This dataset holds realistic environmental concentrations of potential pathogens colonising microplastic beads (nurdles) collected from ten Scottish beaches. Observed measures include concentrations and the present genes of Klebsiella bacteria isolated from the nurdles, as well as the salinity, electrical conductivity, turbidity, and polymer composition of the nurdles. In addition, an infection experiment was conducted to measure the virulence of the Klebsiella bacteria. Galleria melonella larvae were infected with the isolated Klebsiella bacteria and the number of living larvae was recorded.