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
