Reversible decrease in swimming motility accompanies emergent β-lactam antibiotic resistance in Escherichia coli

Within 10 passages of ampicillin or cephalexin selection in Escherichia coli NIST 0056, high-level β-lactam resistance (> 32 ug/mL) emerged alongside dose-dependent swimming motility reductions.

• Why it matters: AMR is hard to detect early because the evolutionary and molecular links between resistance genotypes and phenotypes are not well defined.
• What they did: The study used laboratory evolution under β-lactam pressure, then combined whole-population genome sequencing with swimming motility assays; motility was compared in mixed populations versus individual clones (p < 0.005, Mann-Whitney; two-tailed test).
• The result: After antibiotic removal, motility was restored and AMR-associated mutations decreased in frequency, implying fitness costs; the linked mechanisms included efflux-related acrB and frdD changes and ampC amplification.

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