An improved 24-hour hydrophobic grid membrane filter HGMF method for coliform and Escherichia coli enumeration was developed. The new method, which uses a buffered MUG agar for the E. coli portion of the test, was subjected to a precollaborative validation study against the 3-tube MPN procedure encompassing 375 naturally contaminated and inoculated samples representing 25 food products. The HGMF/MUG method produced coliform and E. coli counts equivalent to the conventional method. The confirmation rate of MUG-positive colonies in this study was 98.1%.
The Hydrophobic Grid-Membrane Filter (HGMF) technique, using an automated counting system, (ISO-GRID™ Sample Processor) was compared against the conventional pour plate technique for aerobic plate counts, and against a spread plate technique for enumerating yeasts and molds in foods. A total of 179 samples, involving five different food types, were compared for aerobic plate counts and 177 samples, representing four different food types, were compared for yeast and mold counts. In all cases, the HGMF counts determined by the Sample Processor were shown to be equivalent to, or greater than, counts obtained using conventional methods.
Effects of prefiltration and enzyme and surfactant treatments on the filterability of foods were examined. The clarification of food suspensions, using a fine wire cloth prefilter, did not affect microbial recovery from the 87 food samples examined. One hundred and nine foods were clarified by prefiltration and then tested for their filterability through a 0.45-μ Hydrophobic Grid Membrane Filter; 68 could be filtered without any additional treatment. Of the remaining 41 foods. 39 were rendered filterable with an appropriate enzyme or surfactant treatment. The application of these procedures greatly enhances the practicality of membrane filtration for microbiological analysis of foods.
A rapid hydrophobic grid-membrane filter (HGMF) method was developed and compared with the Health Protection Branch cultural method for the detection of Salmonella spp. in 798 spiked samples and 265 naturally contaminated samples of food. With the HGMF method, Salmonella spp. were isolated from 618 of the spiked samples and 190 of the naturally contaminated samples. The conventional method recovered Salmonella spp. from 622 spiked samples and 204 unspiked samples. The isolation rates from Salmonella-positive samples for the two methods were not significantly different (94.6% overall for the HGMF method and 96.7% for the conventional approach), but the HGMF results were available in only 2 to 3 days after sample receipt compared with 3 to 4 days by the conventional method.
A new culture medium, LM-137 agar, was developed for use with the ISO-GRID hydrophobic grid membrane filter system for direct presumptive enumeration of Listeria monocytogenes in 24 h. The method was validated against three-replicate, three-dilution most probable number procedures based on enrichment methods specified by the U.S. Department of Agriculture, the Association of Official Analytical Chemists International and the U.S. Food and Drug Administration. The study encompassed meats, dairy products, egg, produce, seafood, and environmental samples. The ISO-GRID filter method produced significantly higher recovery of L. monocytogenes from fermented sausage, hot dogs, pasteurized and raw milk, raw shrimp, and environmental swab samples (P < 0.05). The reference methods yielded significantly higher counts from frozen raw pork and cole slaw (P < 0.05). Confirmation rates of presumptive positive isolates from the filter method ranged from a low of 92% (frozen raw pork) to 100% (most other products). Neither the recovery efficiency nor the confirmation rate were affected by the presence of competing aerobic flora.
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