Study: Shared Office Coffee Machines Are As Gross As You’d Think

Assuming you don’t love rapid bacterial colonization.

Study: Shared Office Coffee Machines Are As Gross As You’d Think
[Top Photo: Noel Hendrickson/Getty Images]

If you work in an office and use the communal coffee maker, chances are you’ve had the following thought at some point or other: Who cleans that thing? Maybe you’ve even joked about it with colleagues, each person laughing with varying degrees of intensity that reflect their own personal level of germaphobia. The uncomfortable answer is: Hopefully someone!


Make that very hopefully, given the results of the “first systematic analysis of coffee machine-associated bacteria,” published recently in Scientific Reports. A research trio from the University of Valencia, in Spain, studied 10 Nespresso machines in both office and home settings and found “significant bacterial diversity”—35 to 67 different genera (or major types), to be exact—just hanging out in the inner drip tray below the discarded capsules. They describe the bacteria’s colonization process as “rapid,” “rich,” and “dynamic.”

So the office machine is as gross as you feared. The good news is that there’s no apparent health risk so long as the tray gets “frequent” maintenance. “The coffee from these kind of machines is perfectly safe,” co-author and biology scholar Manuel Porcar tells Co.Design. “The tray containing the wasted capsules should be cleaned with water and soap, or a few drops of bleach.”

The research team chose to study Nespresso machines given the brand’s popularity. The 10 particular shared and domestic machines tapped for this sample were used, on average, anywhere from twice to 12 times a day. Porcar and colleagues analyzed the drip trays, which catch the drippings of used coffee capsules, with a variety of techniques that included culturing and electron microscopy. Among the loads of bacteria spotted in the trays, two main taxa showed up in moderate to high abundance in nine of the machines: Enterococcus and Pseudomonas.

The bacterial profile of nine (out of 10 tested) home and office Nespresso machines shows the prevalence of Enterococcus and Pseudomonas.Via Scientific Reports

As a controlled follow-up, the researchers also studied the bacterial build-up in a brand new Inissia Krups machine (also made by Nespresso), which got three uses a day over two months. Regular analysis revealed high variation and instability among the bacteria during the first month, steadying out into a “more balanced bacterial composition” afterward. After two months the tray’s bacterial profile was again dominated by Enterococcus and Pseudomonas—a sign that the machine itself, rather than the user community, shaped the microbial outcomes.

“I want to stress that bacteria accumulate in the leach tray, not in the coffee itself,” he says. “Nespresso coffee is microbiologically flawless.”

Images of bacteria from the lab-tested drip tray sample taken after 4 (Image A), 8 (B), 14 (C), and 21 (D) days.Via Scientific Reports

Though the study only considered Nespresso machines, Porcar says it’s logical to suspect that other capsule-based coffee makers give way to similar bacterial build-up. He also says it’s “very likely” that at least some of the bacteria are pathogenic, or potentially harmful to health. But, he notes, the same is true of any environment that’s home to food waste—kitchen towels, for instance, or refrigerators—with the consistent lesson being: wash often.

Better design could help people keep their machines clean. Porcar says the researchers informed Nespresso that drip trays should have rounded corners rather than squared ones, which are tougher to clean and therefore more likely to become bacterial shelters. As for why coffee-maker bacteria had escaped scientific attention in the past, he says the presence of caffeine, known to inhibit bacterial growth, might have discouraged any careful study.

“It is indeed surprising that thousands of microbial ecologists drink coffee and never did this kind of analysis before,” he says.

About the author

Eric Jaffe is an editor at CityLab, where he writes about transportation, history, and behavioral science, among other topics, through the lens of urban life. He's also the author of The King's Best Highway (2010) and A Curious Madness (2014).