Bacteria analysis of the bath ducklings

Bath ducks live in humid and warm areas. These are ideal conditions for the growth of biofilms of bacteria and fungi, for example on shower curtains or behind boxes. This is especially true for rubber ducks and other toys used for bathing. Inside the soft toys, lush carpets of bacteria and fungi grow. If a child then squeezes its toy, it is not uncommon for a brown broth to spray out of it. A group of researchers from Eawag, ETH Zurich and Illinois University has now investigated the factors that promote this growth and the types of microorganisms present.

Stubborn hospital germs

For this purpose, the scientists collected used bath toys and characterized the biofilms of bacteria and fungi on the inside. In parallel, they also conducted tests with new rubber ducks. For eleven weeks, they exposed them to conditions that would be realistic in a household - some as a control group only in clean drinking water and some in used bath water together with factors such as soap residues, dirt and sweat, but also bacteria from the human body. Subsequently, all ducklings were cut open and examined in the laboratory. The results do not sound appetizing: between 5 million and 75 million cells per square centimeter were found on the plastic surfaces. There were large differences in the composition of the biofilm communities, especially in the bath ducks used under real conditions, but also between the control groups. Various fungi were found on 60% of the real used and on all control ducks used in dirty water. In 80% of all ducklings, the researchers found representatives of potentially pathogenic bacteria, including Legionella or the rod-shaped bacteria Pseudomonas aeruginosa, known as persistent hospital germs.

The researchers then investigated the causes of the abundant biofilms: First, they examined the tap water. This usually has such low nutrient concentrations that bacteria can only grow minimally. But the ducklings themselves provide a source of nutrients. That's because a lot of organic carbon is released from the soft plastic material - often low-quality polymers. Other important nutrients such as nitrogen and phosphorus, but also additional bacteria, enter the tub during bathing, e.g. from the bather's body, from dirt brought in, or from care products such as shampoos and creams.

Source of gastrointestinal infections?

The lead author of the study is microbiologist Lisa Neu. In her doctoral thesis, she looks - not only at the example of rubber ducks - at how biofilms form on plastic and how the materials influence microbial processes in drinking water. Her supervisor, Frederik Hammes, is not surprised by the results: "There are many forums and blogs on the Internet about dirty bathing ducks, but they have hardly been studied scientifically so far." Yet, Hammes says, the toys are extremely exciting research objects, "because they form the interface between drinking water, plastics, external contaminants and susceptible end users." By vulnerable or sensitive users, Hammes is referring to (young) children, who also like to splash themselves with the broth from the ducklings. "That can boost the immune system. Then it's positive," the researcher says, "but it can also lead to inflammation of the eyes and ears, or gastrointestinal infections."

So it's better not to have any more bath toys in the tub? After each use clean elaborately? Or, as recommended on the Internet, stop the splashing fun before the first use and tape the hole shut? Researcher Hammes sees another way forward: stricter regulations for the polymers used for the duckies. After all, that worked for problematic chemicals, now the release of carbon would have to be taken into account, as is already being done today in tests for plastic drinking water pipes.

The study was funded by the Swiss National Science Foundation as part of a project on bacterial growth in domestic installations (project 163366; Microbi-Homes).

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