HOUSTON: The world's most widely used weed killer may also be indirectly causing death of honey bees, according to a study.Researchers from The University of Texas at Austin in the US found that honey bees exposed to glyphosate , lose some of the beneficial bacteria in their guts and are more susceptible to infection and death from harmful bacteria.Scientists believe this is evidence that glyphosate might be contributing to the decline of honey bees and native bees around the world."We need better guidelines for glyphosate use, especially regarding bee exposure, because right now the guidelines assume bees are not harmed by the herbicide . Our study shows that's not true," said Erick Motta, who led the study published in the journal PNAS.Since glyphosate interferes with an important enzyme found in plants and microorganisms, but not in animals, it has long been assumed to be nontoxic to animals, including humans and bees.The study shows that by altering a bee's gut microbiome -- the ecosystem of bacteria living in the bee's digestive tract, including those that protect it from harmful bacteria -- glyphosate compromises its ability to fight infection.The researchers, including Professor Nancy Moran from the University of Texas at Austin, exposed honey bees to glyphosate at levels known to occur in crop fields, yards and roadsides.They painted the bees' backs with coloured dots so they could be tracked and later recaptured.Three days later, they observed that the herbicide significantly reduced healthy gut microbiota . Of eight dominant species of healthy bacteria in the exposed bees, four were found to be less abundant.The hardest hit bacterial species, Snodgrassella alvi, is a critical microbe that helps bees process food and defend against pathogens, researchers said.The bees with impaired gut microbiomes also were far more likely to die when later exposed to an opportunistic pathogen, Serratia marcescens , compared with bees with healthy guts.Serratia is a widespread opportunistic pathogen that infects bees around the world.About half of bees with a healthy microbiome were still alive eight days after exposure to the pathogen, while only about a tenth of bees whose microbiomes had been altered by exposure to the herbicide were still alive."Studies in humans, bees and other animals have shown that the gut microbiome is a stable community that resists infection by opportunistic invaders," Moran said."So if you disrupt the normal, stable community, you are more susceptible to this invasion of pathogens," he said.Researchers recommend that farmers, landscapers and homeowners avoid spraying glyphosate-based herbicides on flowering plants that bees are likely to visit.