Transcript Acid-Fast staining

Gram Positive Staphylococci – (Note the purple, sphericallyshaped, clustered cells)
Gram Negative Bacilli – (Note the pink, rod-shaped cells)
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)
Spore staining: Spores are stained blue (B); vegetative cells are stained
red (A).
Photo: Courtesy of Western Michigan University
Ex. 5-13 – Casein hydrolysis
on Milk agar
Photo: Courtesy of Austin Community College
Ex. 5-11 - Starch Hydrolysis
Photo: Courtesy of Austin Community College
Phenylethylalcohol (PEA): Is this media selective or differential? Why?
A= Staphlylococcus aureus
B= Escherichia coli
C= S. aureus + E. coli
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.
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
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
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
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
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
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