Which of the following characteristics is NOT typical of Enterobacteriaceae?

• A. They are gram-negative bacteria
• B. They can ferment lactose
• C. They are always pathogenic
• D. They are facultatively anaerobic

Answer: (C)

The characteristic that is not typical of Enterobacteriaceae is that they are always pathogenic. This family of bacteria includes a diverse group of organisms, some of which are indeed pathogenic and can cause diseases in humans and animals, while others are part of the normal flora of the gastrointestinal tract and are not harmful.

For instance, Escherichia coli includes both non-pathogenic strains that are normal inhabitants of the intestines and pathogenic strains that can cause serious illness. Other members, such as Klebsiella and Enterobacter, can also contain pathogenic strains but are frequently found in non-pathogenic forms within the human microbiome.

The other characteristics listed are typical of Enterobacteriaceae: they are gram-negative, meaning they do not retain the crystal violet stain used in the Gram staining procedure; they can ferment lactose, as seen in lactose-positive species like E. coli; and they are facultatively anaerobic, which allows them to grow in environments both with and without oxygen. This adaptability is key to their survival in various environments.