Discovery

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An investigation into the mechanisms of a family of drugs used in humans was underway, with a control experiment being set up in order to (ideally) show no effect of the drug on bacterial cells.

Control experiments can be performed using antibiotic-resistant bacteria, grown on a petri dish containing the antibiotic (to which they are resistant) to select for the species and prevent contamination. 

When the control (resistant bacteria) colonies failed to grow, it first seemed as though this may have been an error.  However despite multiple repeat attempts, the bacteria still did not grow.  

The combined presence of the human drug with the antibiotic on the petri dish proved too potent for the bacteria to survive. 

It became clear that the repurposed drug was interacting synergistically with the antibiotic in an unanticipated manner, supporting it to overcome the resistance barrier.  The drug effectively reinvigorated the antibiotic, killing the bacteria which had previously been able to resist its antibacterial activity.

A novel way to fight antimicrobial resistance had just been discovered...