Question

A definitive test is one that is used to identify a specific bacterial genus or distinguish two closely related organisms. Discuss the results of the definitive tests that allows you to identify proteus vulgaris.

(Down below are the tests, however I want to know which of these are definitive test that identifies proteus vulgaris and why)

Glucose Fermentation Lactose Fermentation Mannitol Fermentation OF Glucose Methyl Red Voges Proskauer Citrate Utilization Indole Production Phenylalanine Deamination Urea Hydrolysis H2S Production Oxidase Nitrate Reduction

Answer 1

Proteus is a gram-negative bacteria belong to the Enterobacteriaceae family. It is present as normal flora of the human gut and is known for causing opportunistic infections such as urinary tract infections (UTI), wound infection, ear infections, etc.

The Lactose fermentation test helps to distinguish between lactose fermentor and non-fermentor in the Enterobacteriaceae family. Proteus is a Lactose non-fermentor in contrast to Escherichia, Enterobacter, and Klebsiella.

Another genus closely related to Proteus is the Salmonella genus. These both can be definitively distinguished by the Urease test. In this test, urea containing medium with phenol red indicator is used. Few bacteria contain urease enzyme which breaks down the urea to form ammonia and carbon dioxide. The ammonia turns the pH of the media alkaline changing its color from yellow to pink. Salmonella is urease-negative whereas Proteus is Urease positive.

The genus Shigella and Proteus can be distinguished by the Citrate utilization test. In this test, the bacteria can utilize the citrate as a sole source of carbon and energy. The catabolism of citrate produces carbon dioxide as one of the end products, which further reacts to sodium ions to form sodium carbonate, in turn, elevating the pH of the media. Bromothymol blue is used as a pH indicator, the growth of bacteria leads to the color change from green to blue. Shigella unable to utilize citrate and shows no color change whereas Proteus is capable of utilizing citrate and shows the color change.

Proteus mirabilis and Proteus vulgaris are the most common species of the proteus genus. Both species have almost identical biochemical properties. The definitive biochemical test to identify between P. mirabilis and P. vulgaris is an indole test. P. mirabilis is Indole-negative whereas P. vulgaris is indole positive.  The indole test is used to identify or distinguish the microbes which are capable to catabolize the tryptophan amino acid. The presence of tryptophanase enzymes in such bacteria allows them to break the tryptophan into indole. Kovac's reagent is used to detect the production of indole by the synthesis of the red-colored compound.

Test	Observations for P. vulgaris
Glucose fermentation	+
Lactose fermentation	-
Mannitol fermentation	-
Methyl Red	+
Voges Proskauer	-
Citrate Utilization	+
Indole	+
Phenylalanine Deamination	+
Urea Hydrolysis	+
H2S Production	+
Oxidase	-
Nitrate Reduction	+

Key

(+) Positive

(-) Negative