Good bacteria are damaged if they are either removed or if they are killed!
What removes them? Any product which foams (forms bubbles in contact with water) removes everything from skin including good bacteria. All exfoliating ingredients remove good bacteria.
What kills them? Good bacteria is killed by harsh preservatives & antibacterial ingredients.
Harsh preservatives are added in products to kill bad bacteria and prevent them from getting spoiled. Preservatives are needed for only those products which have any type of water content. But preservatives cannot differentiate between good & bad bacteria so they end up killing good bacteria on skin as well.
That’s why we use gentle preservatives in our cleansers as they don’t kill good bacteria. They only prevent growth of bacteria so they are safe in cleansers due to short contact time on skin. But they are not safe in products which stay on skin for a long time like moisturizers, that's why our moisturizers are oil based and free of preservatives!