Rappaport-Vassiliadis Medium for Recovery of Salmonellaspp. from Low Microbial Load Foods: Collaborative Study

Twenty-three laboratories participated in a collaborative study to compare the relative effectiveness of Rappaport-Vassiliadis (RV) medium incubated at 42°C, selenite cystine (SC) broth (35°C), and tetrathionate (TT) broth (35 and 43°C) for recovery of Salmonella from the following foods with a low microbial load: dried egg yolk, dry active yeast, ground black pepper, guar gum, and instant nonfat dry milk. For dry active yeast, lauryl tryptose (LT) broth, incubated at 35°C, was used instead of SC broth. All of the foods were artificially inoculated with single Salmonella serovars, that had been lyophilized before inoculation, at high and low target levels of 0.4 and 0.04 colony forming units/g food, respectively. For analysis of 870 test portions, representing all of the foods except yeast, 249 Salmonella-positive test portions were detected by RV medium, 265 by TT broth (43°C), 268 by TT broth (35°C), and 269 by SC broth (35°C). For analysis of 225 test portions of yeast, 79 Salmonella-positive test portions were detected by RV medium, 79 by TT broth (43°C), 84 by TT broth (35°C), and 68 by LT broth (35°C). RV medium was comparable to, or even more effective than, the other selective enrichments for recovery of Salmonella from all of the foods except guar gum. It is recommended that RV (42°C) and TT (35°C) be used with foods that have a low microbial load, except for guar gum for which SC (35°C) and TT (35°C) are recommended.

Twenty-four-hour direct presumptive enumeration of Listeria monocytogenes in food and environmental samples using the ISO-GRID method with LM-137 agar

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.

Enumeration of beta-glucuronidase-positive Escherichia coli in foods by using the ISO-GRID method with SD-39 agar

A study was undertaken to compare beta-glucuronidase-positive Escherichia coli counts produced by the ISO-GRID hydrophobic grid membrane filter method using SD-39 agar (test method) with those produced by AOAC Official Method 990.11, an existing ISO-GRID method using lactose monensin glucuronate agar and buffered MUG agar (reference method). The methods were evaluated using 21 food products, with three independent lots of five replicate samples analyzed per product by both methods. The test and reference methods were statistically equivalent for 19 of the 21 products; frozen, raw ground lamb produced significantly higher counts using the reference method, whereas counts obtained from cottage cheese were significantly higher using the SD-39 agar-based method.

Direct 24-hour presumptive enumeration of Escherichia coli O157:H7 in foods using hydrophobic grid membrane filter followed by serological confirmation: collaborative study

Fifteen laboratories took part in a collaborative study to validate a method for enumerating Escherichia coli O157:H7. The method is based on use of a hydrophobic grid membrane filter and consists of 24 h presumptive enumeration on SD-39 Agar and serological confirmation to yield a confirmed E. coli O157:H7 count. Six food products were analyzed: pasteurized apple cider, pasteurized 2% milk, cottage cheese, cooked ground pork, raw ground beef, and frozen whole egg. The test method produced significantly higher confirmed count results than did the reference method for milk, pork, and beef. Test method results were numerically higher than but statistically equivalent to reference method results for cheese, cider, and egg. The test method produced lower repeatability and reproducibility values than did the reference method for most food/inoculation level combinations and values very similar to those of the reference method for the remaining combinations. Overall, 94% of presumptive positive isolates from the test method were confirmed serologically as E coli O157:H7, and 98% of these were also biochemically typical of E. coli O157:H7 (completed test). Corresponding rates for the reference method were 69 and 98%, respectively. On the basis of the results of this collaborative study and the precollaborative study that preceded it, it is recommended that this method be adopted official first action for enumeration of E. coli O157:H7 in meats, poultry, dairy foods, infant formula, liquid eggs, mayonnaise, and apple cider.

Two-day hydrophobic grid membrane filter method for yeast and mold enumeration in foods using YM-11 agar: collaborative study

Twenty laboratories participated in a collaborative study to validate a 2-day hydrophobic grid membrane filter method using YM-11 agar for enumeration of yeast and mold in foods. Six naturally contaminated food products were included in the study: garlic powder, raw ground beef, walnuts, flour/meal, orange juice, and yogurt. The test method produced significantly higher results than the 5-day pour plate reference method for orange juice and significantly lower, though numerically similar, results for walnuts and yogurt. Differences between the test and reference methods were not significant for garlic powder, raw ground beef, or flour/meal. Repeatability and reproducibility were similar for both the test and reference methods in all cases. The hydrophobic grid membrane filter method for enumeration of yeast and mold in foods has been adopted by AOAC INTERNATIONAL.

Direct Enumeration of Coliforms and Escherichia coli by Hydrophobic Grid Membrane Filter in 24 Hours using MUG

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%.

Improved hydrophobic grid membrane filter method, using EF-18 agar, for detection of Salmonella in foods: collaborative study

A collaborative study was carried out in 30 laboratories to validate improvements to the official final action hydrophobic grid membrane filter (HGMF) screening method for Salmonella in foods, 985.42, by comparing the performance of the improved HGMF method against that of the AOAC/BAM conventional culture method. Six products were included in the collaborative study: milk chocolate, raw deboned poultry meat, black pepper, soy flour, egg yolk powder, and nonfat dry milk. The raw deboned poultry meat was naturally contaminated with Salmonella, and the remaining 5 products were each inoculated in advance with low levels of individual Salmonella serotypes. The AOAC/BAM method produced 11 false negative results and the improved HGMF method produced 18 false negative results. The improved HGMF Salmonella method has been approved interim official first action for all foods to replace the HGMF official final action method, 985.42.

Hydrophobic grid membrane filter method for aerobic plate count in foods: collaborative study

Twenty-one laboratories participated in a collaborative study to validate a hydrophobic grid membrane filter (HGMF) method for aerobic plate count by comparing its performance against the AOAC/APHA pour plate method. Raw milk, raw poultry, whole egg powder, flours, and spices were included in the study. Counts obtained by the HGMF and pour plate methods did not differ significantly, except in the case of whole egg powder, for which the HGMF method produced significantly higher counts. The hydrophobic grid membrane filter method for aerobic plate count in foods has been adopted official first action.

Enumeration of total coliforms, fecal coliforms, and Escherichia coli in foods by hydrophobic grid membrane filter: supplementary report

Raw, comminuted poultry meat was used to determine the specificity of the media and incubation conditions used in the hydrophobic grid membrane filter method. Confirmation rates for target colonies were 100% for total coliforms, 98% for fecal coliforms, and 97-99% for Escherichia coli. The results of total coliform enumeration in 30 pasteurized milk samples by both the hydrophobic grid membrane filter method and AOAC method 46.013-46.016 are also reported.

Enumeration of total coliforms, fecal coliforms, and Escherichia coli in foods by hydrophobic grid membrane filter: collaborative study

A collaborative study was conducted in 18 laboratories to assess the performance of the hydrophobic grid membrane filter method against that of the AOAC official first action method 46.013-46.016 for enumerating total and fecal coliforms and Escherichia coli. The study was carried out on frozen breaded fish, raw comminuted poultry, unroasted walnut pieces, ground black pepper, and cheddar cheese. The hydrophobic grid membrane filter method recovered significantly larger numbers of target bacteria in 7 of the food/analysis combinations: fecal coliforms in fish; E. coli in poultry; fecal coliforms and E. coli in walnuts; and total coliforms, fecal coliforms and E. coli in black pepper. Random error (Sr2) associated with the hydrophobic grid membrane filter method was significantly lower than that of the reference method in over 30% of the paired sample series. The hydrophobic grid membrane filter method for total coliform, fecal coliform, and E. coli enumeration in foods has been adopted official first action.

Enumeration of coliforms in nonfat dry milk and canned custard by hydrophobic grid membrane filter method: collaborative study

A collaborative study was conducted in 14 laboratories to evaluate the performance of the hydrophobic grid membrane filter method against the official final action method, 46.016, for enumeration of coliforms in foods. Twelve artificially contaminated samples of nonfat dry milk and canned custard were analyzed by each laboratory. The random errors (S2r) associated with the hydrophobic grid membrane filter method were significantly lower than those of the official method, and the counts obtained by the new method fell within the 95% confidence interval of the reference method. The coliform counts obtained by the hydrophobic grid membrane filter method did not differ significantly from those obtained by the reference method as measured by a 3-way analysis of variance. The new method has been adopted official first action.

Effect of Stress and Resuscitation on Recovery of Indicator Bacteria from Foods Using Hydrophobic Grid-Membrane Filtration

The effects of stress and resuscitation on selective enumeration of coliforms, Escherichia coli and enterococci in mechanically deboned raw poultry meat and in dried foods were studied using a hydrophobic grid membrane filter (HGMF) technique. The effects of four different stresses, sublethal heating, freezing, acid pH and drying, were examined on 25 to 30 samples per indicator organism for each type of stress. Counts obtained with resuscitation were compared statistically to direct selective counts for each series of samples. Also, both the direct and resuscitative HGMF results were compared to a 5-tube most probable number method for coliforms and E. coli and to a spread plate method for the enterococci. The use of appropriate resuscitation procedures not only produced a significant increase in counts over the direct HGMF procedure, but also yielded HGMF results that were statistically equivalent to those obtained by conventional methods.

Enumeration of Indicator Organisms in Foods Using the Automated Hydrophobic Grid-Membrane Filter Technique

The automated HGMF technique was compared against accepted traditional methodology for the recovery and enumeration of coliforms, Escherichia coli , enterococci and Staphylococcus aureus from a variety of naturally and artificially contaminated foods. The overall ratios of recovery of the HGMF relative to conventional methods were 0.88 for coliforms, 0.80 for E. coli , 0.81 for enterococci and 0.80 for S. aureus . Our results suggest that the automated HGMF system is a viable alternative to conventional most-probable-number and spread plate techniques for the isolation and enumeration of foodborne microorganisms on selective media; however, consideration must be given to modifying procedures for the optimal recovery of stressed cells by this automated membrane filtration technique.

Determination of Aerobic Plate and Yeast and Mold Counts in Foods Using an Automated Hydrophobic Grid-Membrane Filter Technique

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.

Rapid detection of Salmonella spp. in food by use of the ISO-GRID hydrophobic grid membrane filter

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.