International researchers tested 4000 kilometres of China's coastline, finding sediment from estuaries – where fresh water from rivers and streams meets the sea – had levels of antibiotic resistance gene pollution ranging from 1 million to 100 million genes per gram.

Widespread use of antibiotics in human and animal health, and to promote growth in animals farmed for food, has accelerated the process of microbes becoming resistant to them, threatening the ability to treat common infectious diseases.

The results of the research, published in Nature Microbiology, indicated that the vast majority of estuarine bacteria had at least one gene that helped them become resistant to antibiotics.

Researcher and study author Michael Gillings, a professor of molecular evolution at Macquarie University, said more than 200 different resistance genes were detected during the study, "meaning that we're dealing with quite a variety ... that could help estuary bacteria along the path to becoming superbugs".

Antibiotics and resistance genes were contaminating the environment through sewage and waste water, or being released directly into rivers and lakes, Professor Gillings said.

The places tested in China "are just a microcosm of a worldwide problem" and the discoveries likely applied in some respects to estuaries around the world – including Sydney Harbour, Port Stephens and the Hawkesbury River, he said.

"These regions are places where bacteria and polluting antibiotics can meet, promoting the accumulation of antibiotic resistance genes," Professor Gillings said. Humans were potentially exposed to those genes when eating fish, oysters, prawns and crabs sourced from estuaries.

"Those sediments are now a breeding ground for bacteria that have multiple antibiotic resistance genes," he said. "When a bacterial species gets lots of resistance genes ... it's very, very difficult to kill them with any antibiotic at all."

Antibiotic resistance, or antimicrobial resistance, is a serious global threat, causing around 700,000 deaths per year. It's expected that superbugs will kill 10 million people a year by 2050.

"The only thing we can do is slow down this evolution," Professor Gillings said. "[We need to think about] ways of improving water treatment and waste treatment coming out of human activities."

The Australian government's National Antimicrobial Resistance Strategy 2015-2019 aims to ensure appropriate prescribing of medicines, co-ordinate surveillance of antimicrobial resistance and usage, set a research agenda and improve infection prevention and control measures, while the National Centre for Antimicrobial Stewardship and NPS MedicineWise are among the organisations promoting more restrained use of antibiotics.

On any given day in an Australian hospital in 2014, 38 per cent of patients were given antibiotics, but 47 per cent were either inappropriately prescribed or flouting guidelines, the first national report of antimicrobial use and resistance found.

Professor Mark Walker, director of the Australian Infectious Disease Research Centre, said the spread of antimicrobial resistance threatened to send modern medicine back to the time before antibiotics.

"A lot of people used to succumb to infection post surgery when bacteria contaminated a wound and there was no treatment," he said. "The mortality rates for surgery will go up. The mortality rates for cancer will go up, because chemotherapy knocks down the immune system."

Professor Walker said it was a huge but under-appreciated problem that was "creeping up slowly".

"One more antibiotic resistant pathogen's out there and then it's another and another," he said. "I see the problem as a tide that's slowly getting higher and higher and higher, and the medical community is able to keep its head above water. But eventually, when we have more of these pathogens that are resistant to every known antibiotic class, that's when we drown."

Professor Mary-Louise McLaws​, an infection control expert at the University of NSW, said it was important to keep hospitals clean to prevent resistant bacteria transferring between patients and to reduce community demand for antibiotics during winter.

"A very bad cold or chest infection [is] usually viral and antibiotics won't help these infections, in fact the antibiotics could increase resistance in your [good] bacteria."

She also said that with 90 per cent of antimicrobial agent ingredients sold in Europe used for food animal production, and their use set to increase to more than 100,000 tons by 2030, mainly due to Asian food production, the community can demand that animals raised for food had no antibiotics used in their feed.

"There is no scientific evidence that antibiotics are required to prevent infection in food-animal farming," she said.

You can help prevent antibiotic resistance by:

• not requesting antibiotics for colds and flu, which are caused by viruses
• telling your doctor you only want an antibiotic if it is really necessary
• taking the right dose of your antibiotic at the right time, as prescribed by your doctor
• taking your antibiotics for as long as your doctor tells you to – always finish the prescribed course
• taking simple steps to avoid infections and prevent them from spreading.

Source: NPS MedicineWise