Question

how does the bacterium change the media (i.e color of the agar or colonies) after incubation?

Answer 1

Certain bacteria produce acids as a result of fermentation of certain sugars present in the media. Production of acids causes the pH of the media to fall. Fall in pH will lead to chane in colour of the dye that is added to the media. Therefore there is a change in the colour of the media or colonies.

For example, lactose fermenting bacteria produce pink colonies on MacConkey Agar, whereas lactose non fermenting colonies produce colourless colonies. This is due to the change in the colour of the dye, neutral red due to fermentation of lactose in the media.

Some bacteria also produce pigments which change the colour of the media/colonies. For example, Pseudomonas produces pigments pyoverdin( yellow and flourescent pigment) and pyocyanin(blue) that change the colour of the media.