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you are here->home->Biotechnology and Biomedical Engineering->Microbiology Virtual
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Lab I->Selective and Differential Media for Identifying Microorganisms
. Selective and Differential Media for Identifying Microorganisms .
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Theory
Procedure
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Self Evaluation

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3. Materials Required:
Bunsen Burner
Inoculating loop
Media Used:
Mannitol salt agar
MacConkey’s agar
Eosin Methylene Blue Agar
Phenylethyl Alcohol Agar
Hektoen Enteric Agar
Blood Agar
Chocolate Agar
Cultures used:
Staphylococcus aureus
Staphylococcus epidermidis
Escherichia coli
Pseudomonas aeruginosa
Enterobacter aerogenes
Proteus vulgaris
Shigella flexneri
Salmonella typhimurium
Streptococcus pyogenes
Streptococcus pneumoniae
Streptococcus salivarius
Haemophilus influenzae

4. Procedure:
Mannitol salt agar, MacConkey’s agar, EMB agar, Phenylethyl Alcohol agar, Hektoen
Enteric agar, Blood agar and Chocolate agar plates are prepared and sterilized.
Staphylococcus aureus and Staphylococcus epidermidis were inoculated into
MSA plates.
Escherichia coli and Pseudomonas aeruginosa were inoculated into two halves of
the MacConkey’s agar plate.
Escherichia coli and Enterobacter aerogenes were inoculated into EMB agar
plates.
Staphylococcus aureus, Staphylococcus epidermidis and Proteus vulgaris were
inoculated into PEA agar plate.
Shigella flexneri and Salmonella typhimurium were inoculated into HE agar
plates.
Three species of Streptococci namely Streptococcus pyogenes, Streptococcus
pneumoniae and Streptococcus salivarius were inoculated into blood agar plates.
Haemophilus influenzae was inoculated into chocolate agar plate.
All these plates are incubated at 37oC for 24 hours.
Expected Results
1. Mannitol Salt Agar (MSA):
On MSA, pathogenic Staphylococcus aureus ferments mannitol, thereby changing the
colour of the medium from red to yellow.

5. Figure 11: Culture of Staphylococcus aureus on Mannitol Salt Agar
Staphylococcus epidermidis grows on MSA, but does not ferment mannitol (media
remains light pink in color & colonies are colorless).
Figure 12: Culture of Staphylococcus epidermidis on Mannitol Salt Agar
2. MacConkey’s Agar (MAC):

6. In Macconkey’s agar, E.coli ferment lactose and turn pink in color while P.aeroginosa
will be colorless as they are non lactose fermenters.
Figure 13: MacConkey’s agar with lactose fermenters (left) and non-lactose
fermenters (right)
3. Eosin Methylene Blue (EMB) Agar (Levine):
On EMB agar, E.coli produces colonies which are very dark, almost black, when
observed directly against the light. By reflected light, a green sheen can be seen which is
due to the precipitation of methylene blue in the medium from the very high amount of
acid produced from lactose fermentation. Those which form this type of colony are
methyl red-positive lactose-fermenters.

7. Fig 14: Escherichia coli on EMB agar
On EMB agar, Enterobacter aerogenes produces colonies which are less dark. Often a
pink, dark center is seen surrounded by a wide, light-colored, mucoid rim – resulting in a
"fish-eye" type of colony. Those which form this type of colony are methyl red-negative
lactose-fermenters.

8. Fig 15: Enterobacter aerogenes on EMB agar
Non-lactose-fermenting colonies produce no acid from fermentation, so the lighter-
colored alkaline reaction is seen.
Eg: Proteus vulgaris
Fig 16: Proteus vulgaris on EMB agar
4. Phenylethyl Alcohol Agar:
There will be selective growth of Gram positive organisms (S.aureus and S.epidermidis)
in PEA agar.

9. Figure 17: PEA agar plate with selective growth of Gram positive bacteria
5.Hektoen Enteric (HE) Agar:
In Hektoen Enteric agar plates Shigella flexneri develop into green-colored colonies with
darker blue-green centers while Salmonella typhimurium appear as green colonies with
black centers.

10. Figure 18: Growth of and Shigella flexneri and Salmonella typhimurium on
Hektoen Enteric Agar
6. Blood Agar:
In blood agar, Streptococcus pyogenes cause beta hemolysis resulting in complete lysis of

11. red blood cells and hemoglobin. This results in complete clearing of the blood around the
colonies.
Fig19: Beta hemolysis
Streptococcus pneumonia cause alpha hemolysis which results in a greenish-grey
discoloration of the blood around the colonies.

12. Fig 20: Alpha hemolysis
Streptococcus salivarius cause gamma hemolysis resulting in no change in the medium.
Fig 21: Gamma hemolysis
7.Chocolate Agar:

13. Haemophilus influenzae uses the enrichment growth factors and grow on chocolate agar.
Figure 22:Haemophilus influenzae on Chocolate Agar Plate
Differences Encountered in a Real
Laboratory:
In an actual laboratory setting, there are certain important steps that are not necessarily
applicable in a virtual lab:
1. Always wear gloves, and lab coat.
2. Tie your hair properly to prevent any contamination from the culture you are
working with.
3. When you enter the lab switch on the exhaust fans.
4. Switch on the lights of the laminar air flow and blower. Prepare your work space

14. (Laminar Air Flow Cabinet) or lab bench by wiping down the area with
disinfectant.
5. Properly adjust the flame of the Bunsen burner. The proper flame is a small blue
cone; it is not a large plume, nor is it orange.
6. Always label all tubes and plates with:
1. The name of the organism
2. The type of media
3. Your initials
4. The date
7. While flaming the inoculation loop be sure that each segment of metal glows
orange/red-hot before you move the next segment into the flame.
8. Once you have flamed your loop, do not lay it down, blow on it, touch it with your
fingers, or touch it to any surface other than your inoculums. If you do touch the
tip to another surface or blow on it, you will have to re-flame the loop before you
proceed to your experiment.
9. Allow your loop to cool before you try to pick up your organism to avoid killing
the inoculum.
10. When removing the caps from tubes, always keep the caps in your hand. Never
set them on the table, as they could pick up contaminants.
11. Always handle open tubes at an angle near to the flame of the burner; never let
them point directly up, since airborne or other environmental organisms could fall
into the tube and cause contamination.
12. Open the lid of the plate sufficiently (45 degrees) to introduce an inoculation loop
and only for the time it takes to obtain inoculums.
13. Rotate the plate counter clockwise 90 degrees and cross the prior streaks to pick
up some bacteria and spread them into the next quadrant (Repeat in all the four
quadrants;applicable only in the case of quadrant streaking).
14. Streak gently; does not gouge the agar.
15. As soon as the inoculation is completed, flame your loop or needle. Never place a
contaminated tool on your workbench.
16. Turn the inoculated petriplate upside down while keeping it in the incubator.
17. Discard all contaminated materials properly and return your supplies to the proper
storage locations, and clean up your working area.
18. Always disinfect your work area when you are finished.
19. Turn off Bunsen burner and make sure that the regulator of the gas cylinder is
tightened.
20. Switch off the laminar air flow and exhaust fans before leaving the laboratory.
21. Ensure proper hand washing before you leave from the laboratory.

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