An intercalibration study of the use of 4-Methyumbelliferyl-ß-D-glucuronide for the specific enumeration of Escherichia coli in seawater and marine sediments.

A fluorogenic assay for the specific detection of Escherichia coli on the basis of its β-glucuronidase activity (MUG method) was applied to seawater and marine sediments with different contamination levels. The study was carried out in three Mediterranean areas (Malaga-Spain, Nice-France and Palermo-Sicily), using strictly standardized methods (membrane filtration), media (mFC and Chapman-TTC agars) and reagents, to evaluate statistically its sensitivity and specificity according to the origin and contamination of samples, the workers performing the tests and the selected culture media. The results obtained indicate that the MUG method is highly specific (94.5%) and sensitive (90.8%) for the detection of E.coli in marine samples. Its reliability however closely depends on the macroscopic differentiation of colonies typical of the species on agar plates, and on the number of colonies on the filters. Of the 798 typical colonies with a positive MUG reaction, more than 95.5% were identified as E.coli, and the false-positive results mainly corresponded to Citrobacter freudii (1.75%) and Shigella spp. (1.63%). Only 0.56% of total isolates (14.9% of non-typical MUG negative isolates) were confirmed as E. coli from non- typical colonies on both selective media with a negative MUG reaction (false-negative). It was concluded that the MUG method, in conjunction with the membrane filtration technique using mFC agar is faster, easier and cheaper than the standard procedure for the examination of seawater and sediment samples