Antibiotic Disc Assay for Synechocystis sp. PCC6803

This protocol describes how to investigate the integrity of the outer cell wall in the cyanobacterium Synechocystis sp. PCC6803 using antibiotics. It is adapted to the agar diffusion test (Bauer et al., 1966), in which filter paper discs impregnated with specified concentrations of antibiotics were placed on agar plates inoculated with bacteria. The antibiotics we tested, interfering with the biosynthesis/function of bacterial cell walls, will diffuse into the agar and produce a zone of cyanobacterial growth inhibition around the disc(s). The size of the inhibition zone reflects the sensitivity of the strain to the action of antibiotics, e.g., a mutation in a protein functioning within the cell wall or its construction would render the mutant strain more sensitive to the respective antibiotic. The method has proven to be useful for phenotyping a mutant of Synechocystis sp. PCC6803 lacking all three genes encoding Deg proteases. Deletion of these ATP-independent serine proteases was shown to have impact on the outer cell layers of Synechocystis cells (Cheregi et al., 2015).