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Typing quality of Listeria ‘encouraging’

By Joe Whitworth+

07-Feb-2014
Last updated on 07-Feb-2014 at 10:57 GMT

The quality of typing Listeria by Pulsed Field Gel Electrophoresis (PFGE) in EU labs is ‘encouraging’, according to an external assessment.

The method is described as the ‘gold standard’ for high-discriminatory typing of Listeria and it is commonly done with two enzymes (ApaI and AscI) for extra discriminatory power.

About 40% of laboratories did not produce PFGE profiles of sufficiently high quality for inter-laboratory comparison and only 13 were able to produce the raw gel image and perform data analysis.

This comparability relies on the use of correct running conditions, good quality image acquisition and distinct bands.

Profiles were then normalised and interpreted using the specialised software BioNumerics (BN).

Performance was encouraging, although the number of participants (18) could have been higher, said the European Centre for Disease Prevention and Control (ECDC).

Methods used

PFGE, conventional serological typing and polymerase chain reaction (PCR)-based molecular typing were evaluated.

Listeriosis is a serious foodborne disease, with 1,476 confirmed human cases reported in the EU in 2011.

Compared to other foodborne infections under EU surveillance, listeriosis caused the most severe human disease, with 93.6% of the cases hospitalised and 134 fatalities.

18 laboratories participated in the EQA-1 between January and March 2013 and ten strains were selected.

The report presented the first round of the Listeria External Quality Assurance (EQA) scheme for the typing of Listeria monocytogenes (further EQA-1).

It follows an earlier report done for Salmonella .

Labs participation

17 laboratories produced PFGE results, 16 laboratories participated in serotyping. Ten of the 16 labs performed conventional phenotypic serotyping, while seven did molecular PCR-based serotyping.

ECDC graded the gel quality according to the TIFF Quality Grading Guidelines which involved evaluation of a gel using seven parameters.

All participants scored ‘fair’ and above for the ‘Cell suspension’, ‘Lanes’, ‘Restriction’ and ‘Gel background’ parameters.

For the parameter ‘DNA degradation’, three of the participants’ gels had so much smearing that it was impossible to analyse them.

In the category ‘Bands’, four laboratories were given the lowest score (‘poor’) and five were given the second lowest (‘fair’), mostly due to thick or fuzzy bands.

Some labs had problems with the ‘Image acquisition and running conditions’ category. ECDC said this is critical because incorrect running conditions will make the gel impossible to compare with other gels run under correct conditions.

In the serotyping part participants were divided between the conventional serological (63%) and the molecular PCR multiplex (44%) methods.

The correlation in results between these methods is good but the difference in time consumption and cost is considerable.

If serotyping results are required for EU-wide surveillance it is probably more realistic to expand the use of the PCR-based method, said the report.

Whole genome sequence (WGS)-based methods will likely take over from the methods used as laboratories begin to use it. However, no harmonised procedures for WGS data analysis exist for use in routine surveillance and international comparison of Listeria strains.

Source: European Centre for Disease Prevention and Control

Online, DOI: 10.2900/17761

External quality assurance scheme for Listeria monocytogenes typing

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