Oxygen-independent imaging of anaerobes with FAST

Why GFP goes dark without oxygen

Green fluorescent protein and every reporter derived from it (EGFP, mCherry, mNeonGreen and the rest) do not arrive fluorescent. After the chain folds, it has to mature: an internal, self-catalysed reaction forms the chromophore, and the final oxidation step consumes molecular oxygen. Take the oxygen away and the chromophore never forms. In an anaerobic culture or the hypoxic core of a biofilm or spheroid, a fluorescent-protein fusion is expressed but stays invisible, and even at the low-oxygen margin, maturation is so slow that the signal lags far behind the biology you are trying to watch.

How FAST stays bright in anaerobic conditions

FAST does not build a chromophore. It binds a small fluorogen already present in the medium and switches it on the instant it is bound: no oxygen, no maturation, no waiting. The interaction is non-covalent and reversible, so the signal tracks the biology in real time and can be tuned by the amount of fluorogen you add. FAST is also small, a purely monomeric protein of just 14 kDa (a 375 bp gene), so it perturbs your system less than bulkier tags. In practice: put your cells in an anaerobic chamber, add the fluorogen, and image straight away on the microscope, cytometer or plate reader you already use.

From acetogen engineering to the mammalian microbiome: a short history

FAST reached anaerobic biology in 2019, when Terry Papoutsakis and his team at the University of Delaware built the first strongly fluorescent, oxygen-independent reporter for Clostridium, finally bringing real-time imaging and cell sorting to the acetogenic and solventogenic anaerobes that had long defeated GFP. Metabolic engineers adopted it quickly to turn one-carbon feedstocks into fuels and chemicals in gas-fermenting acetogens such as Acetobacterium woodii and Eubacterium limosum. The same freedom from oxygen soon carried FAST beyond bacteria, into methanogenic archaea and even anaerobic protozoa. Because it reports equally under total anoxia and under milder hypoxia, FAST-encoded bacteria have since been used to illuminate the living gut microbiome inside mice and to dissect the biology of the human pathogen Clostridioides difficile. In a handful of years it went from a niche fix for one genus to a general reporter for life without oxygen, from the bioreactor to the gut.

What you can study

• Obligate anaerobes. Visualise protein localisation and characterise gene-expression activity in Clostridium and other strict anaerobes that fluorescent proteins cannot report on.
• Living, multi-species biofilms. Follow community dynamics in bioreactors, where oxygen is depleted within hours and GFP-type signals saturate or vanish.
• Gut microbiome & syntrophic cultures. Track populations for cell sorting, sporulation dynamics and population characterisation in native or synthetic microbiomes.
• Hypoxic tissue and 3D models. Report from the low-oxygen interior of spheroids, organoids and tissue where chromophore maturation stalls.

Recommended fluorogens

^TFLime (green) and ^TFCoral (orange-red) are the validated choices for anaerobic work and were used in the published studies below. For deeper samples where longer wavelengths penetrate better, ^TFCarmine extends the same oxygen-independent readout into the near-infrared. Because binding is oxygen-independent, any ^TFFluorogen works, so the colour choice is yours.

Frequently asked

Can I use FAST in obligate anaerobes such as Clostridium?

Yes, this is one of its defining uses. Published work shows FAST fluorescing brightly in Clostridium acetobutylicum under fully anaerobic conditions.

Does FAST need oxygen at any step?

No. Unlike GFP-family proteins, FAST requires no oxygen to fold, to mature or to fluoresce. Signal appears as soon as the fluorogen is added.

Recent papers: FAST & splitFAST in anaerobes