VIDAS enzyme-linked immunoflourescent assay for detection of Listeria in foods: collaborative study

The VIDAS LIS method and the traditional culture methods for detection of Listeria species in food were evaluated in a multilaboratory comparative study. The 6 foods tested were either naturally contaminated or inoculated with 3 different concentrations of Listeria. Results for each food and each contamination level with the VIDAS LIS method were as good as or better than those obtained with the traditional culture method. Of 1558 samples tested, 935 were positive: 839 by the VIDAS method and 809 by standard culture methods. Overall false negative rates were 10.3 and 13.5% for the VIDAS LIS and culture methods, respectively. The false positive rate for the VIDAS LIS assay was 1.4% based on 9 VIDAS LIS positive assays that did not confirm positive by isolation of Listeria. The agreement between the VIDAS LIS and culture methods for all samples tested was 86%.

Salmonella in foods--a new enrichment procedure for use with the TECRA Salmonella visual immunoassay: collaborative study

A collaborative study was conducted to compare a new enrichment procedure for the TECRA Salmonella Visual Immunoassay with the reference method given in the U.S. Food and Drug Administration's Bacteriological Analytical Manual (BAM 7th Ed.). Three food types (milk powder, black pepper, and soy flour) were analyzed in Australia, and 3 food types (milk chocolate, dried egg, and raw turkey) were analyzed in the United States. Thirty-eight collaborators participated in the study. No significant differences (p > 0.05) were observed for the pairwise comparison of the proportion of positive samples for the TECRA method with that for the reference method. The new enrichment procedure for the TECRA method has been adopted First Action by AOAC INTERNATIONAL.

Dry rehydratable film method for enumerating confirmed Escherichia coli in poultry, meats, and seafood: collaborative study

A rehydratable dry-film plating method for Escherichia coli, the Petrifilm E. coli/Coliform (EC) Count Plate in foods, has been compared with the AOAC INTERNATIONAL most probable number (MPN) method. Eleven laboratories participated in the collaborative study. Three E. coli levels in 8 samples each of frozen raw ground turkey, frozen raw ground beef, and frozen cooked fish were tested in duplicate. Mean log counts for the Petrifilm plate procedure were not significantly different from those for the MPN procedure for cooked fish samples inoculated with low or high inocula levels, for samples of raw turkey inoculated at medium level, and for beef inoculated at low, medium, and high levels. Repeatability and reproducibility variances of the Petrifilm EC Plate method recorded at 24 h were as good as or better than those of the MPN method. The dry rehydratable film method for enumerating confirmed E. coli in poultry, meats, and seafood has been adopted first action by AOAC INTERNATIONAL.

LOCATE enzyme-linked immunosorbent assay for detection of Salmonella in food: collaborative study

A collaborative study was performed in 27 laboratories to validate the enzyme-linked immunosorbent procedure LOCATE for rapid detection of Salmonella in foods. Results were read visually and with a microtiter plate reader. The LOCATE method was compared with the Bacteriological Analytical Manual (BAM)/AOAC INTERNATIONAL culture method for detecting Salmonella in 6 foods: milk chocolate, nonfat dry milk, dried whole egg, soy flour, ground black pepper, and ground raw turkey. Two foods--dried whole egg and black pepper--required repeat rounds because insufficient data sets were produced initially (AOAC INTERNATIONAL stipulates a minimum of 15 sets per food type). Each laboratory tested one or more of the 6 foods. A total of 1 439 samples were analyzed, and no significant differences (P < 0.05) were observed between LOCATE with either visual or reader detection and BAM/AOAC INTERNATIONAL results. The LOCATE screening method with visual or reader detection is recommended for Official First Action Approval.

Comparison of the SimPlate total plate count method with Petrifilm, Redigel, conventional pour-plate methods for enumerating aerobic microorganisms in foods

The SimPlate Total Plate Count (TPC) method, developed by IDEXX Laboratories, Inc., is designed to determine the most probable number of aerobic microorganisms in foods. The 24-h test was compared to the conventional plate count agar (PCA) method, the Petrifilm Aerobic Count plates, and the Redigel Total Count procedure for enumerating microflora in 751 food samples. Results using the SimPlate TPC method were highly correlated (r > or = 0.96) with results from other test methods. Slopes (0.96-0.97) were not significantly different from 1, and y intercepts (-0.03-0.08) were not different from O. The SimPlate has a high counting range (> 1600 most probable number per single dilution), thus requiring fewer dilutions of samples compared to other methods evaluated. Some foods, e.g., raw liver, wheat flour, and nuts, contain enzymes that gave false-positive reactions on SimPlates. Overall, however, the SimPlate TPC method is a suitable alternative to conventional PCA, Petrifilm, and Redigel methods for estimating populations of mesophilic aerobic microorganisms in a wide range of foods.

VIDAS enzyme-linked fluorescent immunoassay for detection of Salmonella in foods: collaborative study

The VIDAS SLM method for detection of Salmonella was compared with the Bacteriological Analytical Manual (BAM)/AOAC culture method in a collaborative study. Twenty laboratories participated in the evaluation. Each laboratory tested one or more of 6 test products: milk chocolate, nonfat dry milk, dried whole egg, soy flour, ground black pepper, and ground raw turkey. No significant differences (P < 0.05) were observed between the 2 methods. The 2 methods were in agreement for 99% of 1544 samples analyzed. Of the 20 samples out of agreement, 8 were VIDAS SLM positive and BAM/AOAC negative, and 12 were VIDAS SLM negative and BAM/AOAC positive. The VIDAS SLM method for detection of Salmonella in foods has been adopted first action by AOAC INTERNATIONAL.

High-sensitivity dry rehydratable film method for enumeration of coliforms in dairy products: collaborative study

A dry-film coliform count plate that is inoculated with 5 mL sample was compared with the Violet Red Bile Agar plate method in a collaborative study by 18 laboratories. Products analyzed were 2% milk, chocolate milk, cream, vanilla ice cream, cottage cheese, and cheese. Collaborators tested blind duplicate uninoculated samples and samples inoculated at low, medium, and high level. Significantly (P < 0.05) higher numbers of coliforms were recovered by the dry-film method from 2% milk samples at the 3 inoculum levels, the chocolate milk at the low- and high-inoculum levels, and the cream at the high-inoculum level. Significantly higher counts were obtained by the agar method for cottage cheese samples at the low-inoculum level. The repeatability standard deviation for the dry-film method was significantly higher for the high-inoculum level chocolate milk sample and the medium-inoculum level cottage cheese. The same statistic was significantly higher for the agar method at all 3 inoculum levels in the 2% milk and the medium-inoculum level cream. The high-sensitivity dry rehydratable film method for enumeration of coliforms in dairy products has been adopted first action by AOAC INTERNATIONAL.

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.

Enzyme-linked immunoassay for detection of Listeria monocytogenes in dairy products, seafoods, and meats: collaborative study

A collaborative study was conducted to evaluate Listeria-Tek, an enzyme-linked immunosorbent assay (ELISA) for detection of Listeria monocytogenes and other Listeria spp. in foods. The present ELISA method was compared to the U.S. Food and Drug Administration culture method for detection of L. monocytogenes in dairy products and seafoods and to the U.S. Department of Agriculture Food Safety and Inspection Service method for detection of L. monocytogenes in meats. Replicate samples of 6 food types (frankfurters, roast beef, Brie cheese, 2% milk, raw shrimp, and crab meat) inoculated with L. monocytogenes and uninoculated control samples were analyzed by the collaborators. L. monocytogenes was identified in 593 samples by the ELISA method and in 574 samples using culture procedures. Identical results were obtained for 506 positive samples and 419 negative samples using the ELISA and culture methods for an overall agreement rate of 85.6%. The enzyme-linked immunoassay for detection of L. monocytogenes in dairy, seafood, and meat products has been adopted first action by AOAC INTERNATIONAL.

Deoxyribonucleic acid hybridization method for the detection of Listeria in dairy products, seafoods, and meats: collaborative study

The method is based on the hybridization of synthetic deoxyribonucleic acid probes to ribosomal ribonucleic acid sequences unique to Listeria. This method was compared to 2 culture methods: the U.S. Food and Drug Administration method for the detection of Listeria in dairy products and seafoods and the U.S. Department of Agriculture, Food Safety and Inspection Service method for Listeria in meats. Six food types with replicate samples containing various concentrations of Listeria were analyzed by the collaborating laboratories. Listeria was detected in 774 samples using the DNAH method and in 772 samples using a culture method. The DNAH and culture methods were in agreement for 668 samples containing Listeria and 306 samples without Listeria. The overall rate of agreement between methods was 82.3%. The method has been adopted first action by AOAC INTERNATIONAL.

Elevated-temperature, colorimetric, monoclonal, enzyme-linked immunosorbent assay for rapid screening of Salmonella in foods: collaborative study

A collaborative study was performed by 30 laboratories in 3 sets of trials to validate a modified colorimetric monoclonal enzyme-linked immunosorbent assay (ELISA) method for Salmonella detection. The modifications to the current methodology included incubation of enrichments and post-enrichments at an elevated temperature, addition of novobiocin to the M-broth post-enrichment, and elimination of the centrifugation and agitation steps. Five artificially contaminated foods (nonfat dry milk, milk chocolate, dried egg, ground black pepper, and soy flour) and 1 naturally contaminated food (raw ground turkey) were analyzed. The artificially contaminated foods were inoculated with individual Salmonella serotypes at a high (10-50 cells/25 g) and low (1-5 cells/25 g) contamination level. Results from the modified ELISA method were compared to the Bacteriological Analytical Manual (BAM)/AOAC culture method. In 2 of the food products, milk chocolate and pepper, a number of laboratories isolated Salmonella from un-inoculated control samples, thus invalidating their data. As a result, there were too few laboratories remaining with valid data, and these foods were repeated. In the completed study, there were 11 false negative results obtained by the modified ELISA method, while there were 28 false negatives produced by the BAM/AOAC procedure. There were 11 ELISA positive assays which could not be confirmed by culture methods. Statistically, there were no differences between the modified, colorimetric, monoclonal ELISA and the reference culture method in all foods except raw turkey, where the ELISA method was more productive. The colorimetric monoclonal enzyme immunoassay (Salmonella-Tek) method for detecting Salmonella in all foods has been adopted first action by AOAC INTERNATIONAL.

Use of an Elevated Temperature and Novobiocin in a Modified Enzyme-linked Immunosorbant Assay for the Improved Recovery of Salmonella from Foods

A modified colorimetric enzyme-linked immunosorbant assay (ELISA) for Salmonella detection was compared to the standard culture method of the Bacteriological Analytical Manual/Association of Official Analytical Chemists (BAM/AOAC) using 20 artificially contaminated foods (1,200 test samples). The modifications to the current methodology consisted of an elevated incubation temperature of 42°C for the tetrathionate selective broth and M-broth postenrichments, as well as addition of 10 μg/ml novobiocin to the M-broth. The microtiter plate as not agitated during assay incubation, and centrifugation steps were eliminated from the protocol. This modified ELISA method was at least as productive as the standard AOAC culture method for the food samples tested. No false-positive reactions were encountered. The false-negative incidence was 1.5% for the immunoassay and 5.3% by the AOAC cultural method. The incidence of agreement between the methods was 96.7%.

Dry rehydratable film for enumeration of total coliforms and Escherichia coli in foods: collaborative study

Rehydratable dry-film plating methods for total coliforms and Escherichia coli in foods have been compared to the AOAC most probable number methods. Fourteen laboratories participated in the collaborative study. Three coliform and E. coli levels in 6 samples of 4 product types (flour, nuts, cheese, and beef with gravy) and in 3 samples of 2 product types (mushrooms and raw turkey) were tested in duplicate by the participants. The mean log counts for the 3 methods were comparable. In general, the repeatability and reproducibility variances of the plating methods were as good as or better than that of the MPN method. The method has been adopted official first action by AOAC.

Colorimetric and fluorometric substrate immunoassays for detection of Salmonella in all foods: comparative study

Two modified fluorescent enzyme immunoassays for the detection of Salmonella in food have been developed. Both of the new procedures, which substitute a colorimetric substrate for the fluorescent substrate and in which results are read visually or with a photometer, are modifications of AOAC method 989.15. The visually read procedure uses the same antibody-coated wells as in method 989.15. The colorimetric end point of the assay is determined by comparing the solution color to a color chart. The assay result may also be read in a photometer, if the solution is first transferred to a transparent microtiter well. The second procedure designed to be read in a photometer substitutes clear, antibody-coated wells for those used in the fluorescent assay. The colorimetric assays employ identical monoclonal antibodies for capture and detection of Salmonella as used in the fluorescent assay. In this comparative study, the performance of each new assay was consistent with the performance of method 989.15. These methods have been adopted official first action by AOAC as alternative methods for the detection of Salmonella in foods.

Dry rehydratable film for enumeration of total aerobic bacteria in foods: collaborative study

A rehydratable dry-film plating procedure for aerobic plate counts has been compared to the standard agar plate method (966.23B and C, 15th ed.; 46.014-46.015, 14th ed.) in a collaborative study by 12 laboratories. Each laboratory analyzed the normal microflora of 3 samples in duplicate for 6 products. The aerobic plate counts ranged from 1.0 x 10(3) to 1.0 x 10(8) cfu/g. The products were flour, nuts, frozen raw shrimp, spice, frozen raw ground turkey, and frozen and refrigerated vegetables. Repeatability standard deviations of the 2 methods did not differ significantly for 13 of 18 test samples. For 1 shrimp and 2 turkey samples, the dry-film method had lower repeatability variances (P less than 0.05) and for 1 spice sample the agar method had lower repeatability variances (P less than 0.05). Relative standard deviations of repeatability were between 1.7 and 15.5% for the dry-film method and 1.2 and 16.0% for the agar method. Relative standard deviations of reproducibility ranged from 2.4 to 23.4% for the dry-film method and 2.3 to 18.8% for the agar method. The dry rehydratable film method has been adopted official first action for determination of the aerobic plate count.

Colorimetric deoxyribonucleic acid hybridization assay for rapid screening of Salmonella in foods: collaborative study

A collaborative study was performed in 11 laboratories to validate a colorimetric DNA hybridization (DNAH) method for rapid detection of Salmonella in foods. The method was compared to the standard culture method for detection of Salmonella in nonfat dry milk, milk chocolate, soy isolate, dried whole egg, ground black pepper, and raw ground turkey. Samples inoculated with high (0.4-2 cells/g) and low (0.04-0.2 cells/g) levels of Salmonella and uninoculated control samples were included in each food group analyzed. There was no significant difference in the proportion of samples positive by DNAH and culture procedure for any of the 6 foods. The colorimetric DNA hybridization assay screening method has been adopted official first action as a rapid screening method for detection of Salmonella in all foods.

Comparison of colorimetric monoclonal enzyme immunoassay screening methods for detection of Salmonella in foods

A colorimetric enzyme immunoassay (EIA) method for detection of Salmonella in foods has been compared to the AOAC colorimetric monoclonal EIA screening method (986.35, 15th ed.; 46.B21-46.B29, 14th ed.). The assays use the same monoclonal antibodies and have similar reactivity toward Salmonella. However, the new assay uses antibody-coated microtiter wells instead of coated magnetic beads to capture Salmonella antigens. Compared with the bead assay, the coated-well assay format requires significantly less time to complete, and was consistently able to detect lower levels of Salmonella in mixed culture. Compared to the standard AOAC culture method for food samples, the plate assay was as productive. No false negatives were obtained by the immunoassay; the false negative rate was 1.1% by the culture method. The rate of agreement between the 2 methods was 99.1%. The official final action bead assay method for Salmonella in foods, 986.35, and the same assay for use with low-moisture foods, 987.11, have been modified official first action to use antibody-coated microtiter strip-wells.

DNA hybridization assay for detection of Salmonella in foods: collaborative study

A collaborative study was performed in 11 laboratories to validate a DNA hybridization (DNAH) procedure for detection of Salmonella in foods. The DNAH procedure was compared to the standard culture method for detection of Salmonella in 6 foods: ground pepper, soy flour, dry whole egg, milk chocolate, nonfat dry milk, and raw deboned turkey. With the exception of turkey which was naturally contaminated, uninoculated and inoculated samples of each food group were analyzed. Results for the DNAH method were significantly better than for the standard culture method at the 5% probability level for the detection of Salmonella in turkey. There was no significant difference between the methods for the other 5 foods. The method has been adopted official first action.

Constitutive expression of tetracycline resistance mediated by a Tn10-like element in Haemophilus parainfluenzae results from a mutation in the repressor gene

The Tn10-like constitutively expressed tetracycline resistance determinant from a Haemophilus parainfluenzae strain was cloned in Escherichia coli. Toxicity resulting from expression on multicopy plasmids necessitated its being cloned on a low-copy plasmid vector or in cells containing the Tn10-encoded repressor. Constitutive expression of tetracycline resistance was found to result from the synthesis of a truncated inactive repressor molecule. Instead of the 23-kilodalton repressor found in other Tn10-containing strains, this determinant encoded a 14.5-kilodalton molecule. The DNA sequence of the 700-base-pair region spanning the repressor gene and promoter-operator regions of the Haemophilus determinant was identical to that of the same region of Tn10, except for the absence of a single T X A base pair in the repressor gene. This deletion leads to premature termination of the protein. Antisera to the repressor suggested that the repressor was also absent in a second independently isolated H. parainfluenzae strain bearing a Tn10-like constitutive tetracycline resistance determinant.

Intracistronic complementation of the tetracycline resistance membrane protein of Tn10

The structural gene region for tetracycline resistance on Tn10 consists of two complementation groups, tetA and tetB (M. S. Curiale and S. B. Levy, J. Bacteriol. 151:209-215, 1982). Using a series of deletion mutants, we have determined that the tetA region is 450 to 600 base pairs long and that the tetB region, which is adjacent to tetA, is 600 to 750 base pairs long. Point mutations in either tetA or tetB affected the amount and size of the inducible inner-membrane Tet protein synthesized in Escherichia coli maxicells. Moreover, deletions in these regions led to the synthesis of an appropriately smaller Tet protein. A single tetracycline-inducible RNA of about 1,200 bases was detected that was homologous with the tetracycline resistance structural gene region. These results indicate that the tetA and tetB complementation regions represent two parts of a single gene encoding two domains of the tetracycline resistance protein Tet.

Integration and partial excision of a cryptic plasmid in Pseudomonas syringae pv. phaseolicola

A virulent strain of Pseudomonas syringae pv. phaseolicola, a pathogen of the common bean Phaseolus vulgaris (L.), was shown to harbor a 98-megadalton cryptic plasmid, pMC7105. After exposure of this strain to the plasmid-curing agent mitomycin C, a colony was isolated which had no detectable extrachromosomal DNA. Hybridization of labeled pMC7105 probe to nitrocellulose filters containing Southern-blotted BamHI cleavage products of cellular DNA revealed that pMC7105 was integrated into the chromosome rather than cured from this strain. Imprecise excision of pMC7105 resulted in the formation of three smaller plasmids of 34, 50, and 58 megadaltons. BamHI and EcoRI fingerprint analyses revealed that these plasmids were excised from a common region of pMC7105. The BamHI fragments of pMC7105 which were not present in the excision plasmids remained integrated and could be detected by hybridization of pMC7105 probe to Southern-blotted cellular DNA from these strains. Certain chromosomal fragments also had homology with the pMC7105 probe. The excision plasmids were stably maintained and neither integration nor excision altered the pathogenicity of these strains.

Two complementation groups mediate tetracycline resistance determined by Tn10

The structural and regulatory tetracycline resistance genes of transposon Tn10 are located on a 2,700-base pair HpaI fragment. We have used eight tetracycline-sensitive mutations in the 2,700-base pair fragment, cloned into two compatible plasmids, to demonstrate that two complementation groups are required for tetracycline resistance. By genetic recombination with plasmids containing the regulatory or structural regions for resistance, we have determined that both complementation groups reside within the structural region. The complementation groups, designated tetA and tetB, are proximal and distal, respectively, to the promoter for the tetracycline resistance structural region. The tetB mutations are in the portion of the structural region that is known to encode the 36,000-molecular-weight, inner-membrane TET protein. The levels of tetracycline resistance expressed during complementation suggest a complex interaction between the products of the tetA and tetB loci.

Detection and characterization of plasmids in Pseudomonas glycinea

Pathogenic strains of Pseudomonas glycinea were shown to possess plasmid deoxyribonucleic acid by dye-buoyant density gradient centrifugation. The size and number of plasmids of four different isolates were determined by neutral sucrose gradient centrifugation. Two isolates were found to harbor a single plasmid; however, they differed in size, having molecular weights of 43 X 10(6) and 54 X 10(6). Two other isolates each contained two different plasmids. Plasmids with molecular weights of 43 X 10(6) and 73 X 10(6) were observed in one isolate, and the other carried plasmids with molecular weights of 25 X 10(6) and 87 X 10(6). An auxotrophic mutant derived from the latter strain was found to contain plasmids of identical size. The plasmids were found to be under stringent control of replication, having plasmid copies of 1.0 to 2.7 per chromosome equivalent. By the dye-cesium chloride technique, the mutant showed twice as much covalently closed circular deoxyribonucleic acid as did the parental strain.

Ribonucleic acid synthesized in meiotic cells of Saccharomyces cerevisiae: effect of culture medium pH

Pulse-labeled ribonucleic acid (RNA) was extracted from polysomes of sporulating cells of Saccharomyces cerevisiae and characterized in sucrose gradients and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Transfer RNA, ribosomal RNA, and heterodisperse RNA, presumed to be messenger RNA, were synthesized during a 20-min pulse at T4 and T6 when labeling was performed in sporulation medium adjusted to pH 6.0. Furthermore, ribosomal RNA was processed into functional ribosomes during the pulse. The specific activity of pulse-labeled RNA of cells labeled in sporulation medium where the pH was unadjusted at T4 (pH 7.8) and T9 (pH 8.6) was 20- to 50-fold lower than RNA from cells labeled at pH 6.0. The low specific activity resulted from a 50-fold reduction in uptake of labeled precursors when the medium pH was greater than 7.2. However, heterodisperse RNA ranging from 4-17S in size and transfer RNA were synthesized during the pulse at T4 (pH 7.8),but the low specific activity of ribosomal RNA prevented a thorough analysis of its synthesis. Cellular impermeability at T9 (pH 8.6) resulted in minimal uptake of label, and an analysis of pulse-labeled transcripts was impossible. A comparison of the percantage of polysomal material indicate, however, that these cells were at least as active in translation as cells pulse-labeled at pH 6.0.