Question

Lab Report-Carbohydrates 1. Purpose 2. Special Media for Isolating Bacteria (Lab #12) a. Why are dyes such as phenol red, eosin or methylene blue added to the media? b. How does the bacterium change the media (i.e color of agar or colonies) after incubation? C. In this experiment, which media are selective, and which are differential? d. How did the results observe on the mannitol salt agar and EMB agar correlate to the Gram reaction of the bacteria? e. What is the purpose of peptone in the media? f. What is the purpose of agar in the media? 8. What does a red color symbolize in an MSA plate? h. What ingredient makes mannitol salt selective? i. J. Clinical Application a. Nutrients agar that contains vancomycin is selective/differential for b. EMB agar that contains sorbitol is used selective/differentiate E. coli 0157. Explain: Nutrient agar with tellurite is selective/differential for C.

Answer 1

a. Eocene methylene blue (EMB) is a toxin compound, that inhibits the growth of gram-positive bacteria. This allows the selection of gram-negative on the plates. Basically Phenol red, eosin, or methylene blue are called indicators. These indicators serve a few important functions. First, they indicate if the pH of the media is changed during bacterial growth, like from acidic to basic. They also indicate the fermentation of carbon sources in the media. They also indicate if the bacteria of choice is growing.

b. Bacteria change the color of media or colony color due to a couple of reasons. First, many bacteria produce colored pigments as secondary metabolites. These change the color of the media and colonies. Second, is the pH change. Bacteria ferment sugar and produce acids. These change the pH of the media and that is indicated by the color change by pH indicators.

c. Mannitol salt agar and EMB both selective and differential media.

d. EMB agar inhibits the growth of gram-positive bacteria. MSA has high salt content, that inhibits the growth of most gram-negative bacteria which are non-halophilic (allows gram-positive such as Staphylococcus)

e. Peptone is an excellent source of amino acids, peptides, and proteins required for the growth of bacteria.

f. Agar is used as a gelling or solidifying agent. It is used to make solid or semi-solid media on Petri plates. This allows colony formation and better characterization of bacteria.

g. MSA contains mannitol sugar. The original color of the media is red due to the presence of a Phenol red indicator. If the bacteria ferment mannitol, pH of the media changes to acidic. The ph indicator will change the color of the media from red to yellow.

h. Mannitol salt media has high salt content (7%). This is inhibitory for most gram-negative bacteria such as coliforms or fecal coliforms.

J.
- Selective for Vancomycin-resistant organisms.
- It is differential that distinguishes how bacteria metabolises and changes the color of the media.
- Selective for gram positive bacteria.