MUG: It's Not Just For Coffee Anymore

Recently, I have been playing around with a special cell culture media that uses a fluorescent indicator to signal the presence of E. coli bacteria. The culture media is a nutrient broth called LST-MUG, and is often referred to by its proprietary name, Hach media. The nutrient broth is similar to other types of cell culture media, with a couple of notable exceptions. First, the media contains surfactants that inhibit the growth of non-coliform organisms. This basically means that everything but E. coli is killed in the broth, allowing only E. coli to grow in the media. Second, the media contains a molecule called 4-methylumbelliferyl-β-D-glucuronide, or MUG. MUG is the part of the media that is responsible for indicating the presence of E. coli.

Under normal conditions, MUG emits very weak fluorescence at a wavelength down around where blue and UV meet, somewhere around 400 nm, when exposed to UV light. When MUG is introduced to E. coli, it is cleaved (or cut) into two separate smaller molecules by an enzyme within the bacteria called β-glucuronidase. One of the products of this cleavage reaction (the glucoronide part) is basically inert and does nothing at all. But the other product of the reaction is 4-methylumbelliferone, a highly fluorescent molecule. When exposed to the same UV light as before, the fluorescence of 4-methylumbelliferone is easily observable as a pale blue glow. In this way, the MUG acts a fluorogenic reporter of the presence of E. coli bacteria.

This bacterial growth media with the fluorescent reporter is commercially available and widely used to detect the presence of E. coli. However, it has some pitfalls. One problem with the media is that the sample to be tested must be introduced to the broth and then incubated, which is best done in a laboratory environment by a trained technician. A more pressing issue with the broth is its response time, which is 16-24 hours. If you are testing a sample of, say, raw beef, you would take a random in-line sample during processing and inoculate the Hach media. Twenty four hours later, a fluorescent signal is observed in the media, indicating that the beef is contaminated with E. coli. But that particular beef product is already on a truck on its way to the grocery store.

One of the projects that we're focusing on at LU's Center for Nanotechnology & Biosensors is a method for making this process much more effective and drastically faster, so that E. coli can be detected efficiently and contaminated products can be kept off of store shelves.