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Your Coffee Machine Is a Breeding Ground for Caffeine-Resistant Bacteria

Caffeine's natural antibacterial properties are no match for these hardy colonizers.
Scanning electron microscope images of drip tray samples taken, clockwise from top-left, after four, eight, 14, and 21 days. Image: University of Valencia

When you're waiting for your morning coffee to finish brewing, the last thing you probably want to think about is all the bacteria that's living inside your machine. Unless you're from the University of Valencia, that is.

A team of researchers set out to examine the bacterial communities that grow in the drip trays of pod-based coffee machines (they chose Nespresso due to its popularity and standardization across machines). Studies have shown that caffeine has natural antibacterial properties, so it stands to reason that your coffee maker is the last place you'd expect to find a diverse microbial community.


Instead, they found that, not only is all the extra coffee that doesn't make it into your cup a great environment for rapid bacterial growth, but that the most prevalent bacteria actually thrived off the caffeine.

In other words, there's still bacteria to be found in your coffee maker's drip tray—it just happens to be the coffee-loving kind.

Their research was published on Monday in Scientific Report, an open access journal from the publishers of Nature. "This is the first systematic analysis of the microbial diversity associated to coffee machines," the researchers claimed.

The team sampled the drip trays from ten different machines, and then monitored the bacterial colonization of a new Nespresso. After two months, the more general pioneering bacteria gave way to a community composed primarily of "coffee-adapted" bacteria.

One of the two most frequent subfamilies of bacteria found in the machines were Pseudomonas—"which is also one of the few reported examples of a caffeine-degrading bacterium" according to the researchers (caffeine degradation is the process of breaking down, or removing, the presence of caffeine biologically without solvents).

The researchers suggest that there's a good reason for studying this sort of thing, besides grossing you out next time you go to dump your office machine's drip pan: the coffee-happy bacteria "may represent a promising tool for biological coffee decaffeination processes and for environmental caffeine decontamination."

In other words, we could learn a thing or two from these hardy colonizers. But hey, maybe clean your machine out every now and then. With soap.