They make bath-time fun for little ones - but rubber ducks could be concealing a deadly health risk.
The researchers stressed though that there was a big difference between the plastic toys exposed to different types of water.
The bacteria found included Legionella and Pseudomonas aeruginosa, a bacterium that is "often implicated in hospital-acquired infections", the authors said in a statement.
The warm and humid environment in bathrooms is an ideal breeding ground with "dense growths of bacteria and fungi" building up inside toys, the Swiss study said.
But regardless, a clear majority, 80% had potentially harmful bacteria cells. Adults supervising or playing with youngsters could also be vulnerable, the academics added.
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The study, aptly titled, "Ugly ducklings-the dark side of plastic materials in contact with potable water", was published in the latest issue of the journal Biofilms and Microbiomes.
Researchers at the Swiss Federal Institute of Aquatic Science and Technology, ETH Zurich and the University of IL, counted microbes [bacteria causing disease] inside 19 bath toys by cutting them open to reveal what the liquid was like inside. They looked at a total of 19 different bath toys obtained from real life situations. She says that they found that it was the chemical carbon structure that the toys were made of that provided growth material for the bacteria. Children particularly enjoy squirting water from bath toys into their faces.
The researchers said: "During bathing, other key nutrients such as nitrogen and phosphorus, as well as additional bacteria, are contributed by the human body (body fluids such as urine and sweat), external contaminants and personal care products".
In some cases, they say exposing children to microbes like this can result in a stronger immune system.