Researchers have looked at the types of Listeria monocytogenes strains isolated from food and people during a 10-year period in Chile.

Results of tests show Listeria monocytogenes strains isolated from 2008 to 2017 in the city of Santiago indicate serotypes 1/2a, 1/2b and 4b are the most common in food products and from clinical cases, according to the study published in the Food Microbiology journal.

A total of 365 Listeria monocytogenes strains were analyzed. They were isolated from 2008 to 2017 from 40 patients and food products including cheese, cooked meat, cooked sausage, fresh fish, fresh vegetables, frozen seafood, frozen vegetables, ham, ice cream, mixed food, pâté, raw meat, raw poultry, raw sausages and smoked fish.

Emerging importance and large outbreaks
In the past decade, Listeria monocytogenes has emerged as a foodborne pathogen of major importance in Chile. In 2008 and 2009, two large listeriosis outbreaks occurred, associated with consumption of soft cheese like Brie and Camembert and sausages or meat products, respectively.

Since then there has been a slight increase in sporadic infections. According to official reports, in 2017 there were 86 sporadic cases of listeriosis, with a mortality rate of 26 percent. Fifty-three of the illnesses were caused by strains belonging to serotype 4b.

Researchers said the study reinforces the need to continue surveillance and implement food safety regulations to prevent outbreaks and sporadic cases of listeriosis in the country.

A previous study showed that in Chile during 2008 and 2009, Listeria monocytogenes serotypes 1/2a, 1/2b and 4b were the most frequently identified in food and clinical strains.

Isolation and detection of Listeria monocytogenes strains in food and clinical samples was done at the Laboratorio de Salud Pública Ambiental y Laboral de la Seremi de Salud de Santiago.

Widespread distribution and genetic diversity
In surveillance from 2008 to 2017, the most common serotype was 4b with 146 or 40 percent of the total followed by 1/2a with 114, 1/2b at 63 and 1/2c with 30. A total of 12 strains were non-typeable.

Using pulsed field gel electrophoresis (PFGE), researchers identified 41 pulsogroups from A1 to O2 and 189 pulsotypes. Most strains from pulsogroups D1, G1, N1, Z1, A2, F2 and N2 were isolated from cheese, frozen seafood, mixed food, raw poultry, pâté, frozen vegetables and raw meat, respectively.

However, all other pulsogroups grouped strains isolated from several food types. This indicates a substantial genetic diversity of strains among the food samples.

A number of pulsotypes contained clinical and food strains. For example, pulsotype 22 contained 12 clinical strains from the 2008 listeriosis outbreak, 32 strains isolated from cheese and one strain from raw poultry.

Clinical strains were isolated from sporadic cases of listeriosis and it was not possible to associate the food strains with disease because of a lack of epidemiological data.

Researchers identified a number of pulsotypes repeatedly found in different years and food products. They also found pulsotypes including strains isolated in the same year but from different foods. This indicates the widespread distribution of some Listeria monocytogenes clonal populations, which may persist over several years.

“Although some clonal populations are persistent and widespread, the genomic diversity of this pathogen in Chilean food products is considerable and changes over time,” said researchers.

Whole genome sequencing (WGS) and single-nucleotide polymorphism (SNP) based phylogenetic analyzes could complement and improve molecular epidemiology and surveillance of the pathogen in Chile.