Viruses on toothbrushes and shower heads: Northwestern University finds 'absolutely wild' diversity

You are not the only one making use of your toothbrush.

Scientists at Northwestern University in the US have reported an "absolutely wild" quantity of viruses residing on the surfaces we rely on every day-specifically, the heads of our showers and the bristles of the brushes that go straight into our mouths.

Some of the microorganisms they detected appear to be entirely new to science.

If that makes you want to dash to the bathroom, bin your toothbrush, and scour the shower from top to bottom, it is worth noting that finding viruses on everyday items is neither unexpected nor automatically a threat to our health.

Microbes in bathrooms: normal, persistent, and hard to shift

Microorganisms such as bacteria, fungi, and viruses thrive in all sorts of man-made settings. By now, they almost seem to have squatter’s rights: even when we clean, they tend to return sooner or later.

That is why dentists and doctors recommend replacing your toothbrush regularly and keeping your bathroom clean-to prevent microbial communities from building up and getting out of hand.

At the same time, total avoidance is unrealistic, which is why it matters to identify the specific microbes we are most often exposed to.

Many people are aware that bacteria are present on countless visible surfaces, but far less is understood about which viruses persist there-even among researchers.

Why Northwestern University examined shower heads and toothbrushes

The virus work followed earlier research in the US that first focused on the bacteria found on shower heads and toothbrushes; afterwards, another team-overlapping in authorship-carried out a dedicated study of viruses.

"We found many viruses that we know very little about and many others that we have never seen before. It's amazing how much untapped biodiversity is all around us," says microbiologist Erica Hartmann from Northwestern.

"And you don't even have to go far to find it; it's right under our noses."

Hartmann originally developed the idea while she was at the University of Colorado, Boulder. She and colleagues wanted to examine microbes on toothbrushes after hearing worries that flushing the toilet could be dispersing harmful pathogens around bathrooms.

In the small study they ran, the bacteria on a typical toothbrush generally mirrored the microbes in our mouths rather than those from toilets-phew-but the question of viruses remained.

"This project started as a curiosity," explains Hartmann.

"We wanted to know what microbes are living in our homes. If you think about indoor environments, surfaces like tables and walls are really difficult for microbes to live on. Microbes prefer environments with water. And where is there water? Inside our shower heads and on our toothbrushes."

Viruses on toothbrushes and shower heads: what the viromes showed

To explore how bacteria and viruses interact in bathroom settings, Hartmann and her Northwestern colleagues analysed the genomes of microbial communities collected from 96 shower heads and 34 toothbrushes.

During the analysis, the researchers identified many bacteriophages-viruses that infect bacterial cells and reproduce inside them.

They also found that the viral communities, or viromes, living on toothbrushes and those on shower heads were strikingly different. The toothbrush samples contained a broader variety of viruses than the shower samples.

That contrast is consistent with the fact that some viruses tend to target particular bacteria, so you would expect the viromes in these two bathroom niches to diverge.

The bacterial sources differed too: toothbrush bacteria largely originate from human mouths, whereas bacteria on shower heads are more environmental in origin, including some known pathogens found in municipal water supplies.

What the findings mean for health and cleaning habits

Although this research offers only a small snapshot, it did indicate that certain viruses and the bacteria associated with them could pose a risk to human health if they were to multiply extensively.

"Whether interactions between phages and their host have noteworthy implications on human health risks such as antimicrobial resistance dissemination in the built environments requires further investigation," the authors conclude.

Even so, Hartmann advises against panicking about these largely harmless organisms.

"Microbes are everywhere, and the vast majority of them will not make us sick," she says.

"The more you attack them with disinfectants, the more they are likely to develop resistance or become more difficult to treat."

The study was published in Frontiers in Microbiomes.