It’s Coming To Get You. The Strange Case of the Pipe-Climbing Bacteria

More about plumbing than you ever knew you didn’t know

Although it may sound like something out of a horror movie, scientists from the University of Virginia have just published research revealing that bacteria can actually climb the pipes under a sink, making it all the way up to the basin.

More of this worrying study in a minute or two, but first, we go way back in time to the 18th century to meet a Scottish gentleman named Alexander Cumming, who was granted a patent for inventing the S-trap (or bend) that was originally designed as part of a flushing toilet, a novel concept in itself at the time.

The idea of the S-trap (so-called because its shape takes the form of a letter “S” lying on its side) was to provide a water-filled seal between bathroom and drains, preventing smelly sewer gas from wafting into the house.

Believe us, you really wouldn’t want that.

In addition to shutting odors out of bathrooms, Cumming was also an organ-builder and clockmaker, with a sufficient reputation in the world of clocks that he was appointed a member of the commission that adjudicated on John Harrison’s “timekeeper for discovering the longitude at sea,” a story told in Dava Sobel’s 1995 best-selling book Longitude.

Cumming’s S-trap has stood the test of time, with the result that many of today’s plumbing fixtures feature an S-, U-, or J-shaped pipe trap.

To add to this lexical soup, we also have P-traps.

Stop that giggling please, it’s nothing to do with urine – the pipework simply takes the shape of a letter “P.”

Just to extend your plumbing know-how, the outlet from a P-trap goes horizontally into the wall, while an S-trap discharges through a vertical pipe into the floor.

Beside acting as a nasty-niff-neutralizer, P-traps beneath sinks are also invaluable captors of small objects, such as jewelry, inadvertently dropped into the sink.

It’s easy to lose sight of the fact, however, that a trap only works when it’s full of water: a lack of aqua would create a direct pathway between the local sewer pipes and your sensitive nose.

It was the possibility of this standing water becoming a bacterial breeding ground, however, that spurred the team of five University of Virginia scientists into action to conduct their research, which was published at the end of last month in the journal Applied and Environmental Microbiology .

They were particularly interested in a big problem that hospitals have, in which sinks have been linked to outbreaks of pathogenic bacteria.

In their new study, the researchers built a lineup of five sinks, all draining into one common pipe, an arrangement typical of many hospitals.

The individual sinks were separated from each other using plexiglass partitions.

Once the plumbing was in place and fully sterilized, the scientists added harmless, fluorescent bacteria (they used a non-pathogenic form of E. coli) to the P-traps.

So what did the research reveal?

Well, when the sink faucets were run once a day, the bacteria in the P-traps were either somewhat flushed away, or remained at least partly in place.

Bacteria tends to form a biofilm – a sticky layer that fastens itself to solid surfaces, rather than hanging around as a suspension in the water.

However, things were rather different when the faucets weren’t opened.

The University of Virginia scientists hypothesized that in addition to being used for hand washing, sinks in hospitals are also convenient places to dispose of unwanted liquids, such as unused intravenous fluids and unwanted beverages.

So, in order to simulate this, the researchers “fed” their bacteria every day with saline solution and something called Triptych Soy Broth (TSB), which may sound like a heavy metal band, but is actually a nutrient used in microbiology labs when there’s a need to grow certain pathogenic bacteria. It’s made to a precise recipe using fixed amounts of Tryptone, Phytone, sodium chloride, dipotassium phosphate, glucose, and distilled water.

While this may sound pretty yucky to you, microbes consider it a bacterial banquet.

In fact, when this solution was introduced into the sinks, it was almost as if the bacteria couldn’t stop themselves reaching out for more.

The biofilm began climbing the pipework at the extraordinary rate of one inch per day, meaning that in just eight days, there was a substantial presence of microbes in the sink’s strainers.

What’s more, after this happened and the faucet was turned on, an aerosol of bacteria was instantly spread from the strainer to the countertop surrounding the sink, from where it could be potentially distributed further, either by individuals touching the surface, or via objects placed upon it.

In a further revelation, the research showed that bacteria could spread from sink to sink, when water carried microbes from one P-trap to another.

These findings are of considerable significance to the healthcare world, of course, but what implications, if any, are there closer to home?

Well, for example, the Irish government’s Health Service Executive advises that any faucet that’s not in regular use should be turned on once a week, to help flush away germs.

Meanwhile, researchers from the University of Arizona have shown that filling a bathroom sink with warm water containing a cup of household bleach, leaving it for 10 minutes, then draining the sink, reduced bacteria around the drain by more than 90% when the procedure was carried out three times a week.

If you’re concerned about the environmental impact of bleach, there are alternatives. For example, the website, which offers “rural American know-how,” suggests mixing 12 cups of water with a quarter cup of lemon juice, and one cup of hydrogen peroxide, to create a perhaps more eco-friendly bleach alternative. ( also shows you how to build a pretty nifty chicken coop.)

Although we’d be the first to agree that it’s definitely not the case that all bacteria is bad (and some of it is very good indeed) there’s value in taking sensible hygiene precautions around the house, especially when someone has been sick.

Let’s keep that pipe-climbing bacteria in its place, shall we?

More reading

A Safer Bleach Alternative

Alexander Cumming

Hospital Sinks May Spread Multidrug-Resistant Bacteria

Hygiene and Infection – Control advice in the home

Pipe-climbing bacteria might spread infection from hospital sinks

P-Trap & S-Trap, what’s the difference?

Reduction of faecal coliform, coliform and heterotrophic plate count bacteria in the household kitchen and bathroom by disinfection with hypochlorite cleaners

Spread from the Sink to the Patient: in situ Study Using Green Fluorescent Protein (GFP) Expressing Escherichia coli to Model Bacterial Dispersion from Hand Washing Sink Trap Reservoirs

Trap (plumbing)

Tryptic soy broth