in
a mixture of other organisms (e+g+, an enteric pathogen in stool)+
The media are supplemented with inhibitors that suppress the growth of unwanted organisms+ These media can be made differential by adding specific ingredients that allow the
identification of an organism in a mixture (e+g+, addition of lactose and a pH indicator to detect lactose fermenting organisms)+
The following are some examples of selective and differential media:
MacConkey agar
This is a selective agar for gram negative bacteria and
differential for lactose-fermenting and lactose nonfermenting bacteria+
The medium consists of digests of peptones, bile
salts, lactose, neutral red, and crystal violet+ The bile
salts and crystal violet inhibit gram positive bacteria+
Bacteria that ferment lactose produce acid, which
precipitates the bile salts and causes a red color in
the neutral red indicator+
This medium can be used to cultivate : vibrio cholera,
E+coli
Preparation
Ingredients:
Enzymatic Digest of Gelatin, Casein and Animal
tissue
Lactose+
Bile Salts
Fig 1.17: MacConkey agar (Thermo Fisher Scientific Inc , 2015)