Gram Positive Staphylococci – (Note the purple, spherically-shaped, 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 aureusB= Escherichia coliC= 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