Enzymatic markers among Enterobacteriaceae: Which of the following genera commonly produce the enzyme β-galactosidase (detectable by ONPG tests and lactose-fermentation profiles)?

Introduction / Context:
β-galactosidase hydrolyzes lactose to glucose and galactose and is central to lactose-fermentation phenotypes. In diagnostic microbiology, ONPG (o-nitrophenyl-β-D-galactopyranoside) testing helps detect β-galactosidase even when lactose permease is lacking, clarifying late or weak lactose-fermenters.

Given Data / Assumptions:

• Genera considered: Citrobacter, Enterobacter, Serratia.
• Question focuses on β-galactosidase presence, not necessarily strong lactose permease activity.
• Standard lab conditions and conventional strains are assumed.

Concept / Approach:
Many Citrobacter and Enterobacter isolates are ONPG positive and lactose-fermenting. Serratia species may ferment lactose slowly or be variable, but commonly possess β-galactosidase detectable by ONPG even when lactose fermentation on plates is delayed. Therefore, across these genera, β-galactosidase production is a recognized trait.

Step-by-Step Solution:
Consider lactose metabolism: requires permease and β-galactosidase.Recall ONPG detects β-galactosidase independently of permease.Map each genus: Citrobacter (+), Enterobacter (+), Serratia (often ONPG + though lactose on plates may be delayed).Select the inclusive option.

Verification / Alternative check:
Commercial identification systems (e.g., API 20E, VITEK) include ONPG/β-gal tests that commonly score positive for these genera.

Why Other Options Are Wrong:
Any single-genus choice would omit other valid β-gal producers; the group-level pattern supports “all of these.”

Common Pitfalls:
Equating plate lactose fermentation with ONPG positivity; some strains are late fermenters due to permease deficits despite β-gal presence.

Final Answer:
all of these