Carbohydrate Catabolism

OF-Glucose--used to identify the oxidative and fermentative abilities of microorganisms: Organism to be tested is stabbed into two tubes of the semi-soft agar medium.  One tube remains open to air and one tube is covered with oil.  Tubes are incubated and results read by noting the color of the bromthymol blue indicator. If glucose is used, acid end-products are generated, the pH is lowered and the bromthymol blue indicator turns yellow. 

The following photos show the results of the OF glucose test performed on A. faecalis, P. aeruginosa, and E. coli, respectively.  Note that A. faecalis is a non-oxidizer, non-fermentative organism, P. aeruginosa is an oxidative organism only, and E. coli is both oxidative and fermentative.

Phenol Red Fermentation Broth-- used to identify acid and gas production from a variety of sugars (i.e. lactose, mannitol, sucrose, etc).  Broths are inoculated and incubated.  The results are determined by the color of the phenol red indicator.  If sugar is fermented, acid end-products are generated and the phenol red indicator turns yellow. 

  Phenol Red Lactose

The tube on the right has been inoculated with E. coli.  The tube on the left is an un-inoculated tube .  Note the positive result for both acid (yellow) and gas (bubble in Durham tube).

Phenol Red Fermentation

Here is another set of tubes illustrating fermentation in phenol red broths.  The tube on the far left (Tube 1) is a very weak positive for acid but no gas. Tube 2 is negative for both acid and gas. Tube 3 is positive for both acid and gas and tube 4 (the far right) is an un-inoculated tube.

Simmons-Citrate Agar: used to identify organisms that can utilize citrate as their sole carbon source.  A slant is inoculated and incubated.  The result is determined by determining the color of the bromthymol blue indicator.  Citrate is used and the pH of the medium rises to turn the tube a bright "Prussian blue" color.

Citrate + organism on the left; Citrate - on the right

MRVP--Methyl Red-Vogues Proskauer:  used to identify organisms that ferment to produce stable acid end-products or neutralized acids into acetoin.