AN INTERLABORATORY STUDY TO FIND AN ALTERNATIVE TO THE MPN TECHNIQUE FOR ENUMERATING ESCHERICHIA-COLI IN SHELLFISH

Nine laboratories in eight countries tested 16 batches of common musse ls (Mytilus edulis) over a 32 week period in order to find an alternat ive to the Most Probable Number (MPN) technique to enumerate E. coli. The alternatives investigated included the 3M Petrifilm system, the Me rck Chromocult agar method and a Malthus conductance technique. The Pe trifilm was found to be unsuitable and was subsequently dropped from t he trial. After 669 analyses, a correlation of 0.83 was observed for l og E. coli counts between the MPN and Chromocult methods and there was no significant evidence that either method tended to give higher read ings than the other. The MPN was slightly better than the Chromocult m ethod for repeatability but the Chromocult was slightly better for rep roducibility. However, the observed differences an probably too small to be of practical importance. On the basis of these data therefore, t he two methods appear equally suitable for E. coli enumeration in shel lfish. There were poor correlations between these methods and the Malt hus technique. A small but significant number of samples tested positi ve on the Malthus instrument but were recorded negative on the MPN and Chromocult tests. Subsequent analysis positively identified E. coli f rom these Malthus assays. After statistical analysis, errors were note d in both the MPN and Chromocult methods but it was found that there w ould be no statistical differences if the Chromocult agar were used as an alternative to the MPN technique. Published by Elsevier Science B. V.