Abstract: A bacterial activity sensing electrode or probe comprises an insulating substrate carrying a conductive pattern as two conductors as a mask-produced thick film. The conductors terminate in bare sensor parts and also in bare connection parts and are otherwise covered by an insulating coating also mask-produced. The substrate has a molded-on cap of suitably inert plastics material to fit directly to a sample bottle with the sensor parts inside and the connector parts offset to one side for making a push-bit electrical connection thereto. Inoculation of a growth medium is via a through-passage in the cap and ruptuable means associated therewith.