I found this forgotten bar of soap after winter at my home in northern Sardinia. It had grown a coat of mould (pictured). What is the mould and how did it grow on soap, which is supposed to keep your hands clean?Patrizia Figoli Turcheteti, Bellaire, Texas, UK
This is a big misconception, that soap just outright kills any little critter we don't like. Soap allows you to wash up, especially with oils and dirt that don't dissolve readily into water. This has nothing to do with it being a germ killer. Even the antimicrobial soaps on the market only kill germs when they are sufficiently concentrated (the suds on your hands may be useless) That soap is growing mold because it has mold growing on it. As simple as that sounds that is the answer.
Whilst I cannot identify the fungus growing on the soap I can easily identify how it got there, bars of soap that we commoly use to wash our hands with are manufactured by reacting fatty oils with a strong alkaline to produce fatty salts that we use to emulsify the dirt on our hands and so remove it. It is essentially a large bar of fat, once a fungus that can break down the fats finds it, it spreads all over its tasty treat.