In the Colilert Quanti-Tray method viewed under a 365 nm UV lamp, what specifically causes certain wells to fluoresce brightly, indicating a positive result for E. coli?

• The hydrolysis of the fluorogenic substrate MUG by the enzyme beta-glucuronidase
• The reaction of free chlorine with the DPD indicator dye
• The fermentation of lactose resulting in an acidic pH shift
• The hydrolysis of the chromogenic substrate ONPG by beta-galactosidase