Listeria monocytogenes causes potentially fatal illness to susceptible people and is found in various foods. It typically enters processed foods via a contaminated processing environment, in which it may have persisted for years. To study the role of raw material as a potential source of contamination of food processing plants by L. monocytogenes, the prevalence and genetic diversity of this species in tonsils of pigs and in raw fish was examined. A total of 14% and 4% of tonsils of pigs and raw fish, respectively, harboured L. monocytogenes. From 38 pig tonsil isolates and 11 raw fish isolates, 24 and nine different types were recovered using pulsed-field-gel electrophoresis (PFGE) typing. The results indicate that a wide variety of L. monocytogenes strains enters pork slaughterhouses and fish processing plants in the raw materials, which are thus potential sources of direct or indirect contamination of processing plants by this pathogen.
Since identical PFGE types were recovered from both raw and processed fish, it is likely that raw fish are an initial source of the L. monocytogenes found in processed fish. Some strains entering a plant along with raw fish may contaminate and persist in the processing environment, causing recurrent contamination of the final products via contact surfaces. Alternatively, L. monocytogenes strains in raw fish may survive non-listericidal processes, resulting in contamination of the final product.
To identify novel factors contributing to survival of L. monocytogenes in food processing environment, the roles of specific genes in stress response were investigated, using flhA and motA that encode flagellar factors involved in cold stress tolerance, and lmo0866, lmo1246, lmo1450, and lmo1722 encoding DEAD-box RNA helicases involved in cold, heat acid, alkali, osmotic, ethanol, and oxidative stress tolerance. Increased relative transcription levels of flhA, motA, lmo0866, lmo1450, and lmo1722, restricted growth of the single gene deletion mutant strains EGD-eΔflhA, EGD-eΔmotA, Δlmo0866, Δlmo1450, and Δlmo1722 at 3°C, and increased minimum growth temperatures of Δlmo0866, Δlmo1450, and Δlmo1722 revealed that FlhA, MotA, Lmo0866, Lmo1450, and Lmo1722 had roles in growth of L. monocytogenes EGD-e under cold stress conditions. The restricted growth of Δlmo0866 in 3.5% ethanol, and its increased maximum growth temperature and growth rate at 42.5°C, indicated that Lmo0866 had roles also in ethanol and heat stress tolerance of strain EGD-e. The role of Lmo1450 in the growth of strain EGD-e under heat, alkali, and oxidative stress conditions was shown by the restricted growth rate of Δlmo1450 at 42.5°C, in pH 9.4, and in 5 mM H2O2. The slightly decreased growth rate and maximum optical density of Δlmo1246 at 3°C indicated that the role of Lmo1246 in cold stress tolerance was negligible. Under all the other conditions, the growth of Δlmo1246 and the wild-type EGD-e were identical, suggesting that Lmo1246 had no role in growth of L. monocytogenes EGD-e under heat, pH osmotic, ethanol, or oxidative stress conditions.
The deletion of flhA, motA, lmo0866, lmo1450, and lmo1722 impaired the motility of strain EGD-e, whereas the motility of Δlmo1246 did not differ from that of the wild type. This indicates that DEAD-box RNA helicases Lmo0866, Lmo1450, and Lmo1722 have roles in motility of strain EGD-e. Moreover, these results suggest that motility and cold stress tolerance of L. monocytogenes are linked, and that motile flagella may be needed for full cold stress tolerance of strain EGD-e.
|Tila||Julkaistu - 2013|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|
- 413 Eläinlääketiede