Why house dust mites thrive on discarded skin scales

Skin scales consist mainly of strands of tough keratin bound loosely together, a bit like rope, alongside various lipids (fats and fatty acids). When a healthy scale is first discarded it is covered with moulds, bacteria and yeasts all living in harmony with its host. These 'freeloaders' soon die out as their environment changes, but are quickly replaced by opportunistic fungi colonising the newly found habitat. A fresh, healthy skin scale is not the perfect food for the mite because it is too rich in lipid content of fats and fatty acids and needs to be 'defatted' by fungal activity. There is an exception however! Discarded scales from people with dry skin or those with atopic dermatitis come with ready-made reduced lipid content more suitable to the mite's needs. This is why researchers have found larger mite populations in beds and bedding in this grouping.

When a skin scale is 'ripe' for eating it is swollen with water from the surrounding atmosphere, carrying bacteria, microorganisms, fungi and lipids that have been attacked by fungi. The whole scale is a moist, nutrient rich food parcel. For tough keratin, the mite's midgut area acidity level is such that keratin can be partially broken down for digestion or passed on to be encased in mite droppings.

For the mite each skin scale bearing-food has to be just right. Too much fungi and it's a mite poison, too little means not enough water. The climate for the scale must be warm, damp, dark and still for the fungi, yeasts, bacteria and microorganisms to survive and grow.

Here it may be appropriate ask why house dust mites consider their droppings as food? Perhaps it is because the droppings contain:

lost nutrients from part digested food

digestive enzymes to help breakdown newly scavenged food

microorganisms that, through incubation and growth, may aid digestion

products that produce important pheromones for a blind mite