However, no isolates were available for sequencing to confirm this hypothesis, said researchers.
Using forensic microbiology, they identified several products from manufacturing firm Sieber that were contaminated with the outbreak genotype.
The Listeria outbreak sickened 77 people since 2012. There was a first peak in the second half of 2013 but most cases occurred since June 2014 with a decrease since August 2015.
Robert-Koch-Instituts (RKI) said products marketed under the name ‘Original Bayerisches Wammerl’ produced by Sieber were linked to the outbreak from 2012 to 2016.
The meat processing plant predominantly supplied grocery stores.
“Altogether, food consumption histories of patients were compatible with our molecular typing results, but we could not prove the producer was the source of the infections,” said researchers.
“After the production plant was shut down, the outbreak strain was isolated from only three more persons who either might have consumed or consumed pork products from this company with a high degree of probability.”
Analysis of L. monocytogenes isolates
At the end of 2015, an outbreak of invasive listeriosis caused by L. monocytogenes serotype 1/2a was reported in southern Germany.
Analysis of the L. monocytogenes isolates from patients revealed the same novel pulsed-field gel electrophoresis (PFGE) pattern.
Researchers analyzed 543 environmental L. monocytogenes isolates of molecular serotype IIa (comprising the conventional serotypes 1/2a and 3a).
Staff of official control labs acquired these isolates from food matrices and processing plants in the affected federal states (Bavaria, Baden-Württemberg and Hesse).
Analysis showed that 26 isolates from food had the same AscI (13a) and ApaI (54) restriction patterns as the human outbreak strain and patterns from 55 isolates showed >90% similarity to the PFGE pattern ApaI 54.
The outbreak investigation revealed 26 isolates from food with the 13a/54 PFGE pattern: 24 either from or associated with food products from the suspected outbreak source and two not epidemiologically linked to the suspected outbreak source.
As of July this year, 84 human isolates had this PFGE pattern and these isolates could be assigned to 78 surveillance cases.
Subsequent whole-genome sequencing (WGS) and core-genome multilocus sequence typing (MLST) resulted in assignment of 57 human isolates to the unique cluster type 1248.
In May 2016, an L. monocytogenes isolate with the 13a/54 PFGE pattern that belonged to the core-genome MLST cluster type 1248 was identified.
This isolate was found at a retail store in Bavaria two months prior in a smoked pork belly sampled by inspectors. It had been manufactured by Sieber, a meat processing plant in the country that distributed food in southern Germany.
Other cluster types and cross contamination
The highly contaminated batch of smoked pork belly (bacterial concentration 1.9 × 105 CFU/g) was recalled and withdrawn from the market.
Follow-up investigations at the plant revealed identical isolates in another batch of smoked pork belly, in vegetarian sausages and in two types of boiled pork sausages. Bacterial concentration was <100 CFU/g.
At the end of May 2016, all food from the producer was banned from sale and withdrawn from the market and the company later filed for insolvency.
Two more core-genome MLST cluster types found in raw meat and sausage were found in the production chain. However, based on PFGE data, researchers could not assign the isolates to any human cases in southern Germany during 2012-16.
An L. monocytogenes isolate from the fecal sample of an employee differed from the outbreak strain.
Investigation in the household of a patient who regularly consumed smoked pork belly distributed by Sieber revealed an isolate in cheese belonging to cluster type 1248 and identical to the patient isolate.
Comparative analyses of unopened packages of the same batch of cheese were negative for L. monocytogenes, suggesting cross-contamination in the household.
“Molecular tracing to find source of protracted invasive listeriosis outbreak, southern Germany, 2012-2016”
Authors: Kleta S, Hammerl JA, Dieckmann R, Malorny B, Borowiak M, Halbedel S, et al