SuperbugsLow-levels of antibiotics can produce high-levels of resistance

A new study by scientists in Sweden indicates that bacteria exposed to small concentrations of antibiotics over time can become highly resistant, a finding the authors say provides an example of how low levels of antibiotics present in many environments may potentially contribute to antibiotic resistance.
In the study, published this week in Nature Communications, researchers from Uppsala University demonstrated that Salmonella exposed repeatedly to an amount of streptomycin that was not strong enough to kill the bacteria or inhibit growth still evolved high-level resistance. The resistance was caused by genetic mutations that haven’t been typically associated with antibiotic resistance and were different from those that develop when the bacteria is exposed to lethal amounts of the drug.
“These results demonstrate how the strength of the selective pressure influences evolutionary trajectories and that even weak selective pressures can cause evolution of high-level resistance,” the authors write.
Evolution of high-level resistance
CIDRAP says that to study the evolution of antibiotic resistance when exposed to low levels of antibiotics, the scientists repeatedly exposed a strain of streptomycin-susceptible Salmonella enterica serovar Typhimurium to an amount of streptomycin that was roughly one-quarter of the minimum inhibitory concentration (MIC)—the lowest concentration of an antibiotic needed to prevent bacterial growth. This amount was enough to reduce the bacterial growth rate by 3 percent. The bacteria were exposed to the streptomycin every 24 hours for 900 generations.
After this repeated exposure, the Salmonella Typhimurium bacteria were found to have varying levels of resistance, but four clones exhibited high levels of streptomycin resistance and were investigated further using whole-genome sequencing (WGS). This method of DNA fingerprinting revealed that the most resistant bacterial strains had acquired mutations in five genes, most of which are not typical resistance genes. In addition, the affected genes were different than the mutated gene found when the scientists exposed a Salmonella Typhimurium strain to an amount of streptomycin well above the MIC.
Individually, only one of these genetic mutations produced a significant increase in streptomycin resistance. But the scientists found that when the five mutant genes were combined they produced a much higher level of streptomycin resistance than expected, an indication that the resistance is being driven by the interaction of these mutations.