And although wastewater treatment plants cut the
number of bacteria overall, the treatment process boosts the
proportion of bacteria resistant to some antibiotics, the research
The study focused on E. coli, a type of bacteria that commonly
inhabits the intestines of many healthy animals, including humans,
although certain strains can cause food poisoning.
Drug resistance among many types of bacteria, including E. coli, is
a growing problem. Previously unaffected bacteria can pick up
resistance genes from other bacteria that carry them, and resistance
can also spread through food crops irrigated with affected water.
“These multi-drug resistant bacteria are now the most frequently
isolated ones in French hospitals, and in many countries,” said
Xavier Bertrand, an author of the study and a microbiologist at
Universite de Franche-Comte in Besancon, France.
Because E. coli commonly lives in the human digestive tract, the
organism finds its way into wastewater, for example, when waste is
flushed down the toilet.
“The extent to which the discharge of (antibiotic-resistant E. coli)
into the environment contributes to its global spread remains
uncertain,” Bertrand wrote in an email to Reuters Health.
For the study, published in Clinical Infectious Diseases, Bertrand
and his team collected and analyzed samples from 11 sites in the
Besancon wastewater network. Two of the sites contained wastewater
from university hospitals. Some of the other wastewater came from
the city, while a fraction was from rainwater. No water in the study
had been used in livestock farming. The samples were collected each
week for 10 weeks.
The researchers found that all samples contained E. coli, and 96
percent contained antibiotic-resistant strains. The average number
of individual E. coli bacteria found in the city wastewater was more
than twice that in the hospital wastewater.
After wastewater had been treated to make it suitable for release
back into the environment, the number of antibiotic-resistant E.
coli dropped by 94 percent. But the proportion of resistant bacteria
doubled during treatment: initially, 0.3 percent of E. coli carried
a gene for resistance, whereas 0.6 percent did after treatment.
Although some towns, such as Wichita Falls, Texas, have turned to
recycling wastewater due to an ongoing drought, experts said the new
results likely don’t mean much for people drinking water from
[to top of second column]
That’s because water in the U.S. undergoes myriad processes with the
aim of eliminating bacteria and other contaminants. Furthermore, the
U.S. has stringent limits for bacteria in treated wastewater, said
John Scott Meschke, a microbiologist at the University of Washington
“We have a very good multi-barrier approach (to water purification
and disinfection) in the U.S.,” Meschke told Reuters Health.
Each wastewater treatment plant must meet specific standards,
including for the number of bacteria and the amount of oxygen the
water contains. Water cannot contain any E. coli in order to qualify
Still, the importance of wastewater as a source of antibiotic
resistance remains to be seen.
“The immediate concern is the risk of bacteria carrying these genes
within recreational water, such as lakes and rivers,” not drinking
water, Meschke said.
Infections in humans caused by resistant bacteria are more difficult
and complicated to treat than infections caused by non-resistant
The U.S. drinking water supply, Meschke said, is disinfected, so
bacteria in tap water “is not something to worry about.”
Clinical Infectious Diseases, online May 1, 2014.
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