Evaluating Alternative Methods for the Detection of Listeria monocytogenes from Medical Nutrition Samples

Introduction: Nutricia manufacture a broad range of medical nutrition products, including matrices inhibitory to bacterial growth, which are routinely tested for the presence of Listeria monocytogenes using an alternative culture method. A method with a shorter time to result for detection of this pathogen was desirable as the ISO method provides a time to negative result in 5 days. The Thermo Scientific™ SureTect™ Listeria monocytogenes PCR Assay is validated for the detection of L. monocytogenesfrom all food types and production environment samples via AOAC-RI PTM and NF Validation by AFNOR Certification, giving a time to negative result in 24-28 hours.

Purpose: To assess the suitability of the PCR assay for use with a range of medical nutrition products while maintaining a short time to result.

Methods: Nine medical nutrition samples were inoculated with 4.3 to 5.0 CFU L. monocytogenes then enriched and tested using the PCR assay and results compared against the ISO 11290-1:1996 method. Samples found to be inhibitory to growth of L. monocytogeneswere tested with two alternative methods: a modified PCR method that included a non-selective regrowth following primary enrichment; and a rapid culture method.

Results:  The PCR assay and ISO 11290-1:1996 method successfully detected L. monocytogenes from seven of nine samples tested. The remaining two samples were found to be inhibitory to growth of L. monocytogenes along with a further four samples identified by Nutricia. The modified PCR method detected L. monocytogenesfrom four of six inhibitory samples tested. The remaining two inhibitory samples were tested with the rapid culture method, achieving 100% sensitivity.

Significance: Alternative PCR methods to detect L. monocytogenes with short time to result (24 hours–4 days) were identified for all medical nutrition products, including samples inhibitory to growth which prevented detection of L. monocytogenes with the 5-day ISO culture based method.