Scientists have genetically modified bacteria living in the human body to produce chemicals that block HIV infection.
E. coli bacteria were modified
Although the research is still at an early stage, they hope it could eventually lead to a practical and cost effective new way to combat the virus.
As of December 2004, there are 39.4 million people living with HIV/AIDS.
The research, by the US National Cancer Institute, is published in Proceedings of the National Academy of Sciences.
Most HIV transmission occurs on the surfaces of the gut and reproductive areas which are normally coated with a layer of bacteria.
The researchers modified one of these bacteria - a form of E.coli - so that it began to secrete proteins that block HIV from infecting its target cells.
When the modified bacteria were introduced in mice, they successfully colonised parts of the lower gut, and were also found in lower concentrations in the vagina.
Writing in the journal, the researchers said there was an urgent need for new ways to prevent the spread of HIV, especially in the developing world.
Scientists have been working on microbicide creams and gels which can be applied to the genitals to block HIV infection.
However, the researchers say the fact they need to be regularly applied before sex is likely to limit their use.
They believe their approach has the potential to offer more lasting protection.
"Bacteria are simple and practical to manufacture, store, distribute and administer, and they are far less expensive than protein-based microbicides," they write.
They also believe the method could be adapted to deliver bacteria secreting different proteins to different parts of the body.
Although primarily designed to prevent new HIV infections, they believe it could also be used, in combination with drug therapy, to treat people already carrying the virus.
Dr Tim Farley, of the World Health Organization, said: "In principle a technique such as this which enhances the body's defences against HIV sounds like a great idea.
"Clearly there are many steps to be completed in the development and clinical testing of the product, and there may be special safety concerns over unexpected side effects due to deliberately colonising the gastrointestinal tract with genetically engineered bacteria."
Lisa Power, of the Terrence Higgins Trust said: "This research is promising and is based on a clever idea, using naturally occurring bacteria to improve resistance to transmission.
"However, we are a very long way off its practical use in humans, and until then, condoms are the best defence we have against HIV and most other sexually transmitted infections."