Drug effect on bacteria. Disc-diffusion assay showing the effects of an antibiotic drug on bacteria. Black discs containing an antibiotic drug are seen, positioned at intervals on an agar culture of bacteria. The discs contain different concentrations of the drug. Zones of inhibition of bacterial growth are seen as clear areas around the discs. The strongest drug effect is at top centre; the least effect at bottom. Existing anti- biotics act to block bacterial growth. However, the search for new antibiotics is urgent because bacteria causing traditional human diseases are becoming resistant to all known antibiotics. Photographed at Microcide Pharmaceuticals, USA.