Your coffee maker might be brewing more than just coffee, scientists in Spain have discovered, reporting that in a small-scale experiment involving 10 Nespresso coffee machines, they've identified entire whole microbiological communities living in the coffee waste – and some of these bacteria can be dangerous to our health.
Overall, the bacteria found in coffee waste were similar across all 10 machines, but were highly variable within each community, with up to 67 genera identified in a single machine. The team from the Universitat de València says these bacteria have to be pretty hardy to survive in coffee machines - the coffee itself is an antibacterial agent, and the machines consistently reach temperatures of up to 96°C.
In previous studies, microorganisms such as Aspergillus tamarii, Trichosporon asahii, and Pseudomonas sp. have been shown to degrade caffeine as part of their lifecycle. This could explain why Pseudomonas sp. was one of the main types of bacteria found alongside Enterococcus sp. in 9 of the 10 coffee machines.
Other species from the Acinetobacter and Bacillus genera have been detected in coffee bean fermentation, and Paenibacillus has been identified in decomposing coffee hulls. The team says that with further research, we could put them to work by clearing caffeine out of marine environments.
Unlike the relatively harmless bacteria mentioned above, certain species of Enterococcus bacteria that were also found in coffee waste could be more damaging to our health. Two of these are commonly found in our intestines - E. faecalis and E. faecium. Unsurprisingly, these bacteria are able survive in a wide variety of conditions, including acidic pH levels and extreme temperatures. In some circumstances, species of Enterococcus bacteria have been known to cause urinary tract infection, meningitis, and bacteraemia.
The researchers think it's likely that these bacteria came from the coffee drinkers themselves - possibly due to unwashed hands - rather than the coffee itself. They found no evidence of bacteria in the capsules before making the coffee.
But don't throw out your coffee maker just yet. The team found negligible amounts of potentially harmful bacteria in the coffee itself and any other parts of the machine. The only place bacterial colonies were forming was the drip tray.
The solution? Clean your drip tray more often. "The presence of bacterial genera with pathogenic properties… suggest the need for frequent maintenance of the capsule container of these machines," the researchers conclude.
The results have been published in Scientific Reports.
"There is probably always going to be a significant bacterial population in an organic 'soup' such as the one present in the coffee leach tray of the capsule machines," study author Manuel Porcar told Quartz, adding that a weekly rinse warm, soapy water would "decrease bacterial densities to very low values".