Question

4. A colony on VJA plate, grown at 37°C for 24 hours, appears completely black and the surrounding medium is pinkish in colour. Answer the following questions for the bacteria that produced this colony. a) What is the cellular morphology (cell shape and gram reaction) of these bacteria? (1 pt) gram positive cocci b) What ingredient(s) provide this colony with a usable source of carbon and energy? (1 pts) c) What reaction is occurring to generate the black colour in the colony? Explain. (2 pts) d) From the above description, what do we know about the species especially due to the incubation time used? (1 pt)

Answer 1

4.

a) Cocci are generally round or spherical in shape and they are Gram positive in nature.

b) The usable source of carbon and energy in VJA comes from D(-)mannitol, peptone and yeast extract.

c) The potassium tellurite aded to VJA is reduced to metallic tellurium by the pathogenic species of coccus. This reaction leads to the generation of the black color in the colony.

d) From the description above it is certain that coccus colonies are present however it probably is not Staphylococcus as it would cause the pinkish or reddish color of phenol red to turn yellow because of the acid produced.