Children conceived using a form of IVF could carry bacterial DNA in their chromosomes, scientists have said.
The researchers added E.coli to sperm
A Spanish team mixed mouse sperm with E.coli bacteria to see if accidental genetic modification could take place.
They then injected the sperm into mice eggs and found some resulting embryos did indeed contain an E.coli gene, New Scientist reports.
A UK expert said no such contamination had been known in human IVF, and was unlikely to cause ill health anyway.
The team from INIA, the Spanish agricultural research agency, used a procedure called ICSI, or intracytoplasmic sperm injection, where an individual sperm is injected directly into the egg.
ICSI is used to help men who have low sperm counts or sperm which does not move very well.
It accounts for around half of IVF procedures in many countries, including the UK and US.
But scientists have also been investigating using ICSI to make genetically modified animals, by mixing DNA with sperm before injecting it into the eggs.
'No need for alarm'
This led the Spanish team, whose work has also been published in Human Reproduction, to look at whether the DNA of children born through ICSI could accidentally be altered if a sperm sample was contaminated with bacteria.
The tests on mice involved mixing mouse sperm with E.coli which contained a gene that codes for a fluorescent protein, so the scientists could track its progress.
In cases where the sperm was fresh and "washed" to separate it from other components of semen, 12% of newly fertilised embryos contained the fluorescent gene, although it was not seen in embryos implanted in female mice.
Where sperm was frozen but not washed, 19% of embryos, and 6% of those which were implanted, had the gene.
The researchers did use high concentrations of bacteria, which experts say would normally be spotted under the microscope by IVF technicians.
Dr Maryse Bonduelle of the Flemish Free University in Brussels, who is studying the health of hundreds of children born through IVF, said: "I don't think there is any need to alarm patients or change procedures for the moment."
He said sperm samples were often contaminated by skin bacteria carried by the donor, and that - as clinics around the world operate to different standards - many should take more precautions to eliminate the potential for children to carry bacterial DNA.
The UK's Human Fertilisation and Embryology Authority said it would review the Spanish study to see if any of its guidelines needed to be changed.
But UK fertility expert Dr Simon Fishel, of Care Fertility, said: "This risk remains theoretical at the moment.
"And even if other DNA was passed on to the children, there is no evidence it would be manifested in any problems."
He added: "There are so many other factors that can affect our way in life that are perhaps even more toxic.
"But that doesn't mean there shouldn't be long-term research, which will hopefully show the risk is negligible."