IMViC basic principle

Indole test E. aerogenes does not have the enzyme to metabolize tryptophan; when Kovac's reagent is added, just a yellow-green ring forms - negative reaction E. coli has the enzyme to breakdown tryptophan and form indole; when Kovac's reagent is added, a bright cherry red ring is formed - positive reaction Tryptone broth contains tryptophan; certain bacteria have the enzyme tryptophanase to metabolize tryptophan to form indole

IMViC - Methyl red test Enterobacter aerogenes ferments glucose to produce alcohol; the pH of the medium does not change and the medium remains yellow - a negative reaction E. coli ferments glucose to produce acids; the acids lower the pH of the medium and methyl red at pH below 5 changes the color of the medium to red - a positive reaction MR-VP broth contains glucose as energy source; medium also contains methyl red, a pH indicator; the original color of the medium is yellow

 

Citrate test Enterobacter aerogenes secretes permease to take up citrate and break it down to form alkaline products such as sodium carbonate; the pH of the medium goes above 7.6; the color of the medium to deep Prussian blue - a positive reaction E. coli is unable to utilize citrate; therefore it does not grow on Simmon's citrate agar medium; the color of the medium remains green - a negative reaction Original Simmon's citrate agar medium that contains citrate as the sole source of carbon; medium also contains bromothymol blue, a pH indicator; since the pH of the medium is 6.8, the original color of the medium is green