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The study examined eight intravenous drugs active against carbapenem-resistant bacteria - ranging from older antibiotics including Colistin to newer ones such as Ceftazidime-avibactam.

For example, colistin, one of the few gram-negative-only antibiotics approved for clinical use, can cause C. difficile-associated diarrhea and pseudomembranous colitis, a potentially life-threatening complication.

The drug also has toxic effects on the liver and kidney, and "thus colistin is typically utilized only as an antibiotic of last resort," the researchers wrote.

County wastewater that were resistant to colistin, a last-resort antibiotic.

Colistin is an old antibiotic that’s considered the last line of defense against certain infections, including those that are resistant to carbapenems, another last-resort class of antibiotics.