WELLINGTON, March 24 -- Commonly used herbicides can increase bacteria resistance to antibiotics, according to a New Zealand-led study out on Tuesday.

Herbicides, used to kill plants, could be tested for killing bacteria, but they had never been tested for other effects on bacteria, said Prof. Jack Heinemann at the University of Canterbury, which led the study.

In the first study of its kind in the world, the team investigated what happened to species of disease-causing bacteria when they were exposed to common herbicides such as Roundup, Kamba and 2,4-D.

"We found that exposure to some very common herbicides can cause bacteria to change their response to antibiotics," Heinemann said in a statement.

"They often become antibiotic resistant, but we also saw increased susceptibility or no effect. In most cases, we saw increased resistance even to important clinical antibiotics," he said.

The effects found were relevant wherever people or animals were exposed to herbicides at the range of concentrations achieved where they are applied.

That could include farm animals in rural areas and potentially children and pets in urban areas.

Antibiotic resistance was a serious and growing problem for human and animal health, said Heinemann.

New antibiotics were hard to find and can take decades to become available, while the effects of chemicals such as herbicides could conflict with measures taken to slow the spread of antibiotic resistance.

The research team included researchers from Mexico, and New Zealand's Lincoln and Massey universities.