MS : Received 21 August 2003/Accepted 5 December 2003 ABSTRACT

Transcription

1 87 Journal of Food Protection, Vol. 67, No. 5, 4, Pages Relative Effectiveness of the Bacteriological Analytical Manual Method for the Recovery of Salmonella from Whole Cantaloupes and Cantaloupe Rinses with Selected Preenrichment Media and Rapid Methods THOMAS S. HAMMACK,* IRIS E. VALENTIN-BON, ANDREW P. JACOBSON, AND WALLACE H. ANDREWS Division of Microbiological Studies, Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College Park, Maryland 74, USA MS 3-36: Received August 3/Accepted 5 December 3 ABSTRACT Soak and rinse methods were compared for the recovery of Salmonella from whole cantaloupes. Cantaloupes were surface inoculated with Salmonella cell suspensions and stored for 4 days at to 68C. Cantaloupes were placed in sterile plastic bags with a nonselective preenrichment at a :.5 cantaloupe weight-to- volume ratio. The cantaloupe s were shaken for 5 min at rpm after which 5-ml aliquots (rinse) were removed from the bags. The 5-ml rinses were preenriched in 5-ml portions of the same uninoculated type at 358C for 4 h (rinse method). The remaining cantaloupe s were incubated at 358C for 4 h (soak method). The preenrichment s used were buffered peptone water (BPW), modi ed BPW, lactose (LAC), and Universal Preenrichment (UP). The Bacteriological Analytical Manual Salmonella culture method was compared with the following rapid methods: the TECRA Unique Salmonella method, the ICS/SLM method, and the SLM method. The soak method detected signi cantly more Salmonella-positive cantaloupes (P,.5) than did the rinse method: 367 Salmonella-positive cantaloupes of 54 test cantaloupes by the soak method and 4 Salmonella-positive cantaloupes of 54 test cantaloupes by the rinse method. Overall, BPW, LAC, and UP s were equivalent for the recovery of Salmonella from cantaloupes. Both the ICS/SLM and TECRA Unique Salmonella methods detected signi cantly fewer Salmonella-positive cantaloupes than did the culture method: the ICS/SLM method detected 3 of 5 Salmonella-positive cantaloupes (6 tested) and the TECRA Unique Salmonella method detected 6 of 9 Salmonella-positive cantaloupes (6 tested). The SLM and culture methods were equivalent: both methods detected 37 of 37 Salmonella-positive cantaloupes (6 tested). Cantaloupes have been implicated in Salmonella and Escherichia coli O57:H7 outbreaks (4, 6, 9, 4, ). Produce surveys have also revealed these pathogens on both domestic and imported cantaloupes (3, 5). The susceptibility of cantaloupes to contamination by enteric pathogens is due, in most part, to the fact that they grow directly on the ground. Potential sources of contamination include runoff from livestock areas, contaminated irrigation water, fertilizer (noncomposted or semicomposted manure), and/ or fecal contamination (7). Moreover, cross-contamination of large numbers of cantaloupes can occur during washing and cooling procedures, where contaminated melons can cross-contaminate uncontaminated fruit through contact with contaminated wash water (7). Since potential contamination is not limited to the enteric pathogens Salmonella and E. coli O57:H7, the U.S. Food and Drug Administra- * Author for correspondence. Tel: ; Fax: ; Thomas.Hammack@fda.hhs.gov. The use of the commercial rapid method test kits, reported here, does not constitute an endorsement of any these kits, implied or explicit, by the U.S. Food and Drug Administration (USFDA), nor does the exclusion of other kits necessarily indicate their unsuitability by USFDA. These kits were chosen because they can detect Salmonella in foods in 3 days or less and for their ease of use in this laboratory. tion (FDA) has expanded its testing of cantaloupes to include Shigella spp. (, 4, 5). Salmonella spp., E. coli O57:H7, and Shigella spp. do not have a common cultural method. Thus, foods that are screened for the presence of these organisms cannot be cultured in a common preenrichment-enrichment medium. Moreover, since an analyst cannot know if a pathogen is either homogeneously or heterogeneously distributed on the surface of a cantaloupe, it is necessary to analyze the whole cantaloupe. Preenrichment of whole cantaloupes (.5 to 3 kg each) at a :9 sample-to- ratio is impractical because the necessary volumes of preenrichment s exceed the incubator space found in most laboratories. To resolve this dilemma, the FDA has used a method where melons are rinsed with a nonnutritive dilution buffer. Separate aliquots are withdrawn from the rinse and preenriched or enriched with separate media for each of the three pathogens (4, 5). Moreover, up to 5 of these individual rinses may be composited so that a single cultural enrichment represents 5 individual cantaloupes. With a rinse method, many more cantaloupes can be examined, thereby increasing the likelihood of identifying contaminated melons. Yet the success of a rinse method depends on two critical factors: (i) that the target pathogen can be dislodged from the food surface even if it is rmly established in a bio lm and

2 J. Food Prot., Vol. 67, No. 5 BACTERIOLOGICAL ANALYTICAL MANUAL EFFECTIVENESS 87 (ii) that the pathogen is present in large enough numbers to be detected. Both of these factors limit the success of a rinse method. Thus, the effectiveness of rinse methods has been reported to be poor (3, 8, 6 8). The objective of this study was to determine the relative effectiveness of soak and rinse methods for the recovery of Salmonella from cantaloupes with different preenrichment media: buffered peptone water (BPW), lactose (LAC), modi ed BPW (mbpw), and Universal Preenrichment (UP). The preenrichment media were chosen for the following reasons: (i) BPW is recommended by the International Organization for Standardization for use in its Salmonella method (5), (ii) LAC and UP s are recommended for use with select foods by the FDA in its Bacteriological Analytical Manual (BAM) Salmonella method (6), and (iii) mbpw was used because it is part of the TECRA Unique Salmonella test kit system (5). A secondary objective of this study was to determine which, if any, of three selected rapid methods could be used reliably to detect Salmonella on surface-contaminated cantaloupes. Rapid methods are important because they save time and are particularly useful for the analysis of highly perishable foods. Rapid methods can be used to identify contaminated foods, while screening out noncontaminated foods, in 3 days or less compared with 5 days with the traditional Salmonella culture method (5, 6). The three rapid methods evaluated in this study were the ICS/ SLM, SLM, and TECRA Salmonella Unique methods (5). These rapid methods were chosen because they have been validated through collaborative study by the Association of Of cial and Analytical Chemists International (AOAC) and can provide presumptive-positive results in 3 days or less (5). MATERIALS AND METHODS Source of cantaloupes. Western cantaloupes ( count per case; kg per melon) were purchased from a produce wholesaler located in the Washington, D.C., metropolitan area. Source of inocula. Salmonella (isolated from dry noodles), Salmonella (isolated from alfalfa seeds), and Salmonella Oranienburg (isolated from cantaloupes) were obtained from the stock culture collection of the Division of Microbiological Studies, Center for Food Safety and Applied Nutrition, FDA. All Salmonella isolates originated from regulatory food samples. Preparation of inocula. Brain heart infusion agar slants were inoculated with Salmonella organisms and incubated for 8 to 4 h at 358C. The brain heart infusion was inoculated from these slants and incubated for 8 to 4 h at 358C. Tenmilliliter aliquots from the were centrifuged at 3,9 3 g, and the pellets were suspended in ml of Butter eld s phosphate buffer (ph 6.8 to 7.). The cells were washed twice with -ml aliquots of Butter eld s phosphate buffer. FIGURE. BAM Salmonella culture method. BPW, buffered peptone water; LAC, lactose ; UP, Universal Preenrichment ; TT, tetrathionate ; RV, Rappaport-Vassiliadis medium; BS, bismuth sul te agar; HE, Hektoen enteric agar; XLD, xylose lysine desoxycholate agar. Surface inoculation of cantaloupes. Individual cantaloupes were spot inoculated on unblemished rind, at a point midway between the stem scar and the anterior end, with -ml Salmonella suspensions (single serovar) at levels that would provide fractionally positive results ( 3 to 4 CFU per cantaloupe). Fractionally positive results are results where some number of the test portions is positive and some number of the test portions is negative, preferably 5% of the total tests are positive per experiment for at least one of the s. Cantaloupes were dried at room temperature for 3 to 4 h in open plastic laboratory totes. The cantaloupes were held overnight at to 68C in the totes, loosely covered with aluminum foil, and then stored for an additional 3 days in open sterile plastic bags (International Bio-Products, Bothell, Wash.) at to 68C. During the 3-day storage period, the cantaloupes rested in open, loosely tting bags that were in an upright position inside open -gal buckets. Cantaloupes were not prewarmed before Salmonella analysis. Preenrichment media was added to the bagged cantaloupes at a :.5 ratio, melon weight-to- volume, with LAC, BPW (mbpw for the TECRA kit), or UP. Cantaloupes were then shaken on a rotary shaker for 5 min at rpm. Immediately after shaking, 5-ml portions were withdrawn from the sample mixtures and added to 5-ml portions of the same (i.e., 5 ml of LAC was added to 5 ml of LAC). Five uninoculated cantaloupes were also shaken and preenriched, as described above, to demonstrate the absence of Salmonella on the uninoculated melons. Cantaloupe mixtures and rinses were incubated for 4 6 h at C (Fig. ).

3 87 HAMMACK ET AL. J. Food Prot., Vol. 67, No. 5 FIGURE. ICS/SLM assay. LAC, lactose ; TT, tetrathionate ; RV, Rappaport-Vassiliadis medium; BS, bismuth sul te agar; HE, Hektoen enteric agar; XLD, xylose lysine desoxycholate agar. Salmonella was isolated from the preenrichments as described below. ICS/SLM. ICS/SLM analyses (biomérieux Vitek, Inc., Hazelwood, Mo.) were performed on the LAC preenrichments, because LAC is the current BAM recommended preenrichment for fresh fruit and produce. The assay was not performed simultaneously on all preenrichment -soak-rinse combinations, because the capacity of the Vitek mini-vidas machine was limited. Cantaloupes were preenriched as described above. After incubation for 4 h at 358C,.8-ml aliquots from each LAC preenrichment were pipetted into the ICS reagent strip. ICS/ SLM analyses were performed as directed by the Of cial Methods of Analysis (OMA) method.9 (5) (Fig. ). In addition to the ICS/SLM analysis, cultural analysis was applied to the ICS/SLM preenrichments. From each LAC preenrichment, -ml and.-ml aliquots were subcultured to -ml portions of tetrathionate (TT) and Rappaport- FIGURE 3. SLM assay. BPW, buffered peptone water; SC, selenite cystine ; TT, tetrathionate ; RV, Rappaport- Vassiliadis medium; BS, bismuth sul te agar; HE, Hektoen enteric agar; XLD, xylose lysine desoxycholate agar. The selective enrichments (SC, TT, and RV) were all incubated for 4 h at their respective temperatures before plating. Vassiliadis (RV) medium, respectively. TT and RV medium were incubated for 4 h at 35 and 48C, respectively. Incubated ICS s were also streaked to xylose lysine desoxycholate (XLD), Hektoen enteric (HE), and bismuth sul te (BS) agar plates. The selective enrichments and selective agar plates were cultured for Salmonella as described below. SLM. analyses (biomérieux Vitek) were performed in BPW, because BPW is recommended by the kit manufacturer, and initial results, with the ICS/SLM assay, showed that the three preenrichment s (BPW, LAC, UP) were equivalent for use in the culture method. Cantaloupes were preenriched as described above. After incubation for 4 h at 358C, -ml aliquots were subcultured to - ml portions of selenite cystine (SC) and TT s (Fig. 3). Aliquots (. ml) were also subcultured to RV medium. The SC was incubated for 6 h at C. TT and RV medium were incubated for 6 h at 4 6.8C. After 6 h, -ml aliquots from each of the selective enrichments (SC, TT, and RV), were subcultured to -ml portions of M. The M s were incubated for 8 h at 4 6.8C.

4 J. Food Prot., Vol. 67, No. 5 BACTERIOLOGICAL ANALYTICAL MANUAL EFFECTIVENESS 873 Isolation of Salmonella from test portions. All cantaloupe preenrichments that were not used with a test kit system were selectively enriched as described below. From each preenrichment, -ml and.-ml aliquots were subcultured to -ml portions of TT and RV medium, respectively. TT and RV medium were incubated for 4 h at 35 and 48C, respectively. All selective enrichments, including those used with the test kit systems, were streaked to XLD, HE, and BS agar plates. The selective agar plates were incubated for 4 h at 358C. After examination, BS agar plates were incubated an additional 4 h (total incubation of 48 6 h). Presumptive-positive colonies were picked to triple sugar iron and lysine iron agar slants, which were incubated for 4 h at 358C. Presumptive-positive triple sugar iron and lysine iron agar slants were con rmed with the appropriate somatic antisera (Becton Dickinson Microbiology Systems, Sparks, Md.). Forty-eighthour BS agar plates were picked only if 4-h plates (BS, HE, and XLD agars) gave atypical results and there were typical presumptive-positive colonies present on the plates. Media and reagents. All media and reagents were prepared as directed in the BAM or in AOAC s OMA (5, 6). Statistical analysis. Data were analyzed with McNemar s chi-square test. Differences in recovery of Salmonella were signi cant at P,.5 (). FIGURE 4. TECRA Unique for Salmonella assay. mbpw, modi- ed buffered peptone water; TT, tetrathionate ; RV, Rappaport-Vassiliadismedium; BS, bismuth sul te agar; HE, Hektoen enteric agar; XLD, xylose lysine desoxycholate agar. The selective enrichments were reincubated for an additional 8 h (total of 4 6 h) at their respective incubation temperatures. In addition to the three selective enrichments, a - ml aliquot was subcultured from each preenrichment to an additional -ml portion of TT. The TT was incubated for 4 h at 358C. These TT (35) tubes were not subcultured to M. The incubated M s were subjected to SLM analysis as directed by the OMA method (5). The selective enrichments were cultured for Salmonella as described below. TECRA. Test portions, examined with the TECRA system (International BioProducts), were preenriched in mbpw for 6 to h at 358C. TECRA analysis was conducted as directed by OMA method.7 (5) (Fig. 4). In addition to the TECRA analysis, cultural analysis was applied to the TECRA preenrichments. From each mbpw preenrichment, -ml and.-ml aliquots were subcultured to -ml portions of TT and RV medium, respectively. The contents of tube 3 of the TECRA test kit, where the antibody-coated dipstick is enriched to increase Salmonella organisms to detectable levels, were streaked to XLD, HE, and BS agar plates. The selective enrichments and selective agar plates were cultured for Salmonella as described below. RESULTS AND DISCUSSION Results in Table show that the soak method was signi cantly (P,.5) more ef cient than the rinse method for the recovery of Salmonella from cantaloupes. For all of the preenrichment s, in all experiments except one, the soak method detected signi cantly more (P,.5) Salmonella-positive cantaloupes than did the rinse method. Results from Table also show that there were no overall differences among the three preenrichment s (BPW, LAC, UP) for the recovery of Salmonella. Signi cant differences could be seen in individual experiments, but the overall average recovery for each was 88 6 Salmonella-positive cantaloupes of cantaloupes tested. When mbpw, LAC, and UP s were compared during the TECRA Unique Salmonella test kit evaluation (mbpw was used with the TECRA Unique Salmonella test kit), there were no signi cant differences among the s in any of the individual experiments. The LAC was signi cantly more productive (P,.5) overall than either the mbpw or UP for the three TECRA Unique Salmonella experiments (Table ). Results in Table show that the ICS/SLM method was ineffective for the recovery of Salmonella from surface-contaminated cantaloupes. In two of three experiments, the ICS/SLM method detected signi cantly fewer Salmonella-positive cantaloupes than did the LAC based culture method. Moreover, overall results show that the ICS was signi cantly less effective than its corresponding preenrichment for the recovery of Salmonella-positive cantaloupes. In all three experiments, the LAC was comparable to both the BPW and UP s for the detection of Salmonella on surface-contaminated cantaloupes (Table ). There were no readily apparent reasons for these observed differences. The SLM assay performed as well as the cul-

5 874 HAMMACK ET AL. J. Food Prot., Vol. 67, No. 5 TABLE. Relative effectiveness of the rinse and soak methods for the recovery of s from surface-contaminated cantaloupes with selected preenrichment media Salmonella-positive test portions per test portions a (CFU/ ml) Soak Lactose Rinse Buffered peptone water Soak Rinse Universal Soak Rinse A b B AB 7 A B c 8 AB Oranienburg Subtotal of Salmonella-positive test portions Oranienburg Subtotal of Salmonella-positive test portions 4 A 9 B d d 3 B a Test portion for soak method equals cantaloupe. Test portion for rinse method equals 5 ml of preenrichment. b Values within a row, for each method (soak or rinse), not having a common letter are signi cantly different (P,.5). c There is no signi cant difference (P..5) between the soak and rinse methods in this instance. In all other instances, the soak method was signi cantly more producive than the rinse method. d Modi ed buffered peptone water was used in conjunction with the TECRA Unique test kit. 5 d d 3 d d d d 6 4 AB B A tural methods with which it was compared (Table 3). Moreover, SLM results, based on the TT-RV enrichment combination, compared favorably to the SC-TT enrichment combination (current method). Both TT and RV medium were superior to SC for the recovery of Salmonella from cantaloupes (Table 4). SC, because of its toxicity, is no longer recommended for the detection of Salmonella in foods, except guar gum, by FDA s BAM (5). These data show that use of RV medium with cantaloupes is indicated. The TECRA Unique Salmonella test kit was not as effective as the culture method. Overall, the TECRA assay detected signi cantly fewer Salmonella-positive cantaloupes than did the cultural method based on the TECRA preenrichment mbpw (Table 5). The TECRA kit is a selfcontained system that does not require the use of an electronic reader. The assay results depend on antibody-coated dipsticks that change color when exposed to Salmonella. We found these dipsticks dif cult to read because even slight color changes indicated the presence of a Salmonellapositive sample. Nonetheless, the assay results closely tracked the Salmonella-positive immunoconcentrateds in tube 3 of the test kit strip (Table 5). Tube 3 is the well in the strip that is used to con rm the kit results. In all cases, for either positive or negative results, from tube 3 was streaked to selective plating agars so that it was possible to determine instances of false-positive and false-negative kit results. The overall false-negative rate for tube 3 TABLE. Relative effectiveness of the ICS/SLM method and the culture method for the recovery of s from surface-contaminated cantaloupes Salmonella-positive test portions per test portions a Soak method Rinse method (CFU/ ml) Culture (lactose ) ICS b SLM Culture ICS (lactose ) SLM Oranienburg A c 5 A 9 Subtotal of Salmonella-positive test portions 5 A 8 B 3 B a Test portion for soak method equals cantaloupe. Test portion for rinse method equals 5 ml of preenrichment. b ICS is the postenrichment used by the system after the sample has been immunoconcentrated. Nonselective preenrichment (lactose ) was incubated for 8 to 4 h at 358C, the ICS procedure was performed, ICS was incubated for 5 to 6 h at 48C, and the SLM test was performed. After withdrawing an 8-ml aliquot from the lactose, for use in the ICS/SLM assay, the lactose was reincubated until it had been incubated for a total of 4 6 h at 358C. The BAM method was followed thereafter. c Values within a row, for each method (soak or rinse), not having a common letter are signi cantly different (P,.5). 9 A B 8 3 B B

6 J. Food Prot., Vol. 67, No. 5 BACTERIOLOGICAL ANALYTICAL MANUAL EFFECTIVENESS 875 TABLE 3. Relative effectiveness of the SLM (with paired selective enrichments) and Salmonella culture method for the recovery of Salmonella from cantaloupes and cantaloupe rinses Salmonella-positive test portions per test portions a Soak method Rinse method (CFU/ ml) Culture LAC (SC/TT4) b (TT4/RV) c (TT35/RV) d,e UP (SC/TT4) (TT4/RV) Culture (TT35/RV) LAC UP Oranienburg B f 8 AB 9 B 8 AB 9 B 8 AB 9 Subtotal of Salmonella-positive test portions A B 8 4 AB A 8 5 a Test portion for soak method equals cantaloupe. Test portion for rinse method equals 5 ml of preenrichment. b SC, selenite cystine incubated at 358C; TT4, tetrathionate incubated at 48C. c RV, Rappaport-Vassiliadis medium incubated at 48C. d Buffered peptone water was the preenrichment used for the SLM method. e TT35, tetrathionate incubated at 358C. f Values within a row, for each method (soak or rinse), not having a common letter are signi cantly different (P,.5). TABLE 4. Relative effectiveness of the SLM (with single selective enrichments) and Salmonella culture method for the recovery of Salmonella from cantaloupes and cantaloupe rinses Salmonella-positive test portions per test portions a Soak method Rinse method (CFU/ ml) Culture (SC) b (TT4) c (RV) d (TT35/RV) e,f (SC) (TT4) (RV) Culture (TT35/RV) Oranienburg B g 4 8 A 7 9 Subtotal of Salmonella-positive test portions 4 A 36 B 35 B 37 B 3 3 A 7 8 A 8 9 a Test portion for soak method equals cantaloupe. Test portion for rinse method equals 5 ml of preenrichment. b SC, selenite cystine incubated at 358C. c TT4, tetrathionate incubated at 48C. d RV, Rappaport-Vassiliadis medium incubated at 48C. e Buffered peptone water. f TT35, tetrathionate incubated at 358C. g Values within a row, for each method (soak or rinse), not having a common letter are signi cantly different (P,.5).

7 876 HAMMACK ET AL. J. Food Prot., Vol. 67, No. 5 TABLE 5. Relative effectiveness of the TECRA Salmonella test kit and Salmonella culture method for the recovery of Salmonella from cantaloupes and cantaloupe rinses Salmonella-positive test portions for test portions a Soak method Rinse method (CFU/ ml) TECRA assay TECRA b mbpw c TECRA kit TECRA mbpw Oranienberg B d 8 Subtotal of Salmonella-positive test portions 6 B 9 B 9 A a Test portion for soak method equals cantaloupe. Test portion for rinse method equals 5 ml of preenrichment. b Modi ed buffered peptone water preenrichment for the TECRA kit. c Tube 3 of the test strip. TECRA recommends streaking from tube 3 for the con rmation of presumptive-positive test results. d Values within a row, for each method (soak or rinse), not having a common letter are signi cantly different (P,.5). B 8 5 A 3 was 34% compared with their mbpw preenrichments of origination. It is possible that the TECRA kit would have performed better if the TECRA procedure for salads had been performed rather than the TECRA procedure for most foods. In that case, the cantaloupe preenrichments would have been incubated at 4 6 8C with the TECRA Salmonella supplement included, instead of being incubated at C without the supplement. Even if the reduced incubation temperature and the lack of the supplement explain the differences in recovery of Salmonella among the three preenrichment s, they do not explain the differences in recovery of Salmonella between the mbpw preenrichments and tube 3. It may be that the antibodies used in the TECRA test were insuf ciently selective to capture the s at the levels found in the preenrichments. The low recovery of Salmonella with the mbpw preenrichments may also have been caused by the reduced preenrichment period (8 6 h for TECRA and 4 6 h for the culture methods). Reduced preenrichment periods have been shown to be less productive than a standard 4- h preenrichment time (). The inoculation levels, necessary for the cantaloupes used in these experiments, were relatively high ( 3 to 4 CFU per cantaloupe). The low recovery, at these inoculation levels, suggests that either the die-off of the organism on the surface of the cantaloupe was rapid or the method was relatively insensitive and needs to be improved. Still, even if the latter case is true, this research clearly demonstrates that the soak method is superior to the rinse method currently in use. The lack of ef cient recovery by the rinse method may have been due to the formation of bio lms adhering to the surface of the melons (8, 3). Work with wooden cutting boards has shown that up to 75% of bacteria were bound to a wooden surface after min (). The same may also be true with cantaloupes. The surface of a cantaloupe is rough on both macroscopic and microscopic scales. One would expect the pathogen to penetrate such a surface to produce pockets of contamination that would not be readily dislodged through shaking. When soaked in a nonselective preenrichment medium, such organisms would be expected to grow out of their con nement and into the medium. Other foods where rinse methods have been shown to be relatively ineffective compared with soak methods include chicken, frog legs, and sh (3, 6 8). In summary, the soak method is superior to the rinse method for the detection of Salmonella on cantaloupes. Neither the ICS/SLM nor the TECRA Unique for Salmonella test kits proved appropriate for use with whole cantaloupes. The SLM kit is equivalent to the Salmonella culture method for the detection of Salmonella on cantaloupes. Future research should focus on enhancing the sensitivity of the method and the validation of a method to composite the selective enrichments so that the amount of incubator space necessary for these analyses can be reduced. Price et al. (9) demonstrated that small portions of incubated preenrichments could be pooled without reducing the sensitivity of the culture method. Although this procedure would not reduce the incubator space necessary to preenrich cantaloupes, it would drastically reduce the subsequent numbers of plating and tube media necessary to isolate and con rm the presence of Salmonella on cantaloupes. Finally, this research demonstrates the inadvisability of validating methods for all foods. The AOAC validates methods through collaborative study and publishes the validated studies in the OMA (5). Microbiological validations include an in-house validation or precollaborative study and a multilaboratory collaborative study. In the precollaborative study, the effectiveness of the method is evaluated for use with foods. In the collaborative study, the robustness of the method is evaluated simultaneously with six foods in multiple laboratories. Although a claim can be made that, through this process, the method has been validated for the 6 foods tested, no such claim can be made for foods not tested. None of the three test kits used in this study have heretofore been evaluated for use with cantaloupes or cantaloupe rinses. Results show that only one of the test kits is appropriate for use with this food. These results show that food testing laboratories should not accept an all foods claim at face value, since no method, however good, can be validated for all foods. A method should always be tested when examining foods for which the method has not been validated.

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