Scientists have the dirt on the rubber ducky: Those cute yellow bath-time toys are — as some parents have long suspected — a haven for nasty bugs.

Swiss and U.S. researchers found that the toys appeared to be a breeding ground for microbes. They counted the microbes swimming in the toys and said the murky liquid released contained “potentially pathogenic bacteria” in four out of every five ducks studied. The bacteria included Legionella and Pseudomonas aeruginosa, a bacterium that is “often implicated in hospital-acquired infections,” the researchers said.

The study by the Swiss Federal Institute of Aquatic Science and Technology, ETH Zurich and the University of Illinois was published in the journal Biofilms and Microbiomes. It’s billed as one of the first in-depth scientific examinations of its kind.

The researchers tested 19 different bath toys and found a strikingly high volume — 75 million cells of bacteria per square centimeter — in the ducks. Tap water doesn’t usually foster the growth of bacteria, they said, but low-quality polymers in the plastic give them the nutrients they need. Bodily fluids — like urine and sweat — as well as contaminants and even soap add microbes and nutrients like nitrogen and phosphorus and create balmy brine for bacteria.

While certain amounts of bacteria can help strengthen immune systems, they can also lead to eye, ear and intestinal infections, the researchers said. Among the vulnerable users: Children “who may enjoy squirting water from bath toys into their faces,” the institute said.

The toys are not the only household item found to be a haven for bacteria. Last year, a German study found that kitchen sponges were a hotbed for germs. And a microbiologist in 2012 found that the average cellphone carried 10 times more bacteria than most toilet seats.

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