palmitate x u2014 c16 is made in a dmso type solvent a volume of this stock solution which makes it possible to obtain a final substrate concentration of 100 mg l is then added into flasks respectively containing tween 20 final concentrations of 0 1 0 2 and 0 6 by volume n octyl u03b2 d glucopyranoside og to final concentrations of 2 g l an og 2 g l and tween 20 mixture 0 1 volume and n octyl u03b2 d thioglucopyranoside otg to a final concentration in the medium of 2 g l the different solutions are agitated and the volumes are introduced into supercooled agar media chromid u00ae salmonella media a summary of the composition of the various media is given in table 5 below microorganisms of the genus salmonella from the applicant 39 s collection were seeded on each of the aforementioned media in accordance with the three dial technique from bacterial suspensions calibrated to 0 5 mcf the strains were chosen for their different level of expression of the esterase activity a strain corresponding to the strain from collection atcc 25922 of e coli serves as negative control for the expression of such an enzyme activity the dishes were incubated for 24 h at 37 u00b0 c then the colonies formed were examined visually after 24 h of incubation the coloration intensities were recorded the results are lodged in table 6 below it is further noted that the media containing og alone or in mixture with tween 20 in.