Prevalence and characterization of Listeria monocytogenes in deboning and slicing areas of Spanish dry-cured ham processing

Evolution of Volatile Compounds during Ripening and Final Sensory Changes of Traditional Raw Ewe’s Milk Cheese “Torta del Casar” Maturated with Selected Protective Lactic Acid Bacteria

In traditional soft ripened cheeses made with raw milk, the use of protective cultures is infrequent. In the present work, the effect of selected (for their activity against Listeria monocytogenes) protective cultures of Lactocaseibacillus casei 116 and Lactococcus garvieae 151 was evaluated, on the evolution of volatile compounds throughout the ripening and on the final sensory characteristics of traditional soft ripened “Torta del Casar” cheese. For this, both strains were separately inoculated in raw cheeses and ripened for 90 days. The selected LAB strains did not affect physicochemical parameters, including texture and color of the ripened cheeses. However, they could have a positive effect on the aroma, for the generation of methyl branched acids and for the reduction in compounds derived from β-oxidation of fatty acids. Thus, these protective cultures, in addition to contributing to their safety, could improve quality of the ripened cheeses.

Antimicrobial activity of essential oils and natural plant extracts against Listeria monocytogenes in a dry-cured ham-based model

BACKGROUND

Listeria monocytogenes is a widespread common contaminant in food production facilities during preparation, storage, and distribution, and minimally processed ready-to-eat products are considered at high risk of contamination by this bacterium. Increased antibiotic resistance has led researchers to search for plant-based natural alternatives to control pathogenic microorganisms. Among these products, essential oils and plant extracts have previously shown antimicrobial activity and are possible alternatives to manage food pathogens. In this study, commercial essential oils (cinnamon, clove, oregano, ginger, and thyme) and plant extracts (pomegranate, acorn, olive, strawberry tree, and dog rose) were tested against L. monocytogenes in a dry-cured ham-based model.

RESULTS

Essential oils and plant extracts were screened by agar diffusion and minimum inhibitory concentration for anti-L. monocytogenes activity. Cinnamon, pomegranate, and strawberry trees returned the strongest results and were therefore evaluated in a dry-cured ham-based medium assay with water activity of 0.93 or 0.95. The 10% essential oil of cinnamon was capable of completely inhibiting bacterial growth, while strawberry tree and pomegranate extract also showed antilisterial activity (P > 0.05). Water activity influenced the bacterial count of L. monocytogenes in a dry-cured ham-based medium.

CONCLUSIONS

There was a reduction in L. monocytogenes with the application of cinnamon essential oil but, because of the negative sensory impact of this particular compound in meat products, we suggest the use of pomegranate or strawberry tree for the biocontrol of Listeria in ready-to-eat products. © 2021 Society of Chemical Industry.

Effect of Enterocins A and B on the Viability and Virulence Gene Expression of Listeria monocytogenes in Sliced Dry-Cured Ham

Dry-cured ham can be contaminated with Listeria monocytogenes during its industrial processing. The use of bacteriocins could ensure the safety of such meat products, but their effect on pathogen physiology is unknown. Therefore, the impact of enterocins A and B on the L. monocytogenes population, and the expression patterns of five genes (inlA, inlB, clpC, fbpA and prfA) related to adhesion/invasion and virulence regulation have been monitored in sliced dry-cured ham during 30 d of storage in refrigeration (4 °C) and temperature-abuse conditions (20 °C). L. monocytogenes strains S2 (serotype 1/2a) and S7-2 (serotype 4b) counts were reduced by 0.5 and 0.6 log units immediately after the application of enterocins A and B, a decrease lower than previously reported. Differences in gene expression were found between the two strains. For strain S2, expression tended to increase for almost all genes up to day seven of storage, whereas this increase was observed immediately after application for strain S7-2; however, overall gene expression was repressed from day one onwards, mainly under temperature-abuse conditions. L. monocytogenes strains investigated in the present work exhibited a mild sensitivity to enterocins A and B in sliced dry-cured ham. Bacteriocins caused changes in the expression patterns of virulence genes associated with adhesion and invasion, although the potential virulence of surviving cells was not enhanced.

Growth and Expression of Virulence Genes of Listeria monocytogenes during the Processing of Dry-Cured Fermented “Salchichón” Manufactured with a Selected Lactilactobacillus sakei

Simple Summary

During the ripening process of the dry-cured fermented sausage “salchichón”, Listeria monocytogenes could fail to be eliminated. In addition, the food safety criterion for L. monocytogenes in the European Union sets up a maximum level of 100 units of this microorganism per gram in ready-to-eat products throughout their shelf-life. Thus, since L. monocytogenes could be present in this product, it is necessary to evaluate the impact of the dry-cured fermented processing in the potential virulence of this pathogen, even considering the possible effect of the usual microbiota (lactic-acid bacteria) of “salchichón”. In this work, the effect of the processing of “salchichón”, inoculated with a selected strain of Lactilactobacillus sakei, on the growth of L. monocytogenes and on the expression of its virulence genes, was evaluated. The processing of “salchichón” provoked a relevant reduction in L. monocytogenes, but this pathogen was not completely eliminated. However, a downregulation in the expression of the tested virulence genes was found, which could suppose a reduction in the pathogenic effect of this microorganism. These findings could be of great interest to consider the dry-cured ripening of “salchichón” as a safe process to control the pathogen L. monocytogenes.

Abstract

The effect of the dry-cured fermented processing of “salchichón” inoculated with a selected strain of Lactilactobacillus sakei (205) on the growth and transcriptional response of three virulence genes (plcA, hly, and iap) of Listeria monocytogenes was evaluated. For this, three different batches of “salchichón” were analyzed: batch B (inoculated only with L. sakei), batch L (inoculated only with L. monocytogenes), and batch L + B (inoculated with both microorganisms). Sausages were ripened for 90 days according to a traditional industrial process. The processing of “salchichón” provoked a reduction in L. monocytogenes counts of around 2 log CFU/g. The downregulation of the expression of the three genes was found at the end of ripening when the water activity (a_w) of “salchichón” was <0.85 a_w. The combined effect on the reduction in L. monocytogenes counts together with the downregulation in the expression of the virulence genes throughout the “salchichón” processing could be of great interest to control the hazard caused by the presence of this pathogenic bacterium.

Effect of the Dry-Cured Fermented Sausage "Salchichón" Processing with a Selected Lactobacillus sakei in Listeria monocytogenes and Microbial Population

In the present work, the effect of processing of dry-cured fermented sausage “salchichón” spiked with the selected Lactobacillus sakei 205 was challenge-tested with low and high levels of L. monocytogenes. The evolution of the natural microbial population throughout the “salchichón” ripening was also evaluated. For this, a total of 150 “salchichón” were elaborated and divided into six equal cases which were inoculated with different levels of L. monocytogenes, and L. sakei 205. Afterwards, sausages were ripened for 90 days according to a typical industrial process. Moisture content (%) and water activity (a_w) decreased throughout the ripening up to values around 26% and 0.78, respectively. No differences for moisture content, a_w, pH, NaCl and nitrite concentration were observed between the analyzed cases. Lactic acid bacteria counts in the L. sakei 205 inoculated cases were always higher than 6 log CFU g⁻¹ during ripening. Enterobacteriaceae counts were reduced during ripening until non-detectable levels at the end of processing. Reductions in L. monocytogenes counts ranged from 1.6 to 2.2 log CFU g⁻¹; therefore, the processing of “salchichón” itself did not allow the growth of this pathogen. Reduction in L. monocytogenes was significantly higher in the cases inoculated with L. sakei 205.

The efficiency of technologies used for epidemiological characterization of Listeria monocytogenes isolates: an update

The characterization of pathogenic bacteria by providing information regarding the identification and source-tracking of the causes of outbreaks is vital for the epidemiological investigations of foodborne diseases. The knowledge of transmission of Listeria monocytogenes (L. monocytogenes) strains from the environment, directly or indirectly (through food processing facilities) to the final food products, due to the complexity of evaluating numerous, affecting parameters is quite limited. The food trade globalization also adds difficulties in tracking the association between the infection occurrence and causative pathogens, aiming to prevent their spread. The occurrence of listeriosis, a notifiable disease throughout the world, can either be sporadic or outbreak-related. Due to the importance of foodborne outbreaks from a public health aspect and its correspondence enormous economic losses, cross-linked surveillance studies regarding the contamination of foods by L. monocytogenes, besides identifying clusters and tracing the sources of infections on an international-scale to prevent and control L. monocytogenes outbreaks sounds very crucial. Contrary to the conventional typing methods, molecular-based techniques, such as whole-genome sequencing, owing to the capacity to discriminate L. monocytogenes strains down to single nucleotide differences, provide an accurate characterization of strains and tracking the causes of outbreaks. However, routinely using molecular-based methods depends on the required improvements in the affordability, proper timing, and preparing reliable, standardized bioinformatics facilities. This work was conducted to critically review the practical potential of diverse typing methods have been used for the characterization of L. monocytogenes and discuss how they might change the future of efforts for control of listeriosis.

Assessment of the Prevalence and Drug Susceptibility of Listeria monocytogenes Strains Isolated from Various Types of Meat

Listeria monocytogenes are the etiological factor of listeriosis, and their main source for humans is food. The aim of the current study was to assess the contamination of various types of meat and the drug susceptibility of isolated L. monocytogenes. Between 2016–2018, 6000 swabs were taken (2000 annually) from the surface of pork, beef, and poultry. The analysis of intermediate and finished product samples was carried out in accordance with ISO 11290-1 (International Organization for Standardization). The genetic similarity assessment of the isolates obtained was based on the Pulsed Field Gel Electrophoresis (PFGE) method, and drug-sensitivity assessment using the disc-diffusion method. We found 2.1% of collected samples were L. monocytogenes positive. The level of meat contamination varied depending on its matrix. Most L. monocytogenes were isolated from poultry. It was shown that 39 (32.5%) strains were sensitive to all tested antibiotics and eight (6.7%) were resistant to all five tested antimicrobials. Most strains tested were resistant to cotrimoxazole (55; 45.8%) and meropenem (52; 43.3%), followed by erythromycin (48; 40.0%), penicillin (31; 25.8%), and ampicillin (21; 17.5%). High prevalence of this pathogen may be a serious problem, especially when linked with antibiotic resistance and high percentage of serotypes responsible for listeriosis outbreaks.