The laboratory results suggest that probiotic bacteria could be used to stop the spread of HIV through breastfeeding, when standard retroviral drugs are too toxic for newborns, said the scientists from the University of Illinois at Chicago College of Dentistry.
Worldwide, mother-child transmission of HIV results in 800,000 new infections each year.
The team found that Lactobacillus bacteria latched onto the sugar coating on the envelope that encases the virus particle and blocked infection. The bacteria also bound the sugar coating on immune cells, causing them to clump - a feature that could render those harbouring HIV incapable of infecting other cells.
Lin Tao, associate professor of oral biology at the UIC College of Dentistry, and colleagues screened hundreds of oral bacteria taken from the saliva of healthy volunteers before identifying six Lactobacillus strains that produced proteins capable of binding the particular type of sugar found on the HIV envelope, called mannose. The binding of the sugar normally enables the bacteria to stick to the mucosal lining of the mouth and digestive tract, forming colonisation.
Further screening identified two strains capable of trapping live HIV viruses by binding with glycoprotein receptors, called gp120, in their viral envelope composed of protein spikes and a sugar 'dome' rich in the mannose sugar. The sugar 'dome' prevents HIV from being recognised by the human immune response, but it is the target of these lactobacilli. Each HIV virus has about 72 such glycoprotein receptors.
"The two strains were found to bind with several varieties of HIV, the related simian immunodeficiency virus, and immune cells that HIV targets for infection, which are also covered with mannose. Further analysis showed that the bacteria inhibited HIV infection of immune cells in the laboratory," said Tao.
One strain secreted abundant mannose-binding protein particles into its surroundings, neutralizing HIV by binding to its sugar coating. The other Lactobacillus strain also neutralized HIV particles but required direct contact with the virus because its mannose-binding proteins reside on its surface.
That requirement made this strain less effective in tackling HIV particles, but it proved remarkably efficient at binding with immune cells, whose surfaces, like HIV, also contain glycoprotein receptors.
The researchers observed that immune cells trapped by lactobacilli formed a clump. That configuration would be expected to immobilize any immune cells harboring HIV and prevent them from infecting other cells.
While HIV exist as many subtypes because of frequent mutations, posing a challenge for vaccines, the sugar coating of the virus remains largely the same, presenting a ready target for lactobacilli to attack no matter whether the virus particle itself is genetically altered, explained the scientists.
"While studies have been done so far only in the laboratory, we believe this work opens up new possibilities for preventing the transmission of HIV through mothers' milk," said Tao. "Unlike standard retroviral drugs, which are too toxic for newborns, lactobacilli are 'friendly' bacteria already inhabiting the human digestive tract and milk products, and so should pose no danger to infants."