In drug discovery labs, the disk diffusion test is performed slightly differently than in diagnostic labs. In this setting, it is not the bacterial strain that must be characterized, but a test extract (eg. a plant or microbial extract). The agar plate is therefore inoculated with a bacterial strain of known phenotype (often an ATCC or NCTC strain), and disks containing the test extract are applied to the surface. Zone of inhibition sizes cannot be used as a semi-quantitative measure of antibacterial potency because different extracts contain molecules with different diffusion characteristics (different molecular sizes, hydrophilicities etc.). Zone of inhibition sizes can be used for the purpose of dereplication though. This is achieved by testing each extract against paired strains of bacteria (eg. streptomycin-susceptible and -resistant strains to identify streptomycin-containing extracts). Paired strains (eg. wild type and target overexpressing strains) can also be used to identify antibacterial mechanism of action.