The European Centre for Disease Prevention and Control (ECDC) said the second External Quality Assessment (EQA) for PFGE typing of Listeria showed the majority of labs were able to produce good results.
Only 14% of the laboratories produced results that were too poor for the inter-laboratory comparison of data. For the software analysis, 18% need to improve their Bionumerics analysis.
Typing methods covered
The EQA covers the Pulsed Field Gel Electrophoresis (PFGE) method, conventional serological typing and PCR-based molecular typing.
The PCR-based method can discriminate between five groups while the phenotypic method discriminates between 14 serotypes.
The majority of labs were able to produce a PFGE gel of sufficiently high quality to allow for comparison with profiles from other labs.
Profiles were then normalised and interpreted using BioNumerics software. Thirteen laboratories (72%) completed the gel analysis, generally in accordance with the guidelines.
Comparability primarily relies on use of correct running conditions, distinct bands and a good quality image acquisition, said ECDC.
The average score for traditional serotyping was 78%, a decrease from EQA-1 mainly attributed to one difficult strain.
“Strain 6 caused some additional problems since the serotyping was not clear when tested with serum from Denka Seiken,” said the ECDC.
“The agglutination with the polyvalent O:V/VI was clear but the monovalent agglutinations with O:VI, O:VII, O:VII and O:IX were not clear. This would lead to an invalid result for the O-antigens in strain 6.”
In the molecular (multiplex PCR-based) serotyping participants obtained an average score of 94%.
Listeriosis is a relatively rare but serious foodborne disease, with 1,642 confirmed human cases reported in EU in 2012.
Compared to other foodborne infections under EU surveillance, listeriosis caused the most severe human disease, with 91.6% of cases hospitalised.
A total of 18 laboratories participated in at least one part of the EQA-2, 14 laboratories (78%) produced PFGE results, 17 labs (94%) participated in the serotyping exercise.
Nine of these 17 labs performed conventional, phenotypic serotyping, while 14 performed molecular, PCR-based serotyping and 13 laboratories (72%) completed PFGE and serotyping.
Fourteen laboratories participated in the Listeria PFGE, submitting raw gel images (TIFF files) and 11 had also analysed the gel using BioNumerics and 91% performed well.
The gels and the profiles for individual strains, varied considerably in quality, said ECDC.
Since 2012 the Unit of Foodborne Infections at Statens Serum Institut in Denmark has been the EQA provider for three lots covering Salmonella, Shiga toxin/verocytotoxin-producing E.coli (STEC/VTEC) and Listeria monocytogenes.
ECDC said in the longer term, whole genome sequence (WGS)-based methods will most likely take over from those used in the EQA as labs begin to implement it.
“At the moment, there are no harmonised procedures for WGS data analysis in routine surveillance and international comparison of Listeria strains, but some laboratories are beginning to proceed with this idea.”