MUG Test (4-Methylumbelliferyl-β-D-Glucuronide)
A UV-fluorescence bench visual for reading the MUG disk test, used to presumptively identify β-glucuronidase-producing E. coli and other Enterobacteriaceae.
MUG disks A-B under 366 nm UV
Observe disks in a darkened environment using a 366 nm long-wave UV light. Electric blue fluorescence is positive.
Electric blue fluorescence develops on the disk within 2 hours; β-glucuronidase hydrolyzed the MUG substrate to produce the fluorescent 4-methylumbelliferyl moiety.
No fluorescence on the disk; β-glucuronidase is absent in this organism.
Electric blue fluorescence develops on the disk within 2 hours; β-glucuronidase hydrolyzed the MUG substrate to produce the fluorescent 4-methylumbelliferyl moiety.
No fluorescence on the disk; β-glucuronidase is absent in this organism.
How to read under UV
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Electric blue fluorescence on disk | β-glucuronidase hydrolyzed MUG substrate; 4-methylumbelliferyl moiety fluoresces at 366 nm | Positive — presumptive E. coli |
| No fluorescence (disk stays white) | β-glucuronidase absent; MUG substrate remains intact and does not fluoresce | Negative — not E. coli pattern |
| Read time | Incubate 35–37 °C for up to 2 hours before UV observation | Do not read after 2 hours |
| UV wavelength | 366 nm long-wave UV only; short-wave UV (254 nm) will not show this reaction | Always use a long-wave UV lamp |
MUG test study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Positive (fluorescent) | E. coli or MUG-positive Enterobacteriaceae | Presumptive E. coli; confirm with IMViC or additional biochemicals. |
| Negative (no fluorescence) | Klebsiella, Enterobacter, Proteus, or most non-E. coli Enterobacteriaceae | Rule out VTEC (O157:H7) separately; they are MUG-negative. |
| Positive limitations | Some non-E. coli species are MUG-positive; use with clinical context | Citrobacter freundii and some Salmonella are occasionally MUG-positive. |
- MUG detects β-glucuronidase, the enzyme that hydrolyzes the fluorescent MUG substrate to 4-methylumbelliferone.
- Positive = electric blue fluorescence under 366 nm UV; negative = no fluorescence.
- Most E. coli (90–95%) are MUG-positive, but verotoxin-producing strains (VTEC/O157:H7) are characteristically MUG-negative.
Best mental model: If the disk glows blue under UV, β-glucuronidase is present and the organism behaves like E. coli. If the disk stays dark, look for other organisms — or be alert for VTEC.