1. Mystery Growth Laboratory Practical
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Types of Media
Agar commonly used for initial culture
Blood
Chocolate
MacConkey
Citrate nutrient agar – for stool cultures helping to identify Salmonella and Shigella species with a ph indicator that turns yellow/orange when acidic and blue when pH is greater than 7.6. Only organisms that can use citrate as their sole source of carbon will grow.
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Types of Media
Broths commonly used for initial culture
Gram-negative broth
Trypticase soy broth
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Most commonly used agars are blood and chocolate. Both of these types of media will contain nutrients for both Gram-positive and Gram-negative bacteria.
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6. MSA – Mannitol Salt Agar
Will develop a yellow halo around colonies of Staphylococcus aureus when they break down the mannitol and turn the ph indicator from red to yellow. The high levels of salt keep most growth to gram positive species.
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MAC – MacConkey Agar
Contains bile salts and crystal violet dye preventing gram positive growth. Colonies will appear pink when lactose fermentation is present. Other characteristics can be noticed with specific species; for instance Proteus vulgaris will show swarming while Pseudomonas aeruginosa will develop a green metallic sheen.
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8. BLD – Blood (usually sheep blood)
Encourages most species to grow; great for initial culture. Show hemolysis that can be seen as a clear ring around a colony producing beta hemolysis and a greenish ring around alpha hemolysis.
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9. EMB – Eosin Methylene Blue
Will show blue colonies with growth from Escherichia coli because of their ability to digest lactose lowering the pH and allowing those colonies to absorb the eosin and methylene blue stain appearing dark blue-purple. Non-lactose fermenters keep the pH elevated and will stay uncolored as they will not be able to absorb the dye.
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10. CNA – Colistin-nalidixic acid Agar
Allows for gram positive growth only due to the addition of colistin and nalidixic acid.
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11. PEA – Phenylethyl Alcohol Agar
Not used in this lab but can be used for selective growth for anaerobic bacteria.
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12. TSA - Trypticase Soy Agar
General purpose media that most bacteria can grow on, allows for colony color and morphology to not be affected by the agar allowing some natural colors to show through like the red growth of Serratia marscens.
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CHOC – Chocolate
An enrichment broth that encourages slow or poor growers like Neisseria sp. and Haemophilus sp.
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14. TSB – Trypticase Soy Broth
Not used in this lab, also a multipurpose growth media, excellent for some liquid samples and wound cultures very similar to TSA but in liquid form.
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UREA BROTH
Contains large amounts of ammonia to raise the pH and a pH indicator that will show when a species can break ammonia down by turning the media from orange/yellow to pink.
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GN BROTH
Not used in this lab; used for the selective enrichment of Salmonella and Shigella species.
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