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Gram Positive Staphylococci – (Note the purple, spherically-shaped, clustered cells)
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Gram Negative Bacilli – (Note the pink, rod-shaped cells)
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Acid-Fast staining: Below is a mixed culture of acid fast bacteria (red; such as the Mycobacteria, B) and non-acid-fast bacteria (blue, A)
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Spore staining: Spores are stained blue (B); vegetative cells are stained red (A).
Photo: Courtesy of Western Michigan University
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Ex. 5-13 – Casein hydrolysis on Milk agar
Photo: Courtesy of Austin Community College
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Ex. 5-11 - Starch Hydrolysis
Photo: Courtesy of Austin Community College
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Phenylethylalcohol (PEA): Is this media selective or differential? Why?
A= Staphlylococcus aureusB= Escherichia coliC= S. aureus + E. coli
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Gelatin Hydrolysis:
Some bacteria have enzymes which breakdown the gelatin (which is protein) to amino acids; as indicated by liquefaction in test tube B.
Remember: Besides the fact that most bacteria are unable to digest agar, agar is superior to gelatin because it remains solid well above room temperature (~25°C). Whereas gelatin begins to melt around 25°C.
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MacConkey Agar: Selective: inhibits growth of Gram positive bacteria. Differential: distinguishes between lactose fermenters and non-fermenters.
Escherichia coli is growing on the right plate. It is Gram negative, ferments lactose, and produces a red coloration on and around its growth.Salmonella typhimurium is growing on the left plate. It is Gram negative, does not ferment lactose, and is colorless .
What is a coliform bacterium?What ingredient makes MacConkey agar selective?What is the pH indicator in this media? What does it do?
Photos: Courtesy of Austin Community College
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Eosin Methylene Blue (EMB) Agar: Lactose fermenters produce colored colonies on EMB and non-lactose fermenters produce colorless colonies (see lab book photos).
The green metallic sheen shown on the plate below is characteristic of Escherichia coli.
Is EMB a selective or differential media?What are the dyes in EMB?
Photo: courtesy of Dr. Kaiser
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Mannitol Salt Agar: 1. Staphlylococcus aureus 2. Staphylococcus epidermidis (there is growth, but its not visible).
Why does S. aureus produce a yellow color?Why doesn’t S. epidermidis produce a yellow color?Is MSA selective or differential? What media components make it selective and/or differential?
Photo: Courtesy of Austin Community College
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Blood Agar
Gamma Hemolysis: No destruction of red blood cells
Is blood agar selective, differential or enriched?
Photo: Courtesy of Dr. Kaiser, C.C. of Baltimore County
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Blood Agar
Alpha Hemolysis: Partial destruction of red blood cells. Indicated by the greenish coloration of the media around the bacterial growth.
Photo: Courtesy of Dr. Kaiser, C.C. of Baltimore County
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Blood Agar
Beta Hemolysis: Complete destruction of red blood cells. Indicated by the clear area around the bacterial growth.
Photo: Courtesy of Dr. Kaiser, C.C. of Baltimore County