Abstract: We describe a method of determining a food-to-biomass ratio in an aqueous fluid the method comprising: providing an aqueous fluid comprising viable biomass and food for said biomass, and wherein there is insufficient available food to sustain all said viable biomass; using a sensor (for example a respirometer or a sensor for sensing an amount of ammonia, ammonium, nitrates or nitrites) to determine an amount of food in said aqueous fluid available to said biomass; determining a measure of viable biomass in said aqueous fluid by measuring polarisability of viable biomass cells in an AC electric field; and determining a food-to-biomass ratio from said amount of food and said measure of viable biomass.

Abstract: We describe a method of selectively testing a liquid sample for the presence of a specific living biological entity, the method comprising: preparing a plurality of culture vessels each for selective detection of the presence of a different living biological entity, wherein preparing a said culture vessel comprises: providing a culture vessel comprising a sealable chamber for receiving and culturing a liquid sample such that presence of said biological entity is detectable by detecting a change in pressure in a head space of said chamber; providing a selective growth medium in said culture vessel, wherein said selective growth medium is in a dry form and selectively facilitates growth of a specific living biological entity, and providing the culture vessel with an identifier to identity said specific biological entity; providing a test system comprising a device to receive said culture vessel and to detect said change in pressure in said head space of said chamber, the device further comprising a temperature