Genetic heterogeneity of Listeria monocytogenes in Northern Italy

Backgroud: Invasive listeriosis is a rare foodborne disease that can cause septicaemia, meningoencephalitis and eventually death in vulnerable population. The ethiologic agent, Listeria monocytogenes, is genetically heterogeneous, therefore discriminating strains is of major importance for prompt outbreak detection and improvement of surveillance. The aim of this study was detecting emerging strains and monitoring their frequency in Lombardy region. Methods: In Lombardy, the mandatory notification system has been integrated since 2005 with a laboratory-based surveillance network based on voluntary referral of clinical strains to the Regional Reference Laboratory. All human L. monocytogenes strains collected (2005-2013), were serotyped and subtyped by Multi-locus Sequence Typing (MLST). Sequence Types (STs) were assigned in accordance to the Listeria MLST database (Pasteur Institute, France). Results: Among the 245 collected strains 49 STs were identified, 36 of them had been previously reported in the same area. The predominant serotypes were 1/2a (59.6%), 4b (26.1%) and 1/2b (10.6%), while the 6 most common STs were ST38 (16.7%), ST1 (11.8%), ST8 (9.4%), ST155 (6.5%), ST2 (5.7%) and ST3 (4.9%), accounting for 55% of the strains. The frequency of ST38 and ST155 led us to hypothesize the occurrence of two outbreaks undetected by local health authorities, during the period 2009-2011. In particular, the identification of ST155 strains began in 2008 (n=1), peaked in 2011 (n=10) and continued in 2013 (n=5) with 3 pregnancy-related cases closely linked in space and time. For these 3 cases an epidemiological investigations was implemented but failed to identify the source. Only in one case (ST8), it has been possible to characterize the implicated food (brique cheese). Finally, ST1, ST2 and ST3 showed a trend of persistence with a wide distribution across time and space. Conclusions: This study reveals the main STs present in the regional area and enhances our understanding of the genetic diversity of circulating L. monocytogenes strains. The monitoring of these STs will provide important information useful for the prevention of listeriosis and for supporting the epidemiological investigations, which specifically for listeriosis are often arduous.