Survival of Escherichia coli O157:H7 during the manufacture and ripening of Cacioricotta goat cheese

Genome engineering of Stx1-and Stx2-converting bacteriophages unveils the virulence of the dairy isolate Escherichia coli O174:H2 strain UC4224

The past decade witnessed the emergence in Shiga toxin-producing Escherichia coli (STEC) infections linked to the consumption of unpasteurized milk and raw milk cheese. The virulence of STEC is primarily attributed to the presence of Shiga toxin genes (stx1 and stx2) carried by Stx-converting bacteriophages, along with the intimin gene eae. Most of the available information pertains to the "Top 7" serotypes associated with STEC infections. The objectives of this study were to characterize and investigate the pathogenicity potential of E. coli UC4224, a STEC O174:H2 strain isolated from semi-hard raw milk cheese and to develop surrogate strains with reduced virulence for use in food-related studies. Complete genome sequence analysis of E. coli UC4224 unveiled the presence of a Stx1a bacteriophage, a Stx2a bacteriophage, the Locus of Adhesion and Autoaggregation (LAA) pathogenicity island, plasmid-encoded virulence genes, and other colonization facilitators. In the Galleria mellonella animal model, E. coli UC4224 demonstrated high pathogenicity potential with an LD₅₀ of 6 CFU/10 μL. Upon engineering E. coli UC4224 to generate single and double mutant derivatives by inactivating stx1a and/or stx2a genes, the LD₅₀ increased by approximately 1 Log-dose in the single mutants and 2 Log-doses in the double mutants. However, infectivity was not completely abolished, suggesting the involvement of other virulence factors contributing to the pathogenicity of STEC O174:H2. Considering the possibility of raw milk cheese serving as a reservoir for STEC, cheesemaking model was developed to evaluate the survival of UC4224 and the adequacy of the respective mutants as reduced-virulence surrogates. All tested strains exhibited the ability to survive the curd cooking step at 48°C and multiplied (3.4 Log CFU) in cheese within the subsequent 24 h. These findings indicate that genomic engineering did not exert any unintended effect on the double stx1-stx2 mutant behaviour, making it as a suitable less-virulent surrogate for conducting studies during food processing.

Escherichia coli Specific Virulence-Gene Markers Analysis for Quality Control of Ovine Cheese in Slovakia

Shiga toxin-producing and extra-intestinal pathogenic Escherichia coli (E. coli) have the potential to spread through faecal waste, resulting in contamination of food and causing foodborne disease outbreaks. With the aim of characterizing unpasteurized ovine cheese in Slovakia, a total of 92 E. coli strains were examined for eleven representative virulence genes typical for (extra-)intestinal pathogenic E. coli and phylogenetic grouping. Phylogenetic groups B1 (36%) and A (32%) were the most dominant, followed by groups C (14%) and D (13%), while the lowest incidence was recorded for F (4%), and E (1%), and 43 (47%) samples carried at least one virulent gene, i.e., potential pathogens. Isolates present in groups E, F and D showed higher presence of virulence genes (100%, 75%, and 67%), versus 55%, 39%, and 28% in commensal B1, C, and A, respectively. Occurrence of papC and fyuA (both 24%) was highest, followed by tsh, iss, stx2, cnf1, kpsII, cvaC, stx1, iutA and eaeA. Nine E. coli strains (almost 10% of all tested and around 21% of our virulence-gene-associated isolates) harboured stx1, stx2 or eae. Ovine cheeses in Slovakia are highly contaminated with E. coli including potentially pathogenic strains capable of causing intestinal and/or extra-intestinal diseases, and thus may pose a threat to public health while unpasteurized.

Survival rate of Escherichia coli O157 in artificially contaminated raw and thermized ewe milk in different Pecorino cheese production processes

Pecorino is a typical Italian cheese, mostly produced in central and southern Italy regions using ewe raw milk and following traditional procedures. The use of raw milk constitutes a risk linked to the potential survival or multiplication of pathogenic microorganisms, as Shiga toxin-producing Escherichia coli (STEC). The aim of this study was to compare different Italian traditional Pecorino production methods to determine if there were any phases that could influence the Escherichia coli O157 survival rate, but also if they could negatively influence lactic acid bacteria survival rate, during the phases of production and ripening. Therefore batches of Pecorino cheese were prepared using different production methods, representing the real and typical cheese production in southern and central Italy regions: 1) heating the milk at 37 °C for about 40 min before curding, 2) heating the milk at 60 °C (thermization) for 13 min, so that the alkaline phosphatase reaction is still positive before curding, 3) cooking curd at 41 °C and 4) at 45 °C, both for 5 min. Our results demonstrated that traditional milk treatments different from pasteurization can help but do not eliminate serious microbiological treats, as E. coli O157, especially if the raw milk is heavily contaminated. The heat treatment at 60 °C applied to raw milk was able to decrease the concentration of E. coli O157 of 1.7 log₁₀CFU/ml and, according to the inactivation slope, it would be further reduced prolonging the heating treatment. The results obtained also showed that, during the Pecorino cheese ripening, E. coli O157 was always enumerable for 60 days, remaining detectable after 90 days of ripening.

High concentration of sodium chloride could induce the viable and culturable states of Escherichia coli O157:H7 and Salmonella enterica serovar Enteritidis

In the present study, Escherichia coli O157:H7 and Salmonella enterica serovar Enteritidis were transferred into Luria-Bertani medium without NaCl (LBWS) and adjusted to various pHs (4, 5, 6 and 7) with lactic acid containing 0·75, 5, 10 and 30% NaCl, and stored at 25°C until the bacterial populations reached below detectable levels on tryptic soy agar (TSA). Although E. coli O157:H7 and S. Enteritidis did not grow on TSA when incubated in LBWS with 30% NaCl for 35 and 7 days, more than 60 and 70% of the bacterial cells were shown to be viable via fluorescent staining with SYTO9 and propidium iodide (PI), respectively, suggesting that a number of cells could be induced into the viable but nonculturable (VBNC) state. These bacteria that were induced into a VBNC state were transferred to a newly prepared tryptic soy broth (TSB) and then incubated at 37°C for several days. After more than 7 days, E. coli O157:H7 and S. Enteritidis regained their culturability. We, therefore, suggest that E. coli O157:H7 and S. Enteritidis entered the VBNC state under the adverse condition of higher salt concentrations and were revived when these conditions were reversed.

Use of Starter Cultures in Foods from Animal Origin to Improve Their Safety

Starter cultures can be defined as preparations with a large number of cells that include a single type or a mixture of two or more microorganisms that are added to foods in order to take advantage of the compounds or products derived from their metabolism or enzymatic activity. In foods from animal origin, starter cultures are widely used in the dairy industry for cheese, yogurt and other fermented dairy products, in the meat industry, mainly for sausage manufacture, and in the fishery industry for fermented fish products. Usually, microorganisms selected as starter culture are isolated from the native microbiota of traditional products since they are well adapted to the environmental conditions of food processing and are responsible to confer specific appearance, texture, aroma and flavour characteristics. The main function of starter cultures used in food from animal origin, mainly represented by lactic acid bacteria, consists in the rapid production of lactic acid, which causes a reduction in pH, inhibiting the growth of pathogenic and spoilage microorganisms, increasing the shelf-life of fermented foods. Also, production of other metabolites (e.g., lactic acid, acetic acid, propionic acid, benzoic acid, hydrogen peroxide or bacteriocins) improves the safety of foods. Since starter cultures have become the predominant microbiota, it allows food processors to control the fermentation processes, excluding the undesirable flora and decreasing hygienic and manufacturing risks due to deficiencies of microbial origin. Also, stater cultures play an important role in the chemical safety of fermented foods by reduction of biogenic amine and polycyclic aromatic hydrocarbons contents. The present review discusses how starter cultures contribute to improve the microbiological and chemical safety in products of animal origin, namely meat, dairy and fishery products.

Genomic Investigation of Virulence Potential in Shiga Toxin Escherichia coli (STEC) Strains From a Semi-Hard Raw Milk Cheese

Shiga-toxin-producing Escherichia coli (STEC) represents a significant cause of foodborne disease. In the last years, an increasing number of STEC infections associated with the consumption of raw and pasteurized milk cheese have been reported, contributing to raise the public awareness. The aim of this study is to evaluate the main genomic features of STEC strains isolated from a semi-hard raw milk cheese, focusing on their pathogenic potential. The analysis of 75 cheese samples collected during the period between April 2019 and January 2020 led to the isolation of seven strains from four stx-positive enrichment. The genome investigation evidenced the persistence of two serotypes, O174:H2 and O116:H48. All strains carried at least one stx gene and were negative for eae gene. The virulence gene pattern was homogeneous among the serogroup/ST and included adherence factors (lpfA, iha, ompT, papC, saa, sab, hra, and hes), enterohemolysin (ehxA), serum resistance (iss, tra), cytotoxin-encoding genes like epeA and espP, and the Locus of Adhesion and Autoaggregation Pathogenicity Islands (LAA PAIs) typically found in Locus of Enterocyte Effacement (LEE)-negative STEC. Genome plasticity indicators, namely, prophagic sequences carrying stx genes and plasmid replicons, were detected, leading to the possibility to share virulence determinants with other strains. Overall, our work adds new knowledge on STEC monitoring in raw milk dairy products, underlining the fundamental role of whole genome sequencing (WGS) for typing these unknown isolates. Since, up to now, some details about STEC pathogenesis mechanism is lacking, the continuous monitoring in order to protect human health and increase knowledge about STEC genetic features becomes essential.

Survival of Selected Pathogenic Bacteria during PDO Pecorino Romano Cheese Ripening

This study was conducted to assess, for the first time, the survival of the pathogenic bacteria Listeria monocytogenes, Salmonella spp., Escherichia coli O157:H7, and Staphylococcus aureus during the ripening of protected designation of origin (PDO) Pecorino Romano cheese. A total of twenty-four cheese-making trials (twelve from raw milk and twelve from thermized milk) were performed under the protocol specified by PDO requirements. Sheep cheese milk was first inoculated before processing with approximately 106 colony-forming unit (CFU) mL−1 of each considered pathogen and the experiment was repeated six times for each selected pathogen. Cheese composition and pathogens count were then evaluated in inoculated raw milk, thermized milk, and cheese after 1, 90, and 150 days of ripening. pH, moisture, water activity, and salt content of cheese were within the range of the commercial PDO Pecorino Romano cheese. All the cheeses made from raw and thermized milk were microbiologically safe after 90 days and 1 day from their production, respectively. In conclusion, when Pecorino Romano cheese is produced under PDO specifications, from raw or thermized milk, a combination of factors including the speed and extent of curd acidification in the first phase of the production, together with an intense salting and a long ripening time, preclude the possibility of growth and survival of L. monocytogenes, Salmonella spp., and E. coli O157:H7. Only S. aureus can be still detectable at such low levels that it does not pose a risk to consumers.

Naked-eye based point-of-care detection of E.coli O157: H7 by a signal-amplified microfluidic aptasensor

Fast and sensitive detection of E.coli O157: H7 is significantly essential for clinical management as well as for transmission prevention during disease outbreaks. Though many types of detection strategies have been implemented for measuring E.coli O157: H7, most of them still rely on complex instruments or tedious/laborious setups, which restrict their applications in resource-limited scenarios. Herein, we introduce an eye-based microfluidic aptasensor (EA-Sensor) for fast detection of E.coli O157: H7 without the assist of any instruments. We demonstrate the perfect coupling of aptamer sensing, hybridization chain reaction (HCR)-amplification and a distance-based visualized readout to quantitatively determine the pathogen concentration. We first used gel-electrophoresis assay to evaluate the system and the results proved that E.coli O157: H7 was well recognized by the aptamer and HCR could increase the signal by about 100 folds. In addition, the Aptamer specificity and signal-amplification ability were verified on the EA-Sensor for sensing E.coli O157: H7 by naked eyes. Furthermore, we demonstrated that E.coli O157: H7 in milk could be accurately and conveniently measured with good performance. With the benefits of operation integration and strategy integration, our EA-Sensor shows advantages of high specificity, easy operation, efficient amplification and visualized readout, which offers a favorable point-of-care tool for E.coli O157: H7 or other pathogen detection in resource-constrained settings.