Treating patients with bacteria may be an effective way of reducing their risk of repeatedly developing painful kidney stones, a study suggests.
Kidney stones can cause severe pain
People naturally carrying the bacterium Oxalobacter formigenes were found to be 70% less likely to have problems.
Researchers at Boston University, in the US, are now investigating the possibility of using the bacteria as a "probiotic" treatment.
The study features in the Journal of the American Society of Nephrology.
Kidney stones are small, hard lumps formed of waste products contained in the urine.
They normally range in size from a grain of sand to a pearl. They can be smooth or jagged, and are usually yellow or brown.
Once a kidney stone has formed in a kidney it may travel down through the other parts of the urinary system, where they can slow the flow of urine, cause infection, severe pain and even lead to kidney failure.
About three in 20 men and one in 20 women in the UK will develop a kidney stone at some point in their lifetime.
They are most likely to occur in people aged 20 to 40.
Up to 80% of kidney stones are predominately composed of a compound called calcium oxalate.
O. formigenes breaks down oxalate in the intestinal tract and is present in a large proportion of the normal adult population.
The Boston team compared 247 patients with recurrent calcium oxalate kidney stones with 259 people with no history of the condition.
They found just 17% of the kidney stones group were colonised with O. formigenes, compared with 38% of healthy group.
Researcher Professor David Kaufman: "Our findings are of potential clinical importance.
"The possibility of using the bacterium as a probiotic is currently in the early stages of investigation."
Derek Machin, clinical director of urology at University Hospital, Aintree, said an effective treatment for recurrent kidney stones would be a significant step forward.
He said bigger kidney stones were currently treated by using shock waves to break them up, but this was not always completely effective.
Passing a stone in the urine intact can be extremely painful, and even getting rid of the smaller pieces created by shock treatment could cause significant pain.
"For some people kidney stones can be an on-going lifelong problem," he said.
"And in some cases a stone can destroy kidney function before it is even identified."
However, Mr Machin warned that there was much work to be done before clinical trials of a probiotic could be considered.
He said kidney stones had been linked to dehydration and were more common in countries such as Saudi Arabia where the climate is hot and dry.
In instances they may be linked to an unusually high rate of calcium excretion.
However, he said in many cases there was no obvious cause for the condition.
It is a particular problem for airline pilots, who are not allowed to fly if they have a stone.