Commonly used herbicides such as Roundup may be making antibiotics less effective.
Researchers from the University of Canterbury looked at what happened to disease-causing bacteria when they were exposed to herbicides such as Roundup, Kamba and 2,4-D.
Professor Jack Heinemann said the findings were worrying, given the increasing number of disease-causing bacteria that were resistant to antibiotics and the lack of any rules around the use of herbicides in peoples' gardens.
"And even if you were to alert your neighbour, your cat probably would not pay any attention to that and may walk through a lawn," he said.
"The cat will walk back into your house and your child might pet the cat, so if there are also some antibiotics in that household, it may matter."
Dr Heinemann said a herbicide such as Kamba had an even greater effect on disease-causing bacteria if a person exposed to it was also taking aspirin.