Agar plates
DIFFERENT TYPES OF AGAR
Mac Conkey‘s Agar plate
MacConkey (also McConkey) agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation. It contains bile salts (to inhibit most Grampositive bacteria, except Enterococcus and some species of Staphylococcus), crystal violet dye
(which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.
QUALITY CONTROL
Results after 24 hrs at 35º C
Organisms
ATCC
Growth
Colour
Escherichia coli
25922
+
red
Proteus mirabilis
12453
+
colourless
Salmonella typhimurium 14028
+
colourless
Streptococcus faecalis 29212
-
or partial
Uses
Acting as a visual ph indicator, the agar distinguishes those Gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-).
This medium is also known as an "indicator medium" and a "low selective medium". Absence of electrolytes serves to inhibit swarming by Proteus species.
Lac+
By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli,
Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of red/pink colonies.
Eosin methylene blue (EMB) agar –
Eosin methylene blue (EMB) is a selective stain for Gram-negative bacteria. It is a blend of two stains, eosin and methylene blue in the ratio of 6:1. A common application of this stain is in the preparation of EMB agar, a differential microbiological medium, which inhibits the growth of
Gram-positive bacteria and provides a color indicator distinguishing between organisms that ferment lactose (e.g., E. coli) and those that do not (e.g., Salmonella, Shigella).[1] Organisms that ferment lactose display "nucleated colonies" -- colonies with dark centers.[2].
•
•
•
Lactose fermentation produces acids, which lower the pH. This encourages
Mac Conkey‘s Agar plate
MacConkey (also McConkey) agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation. It contains bile salts (to inhibit most Grampositive bacteria, except Enterococcus and some species of Staphylococcus), crystal violet dye
(which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.
QUALITY CONTROL
Results after 24 hrs at 35º C
Organisms
ATCC
Growth
Colour
Escherichia coli
25922
+
red
Proteus mirabilis
12453
+
colourless
Salmonella typhimurium 14028
+
colourless
Streptococcus faecalis 29212
-
or partial
Uses
Acting as a visual ph indicator, the agar distinguishes those Gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-).
This medium is also known as an "indicator medium" and a "low selective medium". Absence of electrolytes serves to inhibit swarming by Proteus species.
Lac+
By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli,
Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of red/pink colonies.
Eosin methylene blue (EMB) agar –
Eosin methylene blue (EMB) is a selective stain for Gram-negative bacteria. It is a blend of two stains, eosin and methylene blue in the ratio of 6:1. A common application of this stain is in the preparation of EMB agar, a differential microbiological medium, which inhibits the growth of
Gram-positive bacteria and provides a color indicator distinguishing between organisms that ferment lactose (e.g., E. coli) and those that do not (e.g., Salmonella, Shigella).[1] Organisms that ferment lactose display "nucleated colonies" -- colonies with dark centers.[2].
•
•
•
Lactose fermentation produces acids, which lower the pH. This encourages