Question

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(1 29) The bacteria cultured on the left side of this phenol red dextrose agar plate metabolically converted what substance in this medium to create the demonstrated colorimetric change? Image gallery (click for full size) (1 30) The bacteria on the right side of this phenol red dextrose agar plate metabolically converted what substance in this medium to create the demonstrated colorimetric change (be specific)? Image gallery (click for full size)

Answer 1

Microbes use a series of enzyme catalysed reactions to generate energy from substrates like carbohydrates, proteins etc. Some microorganisms can ferment sugars( eg glucose) anaerobically, while others use the aerobic pathway to completely break down glucose. Facultative anaerobes are competent to use both anaerobic and aerobic pathways. Some microbes, lacking the ability to oxidize glucose by either, degrade proteins to get energy.

In fermentation, carbohydrates undergo anaerobic degradation and produce various organic acid (for instance, formic, lactic, or acetic acid) along with the production of gases like H₂ or CO₂.

Phenol Red Dextrose Agar is used to study Dextrose (glucose) fermentation of microorganisms. It contains Peptone, Sodium chloride, Dextrose, Agar, and Phenol red indicator. Phenol red is a pH indicator, which is reddish-pink at a neutral pH (7) and changes to yellow at a slightly acidic pH (6.8).

If the test organism can ferment the glucose present in media, the accumulation of organic acids changes the pH of the media to the slightly acidic range. That's why Phenol red changes to yellow colour. This is a positive result. As a negative result, the colour of the media remain unchanged ( the culture is not capable of fermenting glucose )or become deep pink (microbes can digest the protein present in the media and accumulate NH₄OH, that make the environment more basic).

Question 29

The colour of the left side is pink. So, it is basic in nature. That means the microbes growing at this side CAN'T ferment glucose, rather digesting peptones.

Question 30

The colour of the right side is yellow. So, it is acidic in nature. That means the microbes growing at this side CAN ferment glucose and producing organic acids.