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enriched with 225 mL of 3M
Salmonella
Enrichment Base. All matrices were homogenized for 2 minutes
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and incubated at 41.5° ± 1°C for 18-24 hours.
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Prior to analysis, the 3M Petrifilm SALX Plates were hydrated using 2.0 mL of sterile distilled water. After
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hydration, the liquid was spread across of the surface of the plate using a plastic spreader to evenly
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distribute the diluent. The plates were left undisturbed and protected from light for a minimum of 1 hour
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prior to use. For foods with low microbial loads (pasteurized liquid whole egg, dry dog food, cooked
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chicken nuggets and stainless steel environmental surfaces), test portions were streaked in duplicate after
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18 and 24 hours of primary enrichment. For foods that contained a high microbial load (raw ground
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chicken, frozen uncooked shrimp, fresh spinach, raw ground beef and raw ground pork), a 0.1 mL aliquot of
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the primary enrichment was transferred into 9.9 mL of Rappaport-Vassiliadis R10 (RV [R10]) broth at both
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18 and 24 hours of primary enrichment and incubated at 41.5° ± 1°C for 8 and 24 hours. At 8 and 24 hours,
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a loopful of each test portion was streaked in duplicate to 3M Petrifilm SALX Plates and the plates were
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incubated at 41.5° ± 1°C for 24 ± 2 hours. Test portions from both low and high microbial foods were
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confirmed following two procedures, traditional confirmation using secondary selective enrichments and
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an alternative confirmation directly from the 3M Petrifilm SALX Plates.
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Confirmation
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Reference Methods
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Aliquots from the primary enrichment of each test portion were also transferred to the secondary
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selective enrichments specified by each reference method, TT and RV for samples confirmed following
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the FDA/BAM method and TTH and RVS for samples confirmed following the USDA/FSIS-MLG method.
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After incubation, test portions from the secondary selective enrichments were streaked to selective
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agars specified by each reference method and positive samples were confirmed following traditional
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biochemical tests for
Salmonella
species by each reference method including somatic O and flagellar H
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tests. Final confirmation at each time point was achieved by VITEK 2 GN conducted per AOAC Official
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Method 2011.17.
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3M Petrifilm Salmonella Express System
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After incubation of the 3M Petrifilm SALX Plates, the plates were observed for growth of presumptive
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Salmonella
colonies, red to brown colonies with discrete yellow zones and/or gas bubbles. Using a fine
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tip marker, presumptive positive colonies were circled on the plate top film. The film was lifted and a
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3M Petrifilm
Salmonella
Express Confirmation Disk was placed onto the gel. The film was closed and
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using a gloved hand (while practicing good laboratory technique) air bubbles were removed by gently
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applying a sweeping motion with even pressure onto the top of the film with the analyst’s fingers. The
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plates with confirmation disks were incubated at 41.5° ± 1°C for 4 to 5 hours. After incubation, the
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circled presumptive colonies were observed. A color change from red/brown to green blue, blue, dark
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blue or black confirmed the colony as
Salmonella
species. If the circled colony remained red or brown
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it was marked as negative. Typical colonies were picked to TSI and LIA and samples were confirmed
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following traditional biochemical tests for
Salmonella
species by each reference method including
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somatic O and flagellar H tests. Final confirmation was achieved by VITEK 2 GN conducted per AOAC
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Official Method 2011.17.
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