Your Coffee Maker Is A Bacterial Filth Hole

Specialized microbes that process caffeine lurk in your coffee grounds.

Your Coffee Maker Is A Bacterial Filth Hole
Top Photo: Monkew via Shutterstock. Janice Haney Carr/CDC

If you’re ever round at George Clooney’s place and he offers you a cup of coffee, you should probably refuse–unless he regularly empties and scrubs the drip-tray under the used capsules of the Nespresso machines he advertises. That’s because, says a study published in Nature Scientific Reports, the waste container is a bacterial filth-hole.


The study, from researchers Cristina Vilanova, Alba Iglesias, and Manuel Porcar at the University of Valencia, Spain, sampled the contents of the drip-trays of nine different Nespresso-compatible machines. “Nespresso-compatible machines are highly standardized coffee making devices (same capsule type, same basic design, same pressure: 19 bars), and they represent a unique opportunity for a massive biological screening,” say the authors.

Caffeine is naturally antibacterial, but even this couldn’t stop the growth in an ill-maintained drip tray. The team operated the machines for a year and found that the drip trays developed into a cesspool of Enterococcus and Pseudomonas, among other bacteria, as shown in the chart. Some of these are “caffeine degraders,” and some may survive because they have a higher tolerance to caffeine than previously thought.

Flickr user bnpositive

The results ”may shed light on the microbial arsenal of caffeine degraders,” say the authors. This is important, because the degradation of caffeine is essential for decaffeinating the waste products of the coffee processing industry. Coffee husks, for example, are used as an organic fertilizer, but only after the caffeine is destroyed.

The team also took a brand new Krups Inissia machine and primed it with known bacteria, to see how it would flourish. This two-month experiment monitored the colonization of the machine by the bacteria and found that the same two bacteria, Enterococcus and Pseudomonas, came to dominate after the first month.

“Our results show, for the first time, that coffee leach from standard capsule machines is a rich substrate for bacterial growth; that caffeine content does not prevent a rich bacterial biodiversity from rapidly colonizing coffee leach,” says the study, and goes on to recommend regular and thorough cleaning. Rinsing, they say, isn’t enough because of the “fast recovery of the [bacteria] communities.”

But it’s not all bad. The team also thinks that these caffeine-degrading bacteria may prove a “promising tool for biological coffee decaffeination processes, and for environmental caffeine decontamination.”


Clearly, you have to pay as much attention to your drip tray as you do to the rest of the food containers in your kitchen. Given the likely laziness of the typical capsule-coffee consumer, that probably won’t happen. And that might be a good thing, in Darwinian terms at least.

About the author

Previously found writing at, Cult of Mac and Straight No filter.