The method necessitates four successive stages. 1. Pre-enrichment in non-selective liquid medium, i. e. inoculation of bufferd peptone water and incubation; 2. enrichment in selective liquid media; i. e. inoculation of a tetrathionate medium and of a selenite cystine medium with the culture obtained and incubation of these media; 3. plating out and recognition, i. e. from the cultures obtained, inoculation of two selective solid media, incubation and checking for the presence of colonies; 4. confirmation. An annex shows a diagramm of procedure. An annex B gives composition and preparation of culture media and reagents.