Pyelonephritis -- infection of the kidney usually caused by E. coli bacteria and which can start as a urinary tract infection -- causes fever, back pain and vomiting.
About half of people infected with the bacteria require hospitalisation and if not treated with effective antibiotics, it can cause sepsis and lead to death.
In the study, almost 12 per cent of people diagnosed with pyelonephritis had infections resistant to the standard class of antibiotic used in treatment -- fluoroquinolone.
The emergence of infections caused by a specific strain of E. coli that is resistant to additional types of antibiotics, severely limiting treatment options has also been found.
The strain is dubbed as ESBL (extended-spectrum beta-lactamases) for the antibiotic-destroying enzymes it produces.
Currently, there are only a few intravenous antibiotic options to treat ESBL-related infections, and no oral antibiotics that are consistently effective, the researchers said.
"This is a very real example of the threat posed by the emergence of new antibiotic-resistant strains of bacteria, which greatly complicates treatment of infection," said lead author David Talan, Professor at the University of California, Los Angeles.
For the study, the team included 453 persons diagnosed with kidney infection.
The participants were diagnosed between July 2013 and December 2014 in 10 emergency departments at large hospitals in the US.
Further, the study found that the rates of ESBL-related infections varied from 0 per cent to more than 20 per cent, depending on the location of the emergency room and patient risk factors.
About one in three people infected with ESBL-producing E. coli had no traditional risk factors for antibiotic resistance, suggesting the bacterial strain is now endemic and healthy people are also at risk.
About three of every four persons infected with the disease were initially treated with antibiotics ineffective against that particular strain of bacteria, placing them at risk for poor outcomes.
The researchers recommended the development of new medications and guidelines calling for treatment with different types and combinations of antibiotics.
The findings were published in the journal Emerging Infectious Diseases.
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