ERP Micro December 2019

The plate should be read after 24 and 48 hours (in case there are no typical colonies after 24 h). (b) The presence of typical colonies confirms a positive result. (c) If identification of colonies is necessary, use an API ® LIS strip to directly test isolated colonies (without a purification step). In the event of discordant results such as a positive result with the GENE-UP ® test, or no confirmation using a plate (no typical colonies), the laboratory must take the necessary steps to ensure that the results obtained are accurate. It is recommended, for example, to perform the following procedure: (d) Transfer 100 μL from the primary enrichment to 10 mL FB for a secondary enrichment. Incubate at 35 ± 1°C for 24 ± 3 h. (e) Isolate on an ALOA agar plate at 35°C ± 1°C for 24 ± 3 h. The plate should be read after 24 and 48 h (in case there are no typical colonies after 24 h). (f) The presence of typical colonies confirms a positive result. (g) If no typical Listeria colony is identified, repeat steps d and e using 500 µL from the primary enrichment. [External quality control can be performed using one Listeria strain.] (a) Add one isolated colony from a fresh and pure culture in 10 mL of LPT broth. (b) Mix and incubate at 35 ± 1°C or 37 ± 1°C for 18–24 h. (c) Dilute 1/100 of the culture in LPT broth in order to obtain a suspension containing approximately 10 6 cells/mL of the strain. (d) Follow the protocol from the SAMPLE LYSIS section steps to CONFIRMATION OF POSITIVE RESULTS sections. J. Quality Control

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