The soap doesn't kill the fungi and bacteria etc. it forms a paste with water that traps them and then you wash them off.

We use soap for cleaning because it is a detergent: a means of emulsifying insoluble, largely fatty, dirt in water. Its nutritional value is usually irrelevant, but pure traditional soap consists of fatty-acid salts. Because of this, it is completely digestible in modest quantities. You may see a dog scoffing a chunk of soap because it smells appetisingly of fatty acids, but only if it doesn't contain too much scent or lye - sodium hydroxide, which is used in the production process. Missionaries who introduced soap to some tribal communities in Africa were startled to find that members of their congregations would treasure a fatty-tasting bar as a treat, occasionally licking a finger that had been moistened and rubbed on the soap.Toilet soap commonly contains surprising amounts of starches, oils, glycerol and other materials that make it smoother, less aggressive to the skin or simply cheaper to produce. These are all edible too, and moulds are happy to consume them. As long as the soap doesn't contain too much sodium and the air is moist enough, as it well might be in a bathroom, a bar of soap can certainly grow some very contented fungi.I suspect your soap sported a selection of Sardinian domestic moulds. Fusarium, Mucor, even white strains of cheese fungi, such as Penicillium camemberti, might be present. They are probably harmless. Try some if you like...Jon Richfield, Somerset West, South AfricaThough we admire Jon Richfield's desire for direct scientific experimentation, we recommend that you do not eat the soap before finding out exactly what is growing on it - Ed

While I can't help your reader identify the strain of mould on the soap, I can explain how mould can grow on something that is used for cleaning your hands.Soap consists of long-chain organic molecules, with one end that is polar (charged) and the other non-polar (uncharged). The polar end readily dissolves in water, which is also polar, while the non-polar chain readily attaches to grease and oil which are similarly non-polar.The soap therefore acts as a go-between: one end attaches to the oil and the other end wants to be in the water. This enables the oil on your skin to dissolve and be washed off. Without the soap, the polar water molecules would rather stick together than attach themselves to the oil on your hands.Mould can grow on all sorts of apparently uninviting materials, including leather or wallpaper. Soap is no different - it's just another organic material. This is not really a paradox. After all, when you use soap you don't actually leave it on your hands in order for them to stay clean; you wash it off.Simon Iveson, School of Engineering, University of Newcastle, New South Wales, Australia

The mould in our shower seems to grow on the residue of the conditioner rather than the soap or shampoo.Could someone please explain that and if possible suggest a spouse friendly solution (ie that doesn't involve stopping using conditioner)?

Who has been stealing stuff from my 'fridge ? My long lost lunch is famous !!