Bugs resistant to antibiotics have been found in rivers, canals and ponds in London, according to new research.
They were found to contain high levels of DNA that cannot be killed by common drugs such as penicillin, erythromycin and tetracycline.
The potentially deadly genes come from human and animal waste, including wild ducks.
Researchers say they could pose a threat to revellers cooling off in the summer heatwave. They could even be present in drinking water.
The Regent’s Canal, Regent’s Park Pond and the Serpentine also contained them – but the Thames had the most. It is another reason to resist the urge to dive in.
They may help bacteria mutate into untreatable illnesse. Better ways of filtering antibiotic residue out of waste is needed to stop the DNA spreading, warn the team.
Each time an antibiotic is taken much of it is excreted into the sewer system – and then into rivers, canals and ponds.
Their presence provides an environment where microbes carrying the genes can multiply quicker – and pass on their resistance.
Project leader Dr Lena Ciric, a civil engineer at University College London, said: “This shows more research is needed into the efficiency of different water treatment methods for antibiotic removal, as none of the treatments currently used were designed to incorporate this.
“This is particularly important in the case of water bodies into which we discharge our treated wastewater, which currently still contains antibiotics.
“It is also important to look into the levels of antibiotics and resistant bacteria in our drinking water sources.”
There is currently no legislation to remove antibiotics or the resistant genes from water sources.
So they may be lurking in small amounts in drinking water – although this would require testing, say the team.
The Thames is likely to have higher levels of antibiotics and resistant genes because a large number of wastewater treatment works discharge into it both upstream and in London.
Antibiotics entering the sewer system are diluted through flushing, but even low levels can encourage resistance genes to multiply and spread to more microbes.
The researchers developed a DNA-based method which can provide information about the number of each of the resistant genes per litre of water.
They then compared the numbers of the resistant genes in the different London water systems.
Antibiotic resistance has been listed as one of the top 10 threats to human health by the World Health Organisation. Illnesses that were once easy to cure may become deadly.
The drugs fight bacteria in a number of ways, including by killing them or preventing them from spreading. But they also have a major weakness.
Some bacteria acquire genes that protect them from the drug’s attack. This is caused by exposure to low amounts of the drugs over long periods of time.
They survive treatment and reproduce themselves, spreading the key genes more widely so the drug becomes ever less effective.
The team whose findings are published in the Journal of Microbiological Methods are now experimenting with removing antibiotics and the resistant bacteria and genes from water taken from London’s natural water system.
They are doing this using slow sand filtration, which is a form of drinking water treatment.
This technique is used around the world including at Thames Water’s Coppermills Treatment Works which provides drinking water for most of north east London.
They are investigating using different variations of the filtration, with changing proportions of sand and activated carbon and different flow rates.
Three bodies have been recovered from different parts of the Thames this week.