BERN, Switzerland — Scientists now have the dirt on the rubber ducky: Those cute yellow bathtub toys are — as some parents have long suspected — a haven for nasty bugs.

Swiss and American researchers counted the microbes swimming inside the toys and say the murky liquid released when ducks were squeezed contained ‘‘potentially pathogenic bacteria’’ in four out of the five toys studied.

The bacteria found included Legionella and Pseudomonas aeruginosa, a bacterium that is often implicated in hospital-acquired infections.

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

They turned up a strikingly high volume — up to 75 million cells per square centimeter — and variety of bacteria and fungus lurking inside the ducks.

While certain amounts of bacteria can help strengthen kids’ immune systems, they can also lead to eye, ear, and intestinal infections, the researchers said.

ASSOCIATED PRESS