Analytical procedure for use of conductance measurement to estimate Escherichia coli in shellfish

Assays were performed with a Malthus AT Microbiological Analyzer to define an analytical procedure to estimate Escherichia coli counts in live bivalve shellfish by conductance measurement. The growth conditions used (Malthus Coliform Broth at 44°C) were selective for E. coli, and interference was noted only when Klebsiella pneumoniae were at least 100 times as numerous as E. coli. Different sample preparation procedures and seeding conditions were tested to obtain good quality conductance curves. The best results were observed when: (a) meat and shell liquor were diluted 1 : 3 with tryptone salt water and homogenized in a Waring blender for 1 min at 15 000 rev min-1; and (b) the inoculum was taken from the liquid phase of the homogenate 20 min after blending and mixed immediately with the culture medium. Detection parameter threshold values were adjusted (first difference 1.5 μS for the baseline and 3.5 μS for detection, second difference 0.2 μS to improve detection time reliability. The repeatability of conductance measurements was very good (S.D. as % response mean ranged from 1.9 to 3.3) with the protocol used.