Abstract: A method of monitoring the growth of microorganisms in liquid culture involving the detection of pressure change in a gas-tight container such as a blood culture bottle incorporating a flexible diaphragm (septum) by detecting displacement of the septum, is characterised by the position or conformation of the diaphragm being repeatedly sensed using a distance-measuring means, such as a laser, preferably capable of detecting a variation in the position or conformation of the diaphragm to an accuracy of 200.mu. or better, with a portion of the container adjacent the diaphragm conveniently being used as a reference to detect any relative positional or conformational change of the diaphragm. Preferably the laser repeatedly scans a plurality of containers mounted in a holder such as a flat bed or a carousel.