Chromogenic plate assay distinguishing bacteriolytic from bacteriostatic activity of an antibiotic agent

A solid agar plate assay was devised to discriminate bacteriolytic from bacteriostatic activity for a given antibacterial agent. The assay uses a bacterial culture harboring β-galactosidase enzyme as reporter of cellular lysis. When a drop of bacteriolytic compound is placed on the agar, β-galactosidase is released from the bacteria to the external solid medium where it hydrolyzes X-Gal substrate analogue, developing a blue halo at the edge of the inhibition growth zone. The assay was successfully evaluated against several antibiotics with well-known mechanism of action. It was found that bacteriostatic compounds consistently did not display blue halo at the inhibition zone.