Which of the following can be used to differentiate between members of the Enterobacteriaceae family in the laboratory?

• A. Fermentation of lactose
• B. Size of colonies
• C. Shape of cells
• D. Color on XLD agar

Answer: (A)

Fermentation of lactose is a critical biochemical characteristic used to differentiate members of the Enterobacteriaceae family in the laboratory. Many members of this family have distinct abilities to ferment lactose, which can be detected using selective media such as MacConkey agar or EMB agar. For instance, Escherichia coli ferments lactose, producing acid and leading to pink colonies, while non-lactose fermenters such as Salmonella and Shigella do not alter the color of the medium.

This differentiation is significant because it helps microbiologists identify specific strains quickly based on their metabolic capabilities, which is particularly useful in clinical diagnostics for infections or food safety assessments.

While size of colonies, shape of cells, and color on XLD agar can provide some information about bacterial characteristics, they are less definitive in distinguishing between the diverse members of the Enterobacteriaceae family. Colony size can vary due to growth conditions rather than species, cellular shapes are generally similar among this family, and while XLD agar can be useful for certain bacteria, it is not as broadly applicable or discriminative as lactose fermentation tests.