Like a river, urine passing through the kidneys deposits minerals; these can form nephrolithiasis or stones, which can cause irritation and pain. The usual treatment is waiting and lots of water or lithotripsy to break them up into passable sizes.
Many kidney stones are made of calcium and oxalate. Often a change in diet will prevent new ones.
An enzyme is needed to break down oxalate. People with less Oxalobacter formigenes have less of this enzyme and more stones. Bifidobacteria and lactobacilli also enhance this enzyme.
Research indicates that probiotic supplements do reduce oxalate absorption in both animals and humans. Kidney stones are rare in black South Africans. This was attributed to their higher diversity of Bifidobacteria and lactobacilli.
Further reading on kidney stones and probiotics: