A method and a sealable, sterilizable vessel for detecting the presence of microorganisms in a specimen, the vessel containing a liquid culture medium and a sensor with an indicator medium therein. A gas permeable membrane with a gas impermeable removable seal thereon are provided in a wall of the vessel. If the microorganism to be detected is an anaerobic organism, then the gas impermeable seal is left in place. If the microorganism to be detected is an aerobic organism, then the gas impermeable seal is removed so as to allow the passage of oxygen into the vessel. In a preferred embodiment, the gas permeable membrane allows the passage of oxygen into the vessel while at least partly restricting the passage of carbon dioxide to outside of the vessel. Changes in the indicator medium resulting from a change in gas production (CO.sub.2, NH.sub.2, H.sub.2 S, etc.), volatile acid production or pH concentration in the medium, are detected from outside the vessel.