An Assessment of Methods for the Microbiological Analysis of Shellfish

Abstract

The efficiency of membrane filtration (MF) techniques and most probable number (MPN) tube dilution assays for the bacteriological analysis of homogenized oyster and mussel meat was compared. Tests were done on naturally contaminated shellfish and on homogenates seeded with sewage. Digested homogenates (5% trypsin for 20 min at 37°C) were prefiltered (pore size 5 um) and then filtered (pore size 0.45 µm) for MF counts. Undigested homogenates and dilutions were inoculated into tubes with growth medium for MPN tests. Homogenates (with or without trypsin) were used directly in a toplayer method for the detection of coliphages. Trypsin digestion significantly reduced counts of total coliform bacteria, but not of faecal coliform bacteria, enterococci or coliphages. Prefiltration considerably reduced counts of bacteria, and to a lesser extent, also of coliphages. As a result, MPN tests using minerals-modified-glutamate broth yielded higher counts of coliform bacteria than MF tests using mEndo LES and mTec agar. The latter yielded higher counts of faecal coliforms than mFC agar. In the case of enterococci, the MF procedure yielded lower counts than a spread plate test using undigested homogenates.