It is true that some bacteria do glow in and of themselves and many more will glow under UV light to a greater or lesser degree, so the plausibility factor is there. And UV light is used industrially in food production and other processes to detect certain specific bacteria. But we’ve all seen those TV programmes where a ‘clean’ washroom is plunged into darkness, the UV switched on and – euwwwww! But what does it really tell us?
Fluorescence is a form of luminescence. It occurs when a substance absorbs light at one wavelength and re-emits it at a longer wavelength. This property is used in optical brighteners in laundry products – they absorb (invisible) UV light and re-emit (visible) light in the blue part of the spectrum. Many natural molecules exhibit this property including rocks and gems, some vitamins and even your G&T – tonic water fluoresces blue due to the presence of quinine. Many, many molecules can do this but one class of chemical that fluoresces intensely when exposed to ultraviolet light are flavins. A key natural source of flavins is Vitamin B. Uric acid and its salts and soap scum all contain Vitamin B.
Many claim certain species of bacteria accumulate on surfaces where high concentrations of flavins are found so the claim is made by many sellers of hand-held UV systems that UV ‘exposes’ bacteria because bacteria fluoresce. This is not true.
As a rule bacteria do not fluoresce. Some strains of Salmonella and Shigella relevant in foodborne illness can, and many contain materials which fluoresce – but that fluorescence not always visible to the naked eye, either because it’s at the wrong wavelength or there just isn’t enough of it.
Washroom fixtures including toilets, urinals, walls, partitions, mirrors, floors and counter tops will commonly have deposits of compounds derived from urine and soap scum. When you shine ultraviolet light on surfaces with deposits of urea salts or soap scum they absorb it and emit visible light to the naked eye (if viewed in total darkness). Surfaces can be inspected with the use of an appropriate ultraviolet light but this is NOT an accurate indicator of presence of bacteria. But what it can do is indicate areas where bacteria might be found. This is where Rule 1 of hygiene and infection control comes in: You Can’t Disinfect Dirt.
A surface must be visibly clean before you can disinfect it and one that fluoresces under UV most likely isn’t.