Coffee machine bacteria: 35 to 67 genera in a single drip tray

Pull the tray out from under a capsule machine that has been running a year and what comes out is not grounds. It is sludge: wet, dark, and standing at room temperature since the last cup went through. A team at the Cavanilles Institute of Biodiversity and Evolutionary Biology in Valencia sequenced that sludge out of ten machines and published what was in it in Scientific Reports in 2015. The count of coffee machine bacteria ran from 35 to 67 genera per sample. Before that paper, nobody had systematically looked.

What ten machines had living in them

Nine of the ten were ordinary units, domestic and shared, that had been in operation for at least a year. The models were the Essenza, the Pixie, the CityZ and the Inissia. Reported use ran from 2 to 20 times a day, which spans a quiet household and a busy office corridor. None of them had been chosen for being neglected. They were simply machines that people were using.

The tenth was brand new and was there for a different question entirely. What the team sampled in every case was the leach in the used coffee tray: the liquid that drains out of a spent capsule, pools in the basin underneath, and waits there until somebody remembers the tray exists.

Why the drip tray holds 35 to 67 genera

The method was a census rather than a culture. The team sequenced 16S rRNA gene amplicons across the V1-V3 region on an Ion PGM, which reads the DNA that is present instead of waiting to see what will grow on a plate. That distinction matters, because plating finds the organisms that like laboratory conditions and misses most of the rest. Totals per sample landed between 35 and 67 genera.

Two names came up almost everywhere. Enterococcus and Pseudomonas were moderately to highly abundant in nine of the ten samples analysed. Stenotrophomonas, Sphingobacterium and Acinetobacter showed up often enough to count as regulars, with Coprococcus, Paenibacillus and Agrobacterium further down. The full tables are open access, and the spread between machines is the part that surprises: no two trays held quite the same community.

Caffeine keeps nothing out

The assumption worth retiring first is that caffeine sterilises what it touches. The paper puts it flatly: caffeine content does not prevent a rich bacterial biodiversity from rapidly colonising coffee leach. Bacterial diversity in that basin is not being held back by the compound most people assume is holding it back.

Some bacteria go well past tolerating the stuff. Pseudomonas putida CBB5 was isolated from a soil sample in Coralville, Iowa, by deliberately enriching for microbes that could use caffeine as their sole carbon and nitrogen source. CBB5 came out of soil rather than out of anybody's kitchen, so it is not the organism sitting in the tray. What it shows is what the genus can do when caffeine is the only thing on the menu. The Valencia team went the same direction with their own communities, flagging them as potential drivers of decaffeination and bioremediation.

How fast a new machine fills up

The tenth machine answered the question nobody asks about a new one: how long does it stay new. Early samples were dominated by pioneers such as Pantoea, the opportunists that arrive first and do not hold the ground. Within weeks the community had tilted towards coffee-adapted taxa, the organisms that can actually make a living on spent coffee.

Species richness dropped significantly 14 days in, at a t-test p-value of 0.039. That drop is succession rather than hygiene: an early free-for-all narrowing to whatever suits the habitat best. Every detachable part was rinsed once with tap water on day 28, and the community carried on maturing regardless. One rinse is not a reset, much the same lesson descaling intervals teach about the mineral side of the same machine.

Where coffee machine cleaning usually stops

The authors' recommendation is narrow, and worth reading for what it leaves out. Maintenance should employ bacteriostatic compounds, and avoid contact of the coffee leach with other parts of the machine so the drink is not contaminated. No product, no dose, no schedule. The second half carries the weight: what they describe is residue travelling, which is a different failure from residue existing, and a different one again from the problems brewing temperature usually gets blamed for.

Most cleaning routines are built around scale. Descale every few months, run the cycle, consider the job done. That is the mineral half of machine maintenance, and it does nothing whatsoever for a basin of used coffee grounds standing in its own liquid. A capsule machine makes that easy to miss, because the tray hides behind a flap and gives no warning until it overflows.

Coffee machine cleaning that stops at the boiler leaves the wettest part of the thing untouched. Pull the drip tray out tonight rather than the next time it spills, and look at what is standing in the bottom. If there is liquid in there, it has been at room temperature since the last cup. Empty it, rinse it, and let it dry before it goes back in.

Something worth putting through a clean machine?

A machine that gets emptied and dried earns something better than whatever was on offer at the supermarket. Santo Café roasts a single origin Mexican arabica for exactly that kind of morning, whole bean or ground, in a flat terracotta pouch with a zip that keeps the rest of the bag in decent shape. Brew it on a day the tray is clean and the machine stops being the variable.