Molecular epidemiology and heat resistance of Listeria monocytogenes in meat products and meat-processing plants and listeriosis in Latvia

Abstract

The prevalence, contamination and heat resistance of Listeria monocytogenes were investigated in meat products and meat-processing plants. Moreover, trends of human listeriosis in Latvia were studied over a 10-year period from 1998 to 2007.

A high prevalence (40%) of L. monocytogenes was found in cold-smoked meat products compared with other heat-treated ready-to-eat meat products (0.7%) available in retail markets in Latvia. Pulsed-field gel electrophoresis (PFGE) and serotyping were applied to analyse the diversity of L. monocytogenes isolates present in ready-to-eat (RTE) meat products and meat-processing plants. A high genetic diversity was seen among L. monocytogenes isolates from cold-smoked meat products, suggesting the existence of various sources of contamination at different production stages in the meat-processing environment.

The manufacture of cold-smoked meat products involves no processing steps to eliminate L. monocytogenes, thus, contamination of the raw meat and contamination during processing can both contribute to L. monocytogenes in the finished product. Logistic multivariable regression model was successfully applied to identify the main factors associated with L. monocytogenes contamination during the manufacturing of cold-smoked pork products. Meat brining by injections was a significant factor (odds ratio 10.66; P<0.05) for contamination of products with L. monocytogenes. Of the cold-smoked meat-processing plant environmental samples, most contaminated sites were associated with brining machine and brining area. Long cold-smoking times (≥ 12 h) also had a significant predictive value (odds ratio 24.38; P<0.014) for a sample testing positive for L. monocytogenes.

A genetically diverse population of L. monocytogenes entered the meat-processing plant, where only some of the strains colonized and established a persistent microbial community within the plant over a 5-year period. L. monocytogenes PFGE types belonging to serotypes 1/2a and 4b were isolated from imported, defrosted, raw pork from Germany and Belgium in meat-processing plant B. In total, two L. monocytogenes PFGE types originating from raw meat were found also in finished RTE cold-smoked pork products, whereas one PFGE type was recovered later only from the meat-processing environment. One of the L. monocytogenes PFGE types, belonging to serotype 1/2c, was isolated from RTE cold-smoked meat products and from the feeding teeth of the brining machine, thus showing that improper cleaning, disinfection and poor hygiene design of the brining machine may cause L. monocytogenes contamination over time.

Post-package pasteurization of high- and low-fat content cooked sausages at temperatures higher than 55°C was found to be an effective method of post-process thermal treatment to reduce contamination of L. monocytogenes. However, heating to 55°C, 60°C and 62.5°C may not be practical in the meat industry because the process takes too long to reach a 3-log reduction. The formulation of high-fat content RTE cooked sausages may require modification to maintain product quality.

During the 10-year study period the overall incidence of listeriosis in Latvia was 0.4 per 100 000 population, with the highest incidences recorded in 2000 and 2002 (1.5 and 0.7 per 100 000 population, respectively). The highest incidence of listeriosis in Latvia was observed in 2000, which significantly exceeded incidence levels in all Baltic and Nordic countries, and was the highest among all EU member states during the same period. A marked clustering of human listeriosis cases was observed from September to December 2000, possibly indicating one large outbreak. The lack of serotyping and molecular typing methods for subtyping of L. monocytogenes isolates in the present surveillance system is one of the main reasons why there have been no officially documented listeriosis outbreaks in Latvia to date. Measures to allow the application of appropriate actions at the food industry level need to be implemented to prevent or significantly reduce the real burden of foodborne listeriosis in Latvia.

Listeria monocytogenes on elintarvikeperäinen patogeeni, joka voi aiheuttaa listerioosin ihmisille ja eläimille. Listerioosi on suhteellisen harvinainen, mutta vakava tauti, johon sairastuvat yleensä syntymättömät lapset, vanhukset ja vastustuskyvyltään heikentyneet henkilöt. Listerioosille on ominaista korkea kuolleisuus (20-30%) ja se on yksi eniten kuolemia aiheuttavista elintarvikeperäisistä taudeista kehittyneissä maissa.

Listeria monocytogeneksen esiintyvyyttä, kulkeutumista ja lämmönsietoa tutkittiin lihatuotteissa ja lihanjalostuslaitoksissa. Lisäksi tutkittiin listerioosi-tapausten kehityssuuntaa Latviassa kymmenen vuoden aikana 1998-2007.

Latviassa vähittäismyynnissä olevissa kylmäsavustetuissa lihatuotteista todettiin korkea L. monocytogeneksen esiintyvyys (40 %) verrattuna muihin lämpökäsiteltyihin sellaisenaan syötäviin lihatuotteisiin (0,7 %). Kylmäsavustetuista lihatuotteista eristettiin geneettisesti vaihtelevia L. monocytogenes kantoja, mikä viittaa saastumiseen useista lähteistä tuotannon eri vaiheissa.

Listerioosi on Latviassa ilmoitettava tartuntatauti, mutta elintarvikkeista ja kliinisistä näytteistä eristettyjä L. monocytogeneksia ei säännöllisesti kerätä ja tyypitetä. Tämä rajoittaa listerioosi-epidemioiden tunnistamista ja viivyttää tarkoituksenmukaisten toimenpiteiden aloittamista elintarviketuotannossa, jotta voitaisiin estää tai huomattavasti vähentää listerioosi-tapauksia.