Rubber duck bath toy. After this toy was cut in half after one year of use, it revealed a rich, dark grey biofilm of bacteria, fungi and organic bath residues such as soap residue, dander etc. In one study, scientists found between 5 million and 75 million cells per square centimetre on the plastic surfaces of rubber ducks and 80 percent harboured potentially pathogenic bacteria, including Legionella or the hospital germ Pseudomonas aeruginosa.