Abstract: A centrifuge tube and a kit for detecting the presence or absence of microorganisms, particularly bacteria and yeasts, in a liquid sample of a biological material. The centrifuge tube has a cylinder portion; a frustoconical portion; a capillary at the bottom of the tube; and contains a gel system for detecting microorganisms. The gel system includes a development phase of a gel containing microorganism culture medium and a reagent capable of inducing a detectable variation in optical measurement in the presence of microorganisms and a first phase intimate mixture which includes water originating wholly or partly from the culture medium and water-absorbing polymer particles so that, in the first phase intimate mixture, the polymer particles have a particular concentration and size.

Abstract: A method for detecting the presence or absence of microorganisms belonging to the group which consists of bacteria and yeasts in a liquid sample (5) of a biological material. The method comprises placing the liquid sample (5) in a centrifuge tube (6a, 6b) above a gelled system (10) comprising at least (a) a first so-called development phase (1), i.e. a gel comprising a microorganism culture medium and a reagent for inducing a detectable optical measurement change in the presence of microorganisms, said gel being an intimate mixture of water and water-absorbing polymeric particles that have swelled in such a way that, in said intimate mixture, said polymeric particles have (.alpha.) a dry weight concentration of 0.05-0.2 g/ml, and (.beta.) a swollen-state diameter of 90-320 .mu.