Prebiotics in cosmetics are  actives for the skin which naturally feeds the skin’s healthy microbes. The benefit of prebiotics is that they diminish the number of harmful bacteria and create a balanced skin pH.
Prebiotics help to aid in skin restoration and accelerate the recovery of problematic skin, especially sensitive and overly reactive skin.  Prebiotics consist or specialised plant fibres which act as a food source for the good bacteria.  The balancing of microbes results in improvement in the skin’s moisture levels in the skin and the skin barrier becomes strengthened.  Acne has also been shown to improve with prebiotic usage.
Prebiotics and probiotics differ.  Probiotics are live microorganisms which when supplied in adequate amounts give a health benefit.  In terms of cosmetics it means the beneficial microflora that have the ability to optimise the beneficial flora of the skin.
There are many factors that can influence the skin microbiome, from the food we eat to the products we apply topically.  Over cleansing and over exfoliating compromises the acid mantle and this impacts the skin microbiome.  The microbes of the skin function best when the skin barrier is intact and the skin is well hydrated.