Comparative recovery rates of MPN and Pour Plate methods for the enumeration of faecal streptococci in shellfish

Faecal streptococci in mollusks were detected according to the MPN methodology (Azide Dextrose broth followed by Ethyl Violet Azide broth) and the Pour Plate (PP) technique on two agarized media (KF agar and m-Enterococcus agar). Samples of the bivalve mollusks Donax trunculus, Ensis siliqua and Chamelea gallina were collected in different marine areas and examined. Results showed that the various media and techniques yielded different faecal streptococci counts. The MPN procedure almost always gave lower titres than those recovered according to the PP technique. Accompanying and competitive bacterial flora, more abundant in the highest volumes examined according to this procedure, may explain these lower counts. m-Enterococcus agar generally gave lower yields than KF agar, but it was also more selective for faecal streptococci than the latter medium. Notwithstanding the partitioning of the examined samples, the findings may suggest that different shellfish species may differentially concentrate and/or retain microorganisms present in the environment.