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PFGE ‘gold standard’ for Salmonella typing

By Joe Whitworth+

03-Feb-2014
Last updated on 03-Feb-2014 at 14:29 GMT

Pulsed Field Gel Electrophoresis (PFGE) is the gold standard for high discriminatory typing of Salmonella and the only generic method for typing all serovars, according to an EU report.

The study presented the fourth round of the Salmonella External Quality Assessment (EQA) scheme for typing of Salmonella enterica spp. enterica (further EQA-4).

Pulsed Field Gel Electrophoresis (PFGE), Multiple Locus Variable number of tandem repeat Analysis (MLVA) and phage typing are covered.

“PFGE is still the gold standard for high discriminatory typing of Salmonella and the only generic method for typing of all Salmonella serovars,” found the European Centre for Disease Prevention and Control (ECDC) report.  

Salmonellosis is the second most commonly reported zoonotic disease in EU with 97, 897 cases reported by the 27 EU Member States in 2011.

High PFGE gel quality

The majority (64%) of laboratories were able to produce a PFGE gel of sufficiently high quality to allow for the profiles to be comparable to profiles from other laboratories.

The EQA-4 included PFGE (different serotypes), MLVA (Typhimurium), and two phage typing schemes (Typhimurium and Enteritidis).

MLVA typing

MLVA for typing of S. Typhimurium has been increasingly used over the last decade and considering the need for specialised equipment (capillary electrophoresis), it is promising that half of the laboratories participated.

60% of the laboratories reported correct MLVA profiles for all strains and 87% found the correct profile for at least nine of the ten strains.

27 laboratories participated in the EQA-4 that took place in January–March 2013.

25 laboratories (93%) produced PFGE results, 15 laboratories MLVA results, and 12 and 11 laboratories in phage typing of S. Enteritidis and S. Typhimurium. Eight laboratories completed all parts of the EQA.

Phage typing verdict

Phage typing of the two major Salmonella serotypes, Typhimurium and Enteritidis, was included in the EQA as the only phenotypic method.

A high level was obtained for phage typing of Typhimurium as all laboratories found the correct phage type for at least eight of the ten strains (64% of labs had all correct).

More incorrect phage types were reported for the Enteritidis scheme as 42% of laboratories only found 50–80% correct types.

Whole genome sequence (WGS) methods will likely take over, as laboratories use WGS and WGS-based methods, which are harmonised and made applicable for international comparison.

The objectives of the EQA-4 was to assess the quality of PFGE, MLVA and phage typing and the comparability of collected test results between public health national reference laboratories in EU/EEA and EU candidate countries.

“This EQA demonstrated that a majority of the participating laboratories were able to produce good typing results,” said the report.

“Less than half of the laboratories produced results that need to be improved before inter-laboratory exchange of data; however, for the majority of the identified technical issues an acceptable quality is within reach by optimisation of procedures in laboratories, trouble-shooting assistance, and training.”

Source: European Centre for Disease Prevention and Control

Online, doi: 10.2900/93836

Fourth external quality assessment scheme for Salmonella typing