Handwashing may be responsible for bacteria that live and grow in domestic sinks, researchers say.

In what they say is the largest study of sink bacteria conducted outside of hospitals, scientists at the University of Reading discovered communities of similar bacteria that largely remain in drains after hand washing.

Researchers found that there are significant differences between families of dominant bacteria depending on the location in the sink drains.

They also found that plumbing systems such as P-trap or U-bend provide ideal environments for bacteria to grow.

Hyun Soon Gweon, lecturer in bioinformatics for genomics at the University of Reading, said: “The mantra to ‘wash your hands’ to fight coronavirus transmission has highlighted the importance of not only good hand hygiene, but also the need for well-designed and regularly cleaned sinks.

“Our study reveals that the significant difference in bacterial families between different buildings shows that a number of factors including occupancy and building design may have a big influence on the types of bacteria we come into contact with.”

This means that we need to be very aware that what we are putting down our sinks is affecting the bacterial community underneath

Samples were taken from 123 sinks around non-clinical settings at the University of Reading – such as toilets and bathrooms in teaching, research and social spaces.

They showed that sinks have a distinct microbiome dominated by certain bacteria.

The plumbing area beneath sinks revealed microbial communities dominated by a group of bacteria called Proteobacteria.

This included pathogens such as Salmonella and E. coli, which can cause serious disease, although the proportion of bacteria from that family was low.

While higher concentrations of the common Moraxellaceae and Burkholderiaceae bacteria which can cause infection were found, these are mostly harmless to humans.

According to the study published in Environmental DNA, the type of plumbing system had a significant effect on which family was more abundant.

While below-sink strainers were found to have Moraxellaceae bacteria, P-trap sinks had higher amounts of Burkholderiaceae.

Lead author of the study Zoe Withey, a PhD researcher at the University of Reading, said: “The bacteria that live in our sink drains are shaped by what we are directly putting down them.

“While we expected that bacteria from the gut would have a greater impact, caused by the wider environment of a bathroom, it seems that by and large the bacteria living on the skin of our hands are feeding the community in the drains beneath sinks.

“This means that we need to be very aware that what we are putting down our sinks is affecting the bacterial community underneath.

“These areas may not be reached during routine cleaning, and this could lead to communities containing hardier, resistant microbes.”

The researchers point out that all the sinks where samples were taken were regularly cleaned.

Dr Gweon added: “We hope our findings will remind people that the bacteria on your hands often stay alive and capable of growing even after they have been washed off, even in the presence of soap and warm water.

“It is possible to spread bacteria to the surrounding areas of your sink, where they can grow and persist. Reducing transmission of bacteria requires thorough disinfection of the sinks and surrounding areas and not just getting your hands wet.”

The study was conducted in 2019, prior to the coronavirus pandemic, so there is no direct influence of increased handwashing or other hygiene behaviour associated with the pandemic on this study.

But the researchers say the significance of bacteria from the skin means that handwashing will be having a significant effect on the bacterial communities of our sinks.