Which test is most useful for differentiating Shigella spp. from some strains of Escherichia coli?

Which test is most useful for differentiating Shigella spp. from some strains of Escherichia coli?

• A. Oxidase test
• B. ONPG test
• C. Voges-Proskauer test
• D. Methyl red test

Answer: (B)

The ONPG test is particularly valuable for differentiating Shigella species from certain strains of Escherichia coli, specifically those that are lactose non-fermenters. Shigella does not ferment lactose, which is a key characteristic that helps in its identification. However, some strains of E. coli, such as those that are lactose-negative, may not exhibit significant fermentation as well. The ONPG test assesses the ability of an organism to produce beta-galactosidase, an enzyme that allows the breakdown of the disaccharide ONPG (ortho-nitrophenyl-β-galactoside) into simple sugars, even in the absence of lactose fermentation. When an organism can hydrolyze ONPG, a yellow color develops in the medium, which indicates the presence of beta-galactosidase. In this context, Shigella spp. will not produce a positive result on the ONPG test, while some lactose-negative E. coli strains may show a positive result. This distinction enables microbiologists to differentiate between these two groups effectively.

The ONPG test is particularly valuable for differentiating Shigella species from certain strains of Escherichia coli, specifically those that are lactose non-fermenters. Shigella does not ferment lactose, which is a key characteristic that helps in its identification. However, some strains of E. coli, such as those that are lactose-negative, may not exhibit significant fermentation as well.

The ONPG test assesses the ability of an organism to produce beta-galactosidase, an enzyme that allows the breakdown of the disaccharide ONPG (ortho-nitrophenyl-β-galactoside) into simple sugars, even in the absence of lactose fermentation. When an organism can hydrolyze ONPG, a yellow color develops in the medium, which indicates the presence of beta-galactosidase.

In this context, Shigella spp. will not produce a positive result on the ONPG test, while some lactose-negative E. coli strains may show a positive result. This distinction enables microbiologists to differentiate between these two groups effectively.