Population diversity of Listeria monocytogenes in quargel (acid curd cheese) lots recalled during the multinational listeriosis outbreak 2009/2010

It has been possible to determine the genotype diversity of Listeria monocytogenes in the actual cheese lots of acid curd cheese that caused a multinational outbreak between 2009 and 2010. Following product recall in January 2010 all lots were investigated. A total of 422 L. monocytogenes isolates were characterized by genotyping. In a first approach the PCR serogroups were defined by multiplex-PCR assays. Subsequently, the isolates were subtyped by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). Sequence types were assigned by submitting the DNA sequences to the Listeria MLST database at the Institute Pasteur. The serogroup PCR resulted in a homogeneous 1/2a - 3a (genetic linage II) cluster. The generated PFGE patterns divided the strains into two clusters (type 1 and 2) diverging at a homogeneity level of 74%. PFGE-type 2 was predominant, accounting for 98.3% (n = 415/422) of the isolates and was isolated during the whole period of acid curd cheese processing (01.12.2009-13.01.2010). 1.7% of all tested L. monocytogenes isolates (n = 7/422) belonged to PFGE-type 1 and were isolated from 28% of all cheese lots (n = 5/18) produced between the time span of 08.12.2009 to 13.01.2010. Furthermore, PFGE-type 1 and 2 showed the same PFGE patterns as the human outbreak strains (clone 1 and clone 2).