Eosin$/$Methylene Blue

Selective component in EMB: Eosin and methylene blue are the selective components in EMB agar. They inhibit the growth of Gram$-$positive bacteria, making EMB selective for Gram$-$negative bacteria.

Positive for Motility

Growth away from stab line: Motility is indicated by growth spreading away from the stab line. This is because motile bacteria are able to move through the medium, while non$-$motile bacteria remain localized along the stab line.

Lactose Fermentation

Differential aspect of MacConkey: Lactose fermentation is the differential aspect of MacConkey agar. If the bacteria ferment lactose, the colonies typically appear pink due to acid production, while non$-$fermenters produce colorless colonies.

Positive for H$2$S

Blackening of the media: Hydrogen sulfide $($H$2$S$)$ production causes blackening of the media. This reaction is seen in certain bacterial species that reduce sulfur compounds to H$2$S$,$ which reacts with iron salts in the medium to form black precipitates.

Halo around bacteria on plate

Positive for Amylase: A halo around the bacterial growth on an agar plate indicates the breakdown of starch by amylase. The starch in the medium is hydrolyzed, creating a clear zone or halo around the colony.