Survival of Listeria monocytogenes in Galotyri, a traditional Greek soft acid-curd cheese, stored aerobically at 4°C and 12°C

Microbiological Assessment of Dairy Products Produced by Small-Scale Dairy Producers in Serbia

The microbiological quality of dairy products from small-scale producers in Serbia was analysed. A total of 302 dairy products [raw (n = 111) and pasteurized milk cheeses (n = 79) and kajmak (n = 112)], were collected and tested for the presence of pathogens, Listeria monocytogenes and Salmonella spp., and enumerated for Coagulase-positive staphylococci (CPS), Escherichia coli, and yeasts and moulds. None of the samples tested positive for Salmonella spp., while L. monocytogenes was recovered from one raw milk cheese and five kajmak samples. Raw milk cheese and kajmak also had higher levels of indicator microorganisms, namely E. coli and yeast and moulds. Molecular serotyping grouped L. monocytogenes isolates into serogroups 1 (1/2a and 3a) and 3 (1/2b, 3b, and 7). When exposed to eight antibiotics, L. monocytogenes isolates were mostly sensitive, with the exception of oxacillin and reduced susceptibility to clindamycin, penicillin G, and trimethoprim/sulfamethoxazole, emphasizing the importance of continuous surveillance for antimicrobial resistance. Samples that tested positive for Listeria spp. also had higher loads of indicator microorganisms, namely E. coli and yeast and moulds, suggesting lapses in hygiene practices during production. Collectively, these data emphasize the need for improved food safety and hygiene practices among small-scale dairy producers. This is crucial to reduce the microbial contamination and improve both the quality and safety of dairy products in the Serbian market.

Pilot-Scale Production of Traditional Galotyri PDO Cheese from Boiled Ewes’ Milk Fermented with the Aid of Greek Indigenous Lactococcus lactis subsp. cremoris Starter and Lactiplantibacillus plantarum Adjunct Strains

The performance of a mixed thermophilic and mesophilic starter culture consisting of Streptococcus thermophilus ST1 and the Greek indigenous nisin-A-producing Lactococcus lactis subsp. cremoris M78 was evaluated in the absence (A: ST1+M78) or presence (B: ST1+M78+H25) of Lactiplantibacillus plantarum H25—another indigenous ripening strain—under real cheesemaking conditions. Three pilot-scale trials of fresh (6-day-old) Galotyri PDO cheese were made from boiled milk by an artisanal method using simple equipment, followed by cold ripening of the A1–A3 and B1–B3 cheeses at 4 °C for 30 days. All of the cheeses were analyzed microbiologically and for pH, gross composition, proteolysis, sugar and organic acid contents, and sensorial attributes before and after ripening. The artisanal (PDO) Galotyri manufacturing method did not ensure optimal growth of the ST1+M78 starter as regards the constant ability of the thermophilic strain ST1 to act as the primary milk acidifier under ambient (20–30 °C) fermentation conditions. Consequently, major trial-dependent microbial and biochemical differences between the Acheeses, and generally extended to the Bcheeses, were found. However, high-quality Galotyri was produced when either starter strain predominated in the fresh cheeses; only trial A1 had microbiological and sensory defects due to an outgrowth of post-thermal Gram-negative bacterial contaminants in the acidified curd. The H25 adjunct strain, which grew above 7 to 9 log CFU/g depending on the trial, had minor effects on the cheese’s pH, gross composition, and proteolysis, but it improved the texture, flavor, and the bacteriological quality of the Bcheeses during processing, and it exerted antifungal effects in the ripened cheeses.

Characterization and Antibiotic Resistance of Listeria monocytogenes Strains Isolated from Greek Myzithra Soft Whey Cheese and Related Food Processing Surfaces over Two-and-a-Half Years of Safety Monitoring in a Cheese Processing Facility

Listeriosis is a serious infectious disease with one of the highest case fatality rates (ca. 20%) among the diseases manifested from bacterial foodborne pathogens in humans, while dairy products are often implicated as sources of human infection with Listeria monocytogenes. In this study, we characterized phenotypically and genetically by whole-genome sequencing (WGS) 54 L. monocytogenes strains isolated from Myzithra, a traditional Greek soft whey cheese (48 isolates), and swabs collected from surfaces of a cheese processing plant (six isolates) in the Epirus region of Greece. All but one strain of L. monocytogenes belonged to the polymerase chain reaction (PCR) serogroups IIa (16.7%) and IIb (81.5%), corresponding to serotypes 1/2a, 3a and 1/2b, 3b, 7, respectively. The latter was identified as a PCR-serogroup IVb strain (1.8%) of serotypes 4b, 4d, 4e. Bioinformatics analysis revealed the presence of five sequence types (STs) and clonal complexes (CCs); ST1, ST3, ST121, ST 155, ST398 and CC1, CC3, CC121, CC155, CC398 were thus detected in 1.9, 83.3, 11.0, 1.9, and 1.9% of the L. monocytogenes isolates, respectively. Antibiograms of the pathogen against a panel of seven selected antibiotics (erythromycin, tetracycline, benzylpenicillin, trimethoprim-sulfamethoxazole, ampicillin, ciprofloxacin, and meropenem) showed that 50 strains (92.6%), the six surface isolates also included, were intermediately resistant to ciprofloxacin and susceptible to the rest of the six antimicrobial agents tested, whereas strong resistance against the use of a single from three implicated antibiotics was recorded to four strains (7.4%) of the pathogen isolated from Myzithra cheese samples. Thence, the minimum inhibitory concentrations (MICs) were determined for erythromycin (MIC = 0.19 μg/mL), ciprofloxacin (MIC ≥ 0.19 μg/mL), and meropenem (MIC = 0.64 μg/mL), and finally, just one strain was deemed resistant to the latter antibiotic. The phylogenetic positions of the L. monocytogenes strains and their genetic variability were determined through WGS, whilst also stress response and virulence gene analysis for the isolates was conducted. Findings of this work should be useful as they could be utilized for epidemiological investigations of L. monocytogenes in the food processing environment, revealing possible contamination scenarios, and acquired antimicrobial resistance along the food production chain.

Occurrence of Listeria spp. in Soft Cheese and Ice Cream: Effect of Probiotic Bifidobacterium spp. on Survival of Listeria monocytogenes in Soft Cheese

Listeria monocytogenes is one of the most important emerging foodborne pathogens. The objectives of this work were to investigate the incidence of Listeria spp. and L. monocytogenes in soft cheese and ice cream in Assiut city, Egypt, and to examine the effect of some probiotic Bifidobacterium spp. (Bifidobacterium breve, Bifidobacterium animalis, or a mixture of the two) on the viability of L. monocytogenes in soft cheese. The existence of Listeria spp. and L. monocytogenes was examined in 30 samples of soft cheese and 30 samples of ice cream. Bacteriological analyses and molecular identification (using 16S rRNA gene and hlyA gene for Listeria spp. and L. monocytogenes, respectively) were performed on those samples. Additionally, Bifidobacterium spp. were incorporated in the making of soft cheese to study their inhibitory impacts on L. monocytogenes. Out of 60 samples of soft cheese and ice cream, 25 samples showed Listeria spp., while L. monocytogenes was found in only 2 soft cheese samples. Approximately 37% of soft cheese samples (11 out of 30) had Listeria spp. with about 18.0% (2 out of 11) exhibiting L. monocytogenes. In ice cream samples, Listeria spp. was presented by 47% (14 out of 30), while L. monocytogenes was not exhibited. Moreover, the addition of B. animalis to soft cheese in a concentration of 5% or combined with B. breve with a concentration of 2.5% for each resulted in decreasing L. monocytogenes efficiently during the ripening of soft cheese for 28 d. Listeria spp. is widely found in milk products. Probiotic bacteria, such as Bifidobacterium spp., can be utilized as a natural antimicrobial to preserve food and dairy products.

Microbiological Characterization of Greek Galotyri Cheese PDO Products Relative to Whether They Are Marketed Fresh or Ripened

Galotyri is the most popular traditional Greek PDO soft acid-curd cheese. This study compared the microbial numbers and types and characterized the lactic acid bacteria (LAB) biota of two artisan-type Galotyri PDO cheese varieties, one marketed fresh (Brand-K) and the other ripened (Brand-Z). Two retail batches of each cheese variety were analyzed, and a total of 102 LAB isolates were biochemically identified. LAB (7.2–9.3 log CFU/g) prevailed in all cheeses, followed by yeasts (5.8–6.8 log CFU/g). Typical starter strains of Streptococcus thermophilus and Lactobacillus delbrueckii were the most abundant species in all batches. However, the fresh Brand-K cheeses had 1–3 log units higher thermophilic starter LAB counts than the ripened Brand-Z cheeses, which contained a more diverse viable LAB biota comprising Lacticaseibacillus paracasei, Leuconostocmesenteroides, Lentilactobacillus (L. diolivorans, L. kefiri, L. hilgardii), Pediococcusinopinatus/parvulus, few spontaneous nonstarter thermophilic streptococci and lactobacilli, and Enterococcus faecium and E. faecalis at higher subdominant levels.Conversely, the fresh Brand-K cheeses were enriched in members of the Lactiplantibacillus plantarum group; other LAB species were sporadically isolated, including Lactococcus lactis. All retail cheeses were safe (pH 3.9–4.0). No Salmonella spp. or Listeria monocytogenes were detected in 25-g samples by culture enrichment; however, Listeria innocua and coagulase-positive staphylococci (850 CFU/g) survived in one ripened batch. Gram-negative bacteria were <100 CFU/g in all cheeses. In conclusion, ripening reduced the starter LAB viability but increased the nonstarter LAB species diversity in the present Galotyri PDO market cheeses.

Inhibitory Effect of Lactiplantibacillusplantarum and Lactococcus lactis Autochtonous Strains against Listeria monocytogenes in a Laboratory Cheese Model

In the present study, six Lactococcus lactis and seven Lactiplantibacillus plantarum strains isolated from artisanal Sardinian dairy products were evaluated for their efficacy in controlling the growth of Listeria monocytogenes during the storage of miniature fresh cheese manufactured on a laboratory scale to exploit their possible use as biopreservatives. The strains were tested for antimicrobial activity and some technological characteristics before using them in miniature fresh cheese to evaluate their in situ antilisterial effect. Our results showed that five strains (L. lactis 16FS16-9/20234-11FS16 and Lpb. plantarum 1/14537-4A/20045) could be considered suitable candidates for use as protective cultures in fresh cheese manufacture since they significantly lowered the pathogen counts by 3–4 log units compared to the control; however, all strains tested were capable of decreasing L. monocytogenes numbers. Our results suggest that the single and combined action of the acidifying power and the production of bacteriocin of these strains was capable of controlling and/or reducing the growth of L. monocytogenes. Considering their technological characteristics, they might be used as starter/adjunct cultures to increase the safety of the products, perhaps in association with other antimicrobial hurdles.

Microbiological and Metagenomic Characterization of a Retail Delicatessen Galotyri-Like Fresh Acid-Curd Cheese Product

This study evaluated the microbial quality, safety, and ecology of a retail delicatessen Galotyri-like fresh acid-curd cheese traditionally produced by mixing fresh natural Greek yogurt with ‘Myzithrenio’, a naturally fermented and ripened whey cheese variety. Five retail cheese batches (mean pH 4.1) were analyzed for total and selective microbial counts, and 150 presumptive isolates of lactic acid bacteria (LAB) were characterized biochemically. Additionally, the most and the least diversified batches were subjected to a culture-independent 16S rRNA gene sequencing analysis. LAB prevailed in all cheeses followed by yeasts. Enterobacteria, pseudomonads, and staphylococci were present as <100 viable cells/g of cheese. The yogurt starters Streptococcus thermophilus and Lactobacillus delbrueckii were the most abundant LAB isolates, followed by nonstarter strains of Lactiplantibacillus, Lacticaseibacillus, Enterococcus faecium, E. faecalis, and Leuconostoc mesenteroides, whose isolation frequency was batch-dependent. Lactococcus lactis isolates were sporadic, except for one cheese batch. However, Lactococcus lactis, Enterobacteriaceae, Vibrionaceae, Salinivibrio, and Shewanellaceae were detected at fairly high relative abundances culture-independently, despite the fact that their viable counts in the cheeses were low or undetectable. Metagenomics confirmed the prevalence of S. thermophilus and Lb. delbrueckii. Overall, this delicatessen Galotyri-like cheese product was shown to be a rich pool of indigenous nonstarter LAB strains, which deserve further biotechnological investigation.

Determination of the growth potential of Listeria monocytogenes in various types of Belgian artisanal cheeses by challenge tests

Cheese potentially allowing the growth of Listeria monocytogenes must be free of the pathogen in 25 g before being put on the market, while 100 cfu/g is tolerated when the pathogen is unable to grow. Challenge tests were performed in order to assess the growth potential of L. monocytogenes in at least one batch of 32 Belgian cheese varieties from 32 factories. All varieties were grouped in four categories: unripened acid-curd cheeses, mold-ripened soft cheeses, smear-ripened soft cheeses and ripened semi-hard cheeses. Associated microflora and cheese physicochemical characteristics were also studied. A cocktail of three strains was used to inoculate cheese on the first day of shelf-life, and samples were stored until the end of shelf-life at 7-9 °C. Growth potential was considered as the difference (a) between median contamination at the end and at the beginning of the test or (b) between the highest value at the end of the test and the lowest value at its beginning. L. monocytogenes always decreased in unripened acid-curd cheeses but showed extended growth in 21 out of 25 batches of ripened soft cheese. Contrasting results were obtained for semi-hard cheeses, as important intra- and inter-batch variability was observed. For the latter, the recommended method based on medians to calculate the growth potential led to erroneous food safety considerations, and it should always be advised to focus on absolute levels.

Characterization of cottage cheese using Weissella cibaria D30: Physicochemical, antioxidant, and antilisterial properties

This study aimed to evaluate the potential of Weissella cibaria D30 as an adjunct culture in cottage cheese, including an assessment of antioxidant, antilisterial, and compositional parameters. Cottage cheese samples were manufactured using a commercial starter culture and probiotic strains Lactobacillus rhamnosus GG (GG) or W. cibaria D30 (W) and without probiotic (control). Samples were stored at 4 ± 1°C for 28 d. Bacterial cell counts (log cfu/g) of control, GG, and W samples were counted at 0, 7, 14, 21, and 28 d. Counts of W. cibaria D30 in the W samples remained at 6.85 log cfu/g after 28 d. Total solids, fat, protein, ash, and pH were measured and no significant differences were observed in compositional parameters or pH after 28 d of storage in all cheeses except those inoculated to Listeria monocytogenes. To measure the antilisterial effect, Listeria monocytogenes was inoculated into the cottage cheese samples and bacterial cell counts were obtained at 0, 6, 12, 24, 48, 72, 96, 120, and 144 h. Listeria monocytogenes counts were less than the analytical limit of detection (<10 cfu/g) in the inoculated GG and W samples, whereas the counts of L. monocytogenes in the inoculated control sample remained at 3.0 log cfu/g after 144 h. We used the DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS [2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] radical scavenging activity assays to assess antioxidant activity: GG and W samples exhibited significant increases in antioxidant activity compared with the control sample. These results indicate that W. cibaria D30 has potential as an adjunct culture in the dairy industry.

Controlled release of nisin from Neusilin particles to enhance food safety of sour curd cheese

Nisin is frequently added as food additive to soft cheese to increase food safety against foodborne pathogens like Listeria monocytogenes. The goal of this study was the extension of the antimicrobial activity of nisin in sour curd cheese (SCC) by self-releasing adsorbed nisin from Neusilin UFL2 over production-based pH shift. First, the antimicrobial activity of nisin adsorbed to Neusilin UFL2 (UFL2-N) and free nisin was investigated in BHI broth at a pH range from 7.5 to 4.5 for each of six L. monocytogenes field isolates. UFL2-N showed similar minimal inhibition concentration to L. monocytogenes over time as free nisin. Distribution of nebulized, fluorescence-labelled UFL2 was homogenous on SCC surface. Thereafter, SCC surface was inoculated with L. monocytogenes and 0.004, 0.013, 0.026, and 0.132 mg mL-1 UFL2-N or free nisin. In SCC, L. monocytogenes was below quantification limit at 0.132 mg mL-1 UFL2-N or free nisin after 2 days of ripening. Collectively, UFL2-N enabled a slow release and antilisterial activity in vitro as well as in cheese manufacturing.

Traditional Balkan fermented milk products

Traditional fermented milk products have been prepared since ancient time by various civilizations. Despite their long history, popularity, and nutritive and healthy value, the acceleration and industrialization of food production leads to increase of the diversity of fermented milk products in the Balkan Peninsula. As a result of the multitude of food-microbe combinations, there are thousands of different types of fermented milk products - yoghurts, yogurt-like products, and various types of cheeses with proven health benefits. Among those products is the domestic Bulgarian yoghurt "kiselo mlyako", whose anti-aging effect has been scientifically studied yet at the beginning of 20th century. The current review summerizes the wide range of traditional fermented milk products at the Balkan countries, which are the primary source for their production.

Hurdle factors minimizing growth of Listeria monocytogenes while counteracting in situ antilisterial effects of a novel nisin A-producing Lactococcus lactis subsp. cremoris costarter in thermized cheese milks

The capacity of growth, survival, and adaptive responses of an artificial contamination of a three-strain L. monocytogenes cocktail in factory-scale thermized (65 °C, 30 s) Graviera cheese milk (TGCM) was evaluated. Bulk TGCM samples for inoculation were sequentially taken from the cheese making vat before process initiation (CN-LM) and after addition of a commercial starter culture (CSC), the CSC plus the nisin A-producing (NisA+) costarter strain Lactococcus lactis subsp. cremoris M78 (CSC + M78), and all ingredients with the rennet last (CSC + M78-RT). Additional treatments included Listeria-inoculated TGCM samples coinoculated with the NisA+ costarter strain M78 in the absence of the CSC or with the CSC in previously sterilized TGCM to inactivate the background microbiota (CSC-SM). All cultures were incubated at 37 to 42 °C for 6 h, followed by additional 66 h at 22 °C, and 48 h at 12 °C after addition of 2% edible salt. L. monocytogenes failed to grow and declined in all CSC-inoculated treatments after 24 h. In contrast, the pathogen increased by 3.34 and 1.46 log units in the CN-LM and the CSC-SM treatments, respectively, indicating that the background microbiota or the CSC alone failed to suppress it, but they did so synergistically. Supplementation of the CSC with the NisA+ strain M78 did not deliver additional antilisterial effects, because the CSC Streptococcus thermophilus reduced the growth prevalence rates and counteracted the in situ NisA+ activity of the costarter. In the absence of the CSC, however, strain M78 predominated and caused the strongest in situ nisin-A mediated effects, which resulted in the highest listerial inactivation rates after 24 to 72 h at 22 °C. In all TGCM treatments, however, L. monocytogenes displayed a "tailing" survival (1.63 to 1.96 log CFU/mL), confirming that this pathogen is exceptionally tolerant to cheese-related stresses, and thus, can't be easily eliminated.

Viability of and Escherichia coli O157:H7 and Listeria monocytogenes in a delicatessen appetizer (yogurt-based) salad as affected by citrus extract (Citrox©) and storage temperature

The antimicrobial effect of citrus extract (at 1 mL/kg [C1] and 2 mL/kg [C2]) on naturally occurring microbiota and inoculated pathogens (E. coli O157:H7 and L. monocytogenes at ca. 6 log cfu/g) in the traditional Greek yogurt-based salad Tzatziki stored at 4, 10, or 21 °C, was examined. Lactic acid bacteria (LAB) were high (8.0-8.5 log cfu/g) and varied only minimally for both the control (untreated) and the citrus extract-treated salad samples, whereas the higher citrus extract concentration yielded the lowest yeast populations, irrespective of temperature, during the entire storage period. Populations of inoculated E. coli (6 log cfu/g) declined in both untreated and citrus extract-treated samples from day 0-70, 35, and 15 at 4, 10, and 21 °C, respectively. Citrus extract had a significant effect on the survival of the inoculated E. coli O157:H7, with reductions of 2.8-4.8 log cfu/g in the citrus extract-treated samples at the end of the storage period. Our data show that L. monocytogenes survived in both untreated and citrus extract-treated samples during the entire storage period, irrespective of the storage temperature. The higher concentration of citrus extract had a significant effect on the survival of L. monocytogenes in the treated samples, and reductions of 1.5-3.0 logs were noted on final day 70, 35 and 15 at 4, 10 and 21 °C, respectively. The results of our study demonstrated the potential of citrus extract as a natural compound that can control the growth of food-borne pathogenic bacteria, such as E. coli O157:H7 and L. monocytogenes in Tzatziki, a yogurt-based salad.

Predicting and Modelling the Growth of Potentially Pathogenic Bacteria in Coalho Cheese

Coalho is a semihard medium- to high-moisture cheese produced in various states in the northeastern region of Brazil. This study was conducted to predict the growth kinetics (maximum growth rate, Grmax) of Escherichia coli, Listeria monocytogenes, Salmonella, and Staphylococcus aureus using the ComBase predictor with various combinations of temperature, pH, and water activity (a_w) in commercial Coalho cheese samples. The growth of two antibiotic-resistant derivative strains of L. monocytogenes (parental strains ATCC 19115 and ATCC 7644) and S. aureus (parental strains ATCC 13565 and ATCC 19095) was measured in commercial Coalho cheese samples during 14 days of storage as a function of the initial contamination level (3 and 5 log CFU/g) and storage temperature (7.5 and 12°C). The highest Grmax values predicted by ComBase under the various conditions of temperature, pH, and a_w were for L. monocytogenes (0.006 to 0.065 log CFU/g/h) and S. aureus (0.003 to 0.048 log CFU/g/h). The Grmax values predicted by ComBase for E. coli and Salmonella were 0.007 to 0.026 and 0.008 to 0.041 log CFU/g/h, respectively. An experimental challenge in Coalho cheese revealed that the populations of all tested antibiotic-resistant derivative strains of L. monocytogenes and S. aureus increased (>0.5 log CFU/g) by day 14 of storage at 7.5 or 12°C. L. monocytogenes and S. aureus had higher Grmax values in cheese samples stored at 12°C than those stored at 7.5°C. The ComBase growth predictions under the temperature, pH, and a_w conditions in commercial Coalho cheese samples were generally fail-safe for predicting the growth of L. monocytogenes and S. aureus in the actual product. These results indicate that Coalho cheese has pH and a_w characteristics that allow the growth of E. coli, L. monocytogenes, Salmonella, and S. aureus. These cheeses are typically stored at temperatures that do not prevent the growth of these bacteria.

Performance of two alternative methods for Listeria detection throughout Serro Minas cheese ripening

The ability of pathogens to survive cheese ripening is a food-security concern. Therefore, this study aimed to evaluate the performance of two alternative methods of analysis of Listeria during the ripening of artisanal Minas cheese. These methods were tested and compared with the conventional method: Lateral Flow System™, in cheeses produced on laboratory scale using raw milk collected from different farms and inoculated with Listeria innocua; and VIDAS^®-LMO, in cheese samples collected from different manufacturers in Serro, Minas Gerais, Brazil. These samples were also characterized in terms of lactic acid bacteria, coliforms and physical–chemical analysis. In the inoculated samples, L. innocua was detected by Lateral Flow System™ method with 33% false-negative and 68% accuracy results. L. innocua was only detected in the inoculated samples by the conventional method at 60-days of cheese ripening. L. monocytogenes was not detected by the conventional and the VIDAS^®-LMO methods in cheese samples collected from different manufacturers, which impairs evaluating the performance of this alternative method. We concluded that the conventional method provided a better recovery of L. innocua throughout cheese ripening, being able to detect L. innocua at 60-day, aging period which is required by the current legislation.

Addition to thermized milk of Lactococcus lactis subsp. cremoris M104, a wild, novel nisin a-producing strain, replaces the natural antilisterial activity of the autochthonous raw milk microbiota reduced by thermization

Recent research has shown that mild milk thermization treatments routinely used in traditional Greek cheese production are efficient to inactivate Listeria monocytogenes and other pathogenic or undesirable bacteria, but they also inactivate a great part of the autochthonous antagonistic microbiota of raw milk. Therefore, in this study, the antilisterial activity of raw or thermized (63°C, 30 s) milk in the presence or absence of Lactococcus lactis subsp. cremoris M104, a wild, novel, nisin A-producing (Nis-A+) raw milk isolate, was assessed. Bulk milk samples were taken from a local cheese plant before or after thermization and were inoculated with a five-strain cocktail of L. monocytogenes (approximately 4 log CFU/ml) or with the cocktail, as above, plus the Nis-A+ strain (approximately 6 log CFU/ml) as a bioprotective culture. Heat-sterilized (121°C, 5 min) raw milk inoculated with L. monocytogenes was used as a control treatment. All milk samples were incubated at 37°C for 6 h and then at 18°C for an additional 66 h. L. monocytogenes grew abundantly (>8 log CFU/ml) in heat-sterilized milk, whereas its growth was completely inhibited in all raw milk samples. Conversely, in thermized milk, L. monocytogenes increased by 2 log CFU/ml in the absence of strain M104, whereas its growth was completely inhibited in the presence of strain M104. Furthermore, nisin activity was detected only in milk samples inoculated with strain M104. Thus, postthermal supplementation of thermized bulk milk with bioprotective L. lactis subsp. cremoris cultures replaces the natural antilisterial activity of raw milk reduced by thermization.

Contributions of σ(B) and PrfA to Listeria monocytogenes salt stress under food relevant conditions

Listeria monocytogenes is well known to survive and grow under several stress conditions, including salt stress, which is important for growth in certain foods as well as for host infection. To characterize the contributions, to salt stress response, of transcriptional regulators important for stress response and virulence (i.e., σ(B) and PrfA), we analyzed three L. monocytogenes parent strains and isogenic mutants (ΔsigB, ΔprfA, and ΔsigBΔprfA), representing different serotypes and lineages, for their ability to grow, at 25°C, in BHI with 1.9 M NaCl. With regard to growth rate, only the lineage IV strain presented a significant difference between the parent strain and both of its respective mutants lacking prfA (ΔprfA and ΔsigBΔprfA). Conversely, the lineage I and II parent strains showed significantly shorter lag phase in comparison to their respective ΔsigB mutant strains. Intestinal epithelial cell invasion assay and hemolytic activity assays showed a significant role for σ(B) in the former and for PrfA in the latter. To explore the mechanism that may contribute to the extended lag phase in the ΔsigB mutant strain and survival and growth of the parent strain upon salt shock, whole genome transcription profiling was performed to compare transcript levels between the lineage I, serotype 1/2b, parent strain and its isogenic ΔsigB mutant after 30 min of lag phase growth at 25°C in the presence of 1.9M NaCl (salt shock) without aeration. Microarray data showed significantly higher transcript levels for 173 genes in the parent strain as compared to the ΔsigB strain. Overall, 102 of the 173 σ(B) up-regulated genes had been identified in previous studies, indicating that 71 genes were newly identified as being up-regulated by σ(B) in this study. We hypothesize that, among these genes newly identified as σ(B) up-regulated, four genes (lmo2174, lmo0530, lmo0527 and lmo0529) may play a major role in response to salt stress. Lmo2174 contains domains that facilitate sensing and producing a transduction signal in the form of cyclic di-GMP, which may activate the enzymes Lmo0527, Lmo0529 and Lmo0530, which encode proteins similar to those responsible for synthesis of exopolysaccharides that may protect the cell by changing the cell wall structure during salt stress. Overall, our data showed that σ(B), but not PrfA, contributes to growth under salt stress. Moreover, we show that the σ(B) regulon of a L. monocytogenes lineage I strain challenged with salt shock includes salt stress-specific as well as previously unidentified σ(B) up-regulated genes.

The Microfloras of Traditional Greek Cheeses

Many traditional cheeses are made in Greece. Some of them are, in fact, types of the same cheese variety, whether or not they have different cheesemaking technologies, but are known by different local names. Twenty of them have been granted protected designation of origin status. In the 8th century BCE, Homer described a cheese thought to be the ancestor of feta, the main cheese manufactured in Greece from the ancient times until today. Meanwhile, various cheese types evolved through the centuries, and almost every area in Greece has its own cheesemaking tradition. Some cheese varieties are local, handcrafted products whose production has been handed down from generation to generation, and without interest in their continued production, these varieties will disappear. Other local varieties are made at small factories from pasteurized milk and commercial rennet and starter and are very different from the traditional versions. However, some milk producers still make their cheeses at home or at small dairies from raw milk, without any starter, or sometimes from thermized milk, with traditional yogurt as the starter. Their cheeses are the basis for the information presented in this review.

Survival of Listeria monocytogenes during Production and ripening of traditional Turkish Savak tulum cheese and in synthetic gastric fluid

This study investigated the survival and acid tolerance of Listeria monocytogenes during the 2-day processing stage and 90-day ripening of Savak tulum cheese, a traditional cheese in Turkey. Experimental Savak tulum cheese was produced from raw sheep's milk that was inoculated with a L. monocytogenes mixture consisting of five strains (average 7.0 log CFU/ml) and was ripened at 6°C for 90 days. Microbiological and chemical analyses and acid exposure experiments in synthetic gastric fluid (SGF) (pH 1.5 to 2.5) were carried out on days 1 and 2 during processing and on days 0, 15, 30, 45, 60, and 90 during ripening. The numbers of L. monocytogenes did not decrease during processing, but a total of 4.1 log CFU/g reduction was observed during ripening. Throughout the ripening period, L. monocytogenes cells survived direct 90-min exposures of the cheese samples to SGF. These results suggest that, although the pathogen numbers decreased in Savak tulum cheese ripened at 6°C for 90 days, a sublethal environment may have occurred in the cheese during the production stage, activating the acid-tolerance mechanisms of the pathogen and allowing L. monocytogenes to maintain its viability in the SGF for 90 min.

Modeling the growth of Listeria monocytogenes in mold-ripened cheeses

This study presents possible applications of predictive microbiology to model the safety of mold-ripened cheeses with respect to bacteria of the species Listeria monocytogenes during (1) the ripening of Camembert cheese, (2) cold storage of Camembert cheese at temperatures ranging from 3 to 15°C, and (3) cold storage of blue cheese at temperatures ranging from 3 to 15°C. The primary models used in this study, such as the Baranyi model and modified Gompertz function, were fitted to growth curves. The Baranyi model yielded the most accurate goodness of fit and the growth rates generated by this model were used for secondary modeling (Ratkowsky simple square root and polynomial models). The polynomial model more accurately predicted the influence of temperature on the growth rate, reaching the adjusted coefficients of multiple determination 0.97 and 0.92 for Camembert and blue cheese, respectively. The observed growth rates of L. monocytogenes in mold-ripened cheeses were compared with simulations run with the Pathogen Modeling Program (PMP 7.0, USDA, Wyndmoor, PA) and ComBase Predictor (Institute of Food Research, Norwich, UK). However, the latter predictions proved to be consistently overestimated and contained a significant error level. In addition, a validation process using independent data generated in dairy products from the ComBase database (www.combase.cc) was performed. In conclusion, it was found that L. monocytogenes grows much faster in Camembert than in blue cheese. Both the Baranyi and Gompertz models described this phenomenon accurately, although the Baranyi model contained a smaller error. Secondary modeling and further validation of the generated models highlighted the issue of usability and applicability of predictive models in the food processing industry by elaborating models targeted at a specific product or a group of similar products.

The fate of indigenous microbiota, starter cultures, Escherichia coli, Listeria innocua and Staphylococcus aureus in Danish raw milk and cheeses determined by pyrosequencing and quantitative real time (qRT)-PCR

The purpose of this work was to study the bacterial communities in raw milk and in Danish raw milk cheeses using pyrosequencing of tagged amplicons of the V3 and V4 regions of the 16S rDNA and cDNA. Furthermore, the effects of acidification and ripening starter cultures, cooking temperatures and rate of acidification on survival of added Escherichia coli, Listeria innocua and Staphylococcus aureus in cheeses at different stages of ripening were studied by pyrosequencing and quantitative real time (qRT)-PCR. A high diversity of bacterial species was detected in raw milk. Lactococcus lactis, Streptococcus thermophilus, Lactobacillus casei and Lactobacillus rhamnosus were the main bacteria detected in raw milk and cheeses. Bacteria belonging to the genera Brevibacterium, Staphylococcus, Escherichia, Weissella, Leuconostoc, Pediococcus were also detected in both 16S rDNA and cDNA obtained from raw milk and cheeses. E. coli, which was added to milk used for production of some cheeses, was detected in both DNA and RNA extracted from cheeses at different stages of ripening showing the highest percentage of the total sequence reads at 7 days of ripening and decreased again in the later ripening stages. Growth of E. coli in cheeses appeared to be affected by the cooking temperature and the rate of acidification but not by the ripening starter cultures applied or the indigenous microbiota of raw milk. Growth of L. innocua and S. aureus added to milks was inhibited in all cheeses at different stages of ripening. The use of 16S rRNA gene pyrosequencing and qRT-PCR allows a deeper understanding of the behavior of indigenous microbiota, starter cultures and pathogenic bacteria in raw milk and cheeses.

Technological characterization of bacteriocin producing Lactococcus lactis strains employed to control Listeria monocytogenes in cottage cheese

In recent years, there has been a particular focus on the application of antimicrobial compounds produced by lactic acid bacteria (LAB) as natural preservatives to control the growth of spoilage and pathogenic bacteria in food. Bacteriocins are antimicrobial peptides which can be added to foods in concentrated forms as food preservatives, e.g. additives, or they can be produced in situ by starters or protective cultures. In this study, twenty Lactococcus lactis bacteriocin producers previously isolated from Italian fermented foods were subjected to a variety of physical and biochemical tests in order to identify those with the greatest potential as starter cultures in cheese production. Of these, four strains isolated from cheese (one nisin Z producer, one nisin A producer and two lacticin 481 producers) which fulfilled the desired technological criteria were assessed for their ability to control Listeria monocytogenes. The subsequent application of these bacteriocinogenic strains as starter cultures in Cottage cheese established that the nisin A producing Lact. lactis 40FEL3, and to a lesser extent the lacticin 481 producers 32FL1 and 32FL3, successfully controlled the growth of the pathogen. This is the first study to directly compare the ability of nisin A, nisin Z and lacticin 481 producing strains to control listerial growth during the manufacture and storage of Cottage cheese.

Potential of nisin-incorporated sodium caseinate films to control Listeria in artificially contaminated cheese

A sodium caseinate film containing nisin (1000 IU/cm(2)) was produced and used to control Listeria innocua in an artificially contaminated cheese. Mini red Babybel cheese was chosen as a model semi-soft cheese. L. innocua was both surface- and in-depth inoculated to investigate the effectiveness of the antimicrobial film as a function of the distance from the surface in contact with the film. The presence of the active film resulted in a 1.1 log CFU/g reduction in L. innocua counts in surface-inoculated cheese samples after one week of storage at 4 degrees C as compared to control samples. With regard to in-depth inoculated cheese samples, antimicrobial efficiency was found to be dependent on the distance from the surface in contact with the active films to the cheese matrix. The inactivation rates obtained were 1.1, 0.9 and 0.25 log CFU/g for distances from the contact surface of 1 mm, 2 mm and 3 mm, respectively. Our study demonstrates the potential application of sodium caseinate films containing nisin as a promising method to overcome problems associated with post-process contamination, thereby extending the shelf life and possibly enhancing the microbial safety of cheeses.

Assuring growth inhibition of listerial contamination during processing and storage of traditional Greek Graviera cheese: compliance with the new European Union regulatory criteria for Listeria monocytogenes

The current microbiological regulatory criteria in the European Union specify a maximum Listeria monocytogenes population of 100 CFU/g allowable in ready-to-eat foods provided the product will not exceed this limit throughout its shelf life. The aim of this study was to validate the manufacturing method for traditional Greek Graviera cheese produced from thermized milk. Initial challenge experiments evaluated the fate of inoculated L. monocytogenes (ca. 4 log CFU/ml, three-strain cocktail) in thermized Graviera cheese milk (TGCM; 63 degrees C for 30 s) in the presence and absence of a product-specific starter culture (SC) in vitro. Milk samples were incubated for 6 h at 37 degrees C and then for 66 h at 18 degrees C. Experiments were conducted to evaluate the fate of a cocktail of three nonpathogenic L. monocytogenes and L. innocua indicator strains inoculated (ca. 3 log CFU/g) in Graviera cheeses commercially manufactured from TGCM+SC. Cheeses were brined, ripened at 18 degrees C and 90% relative humidity for 20 days, and stored at 4 degrees C for up to day 60 under vacuum. In TGCM, L. monocytogenes increased by ca. 2 log units, whereas in TGCM+SC L. monocytogenes growth was retarded (P < 0.05) after a ca. 1-log increase within 6 h at 37 degrees C. Populations of Listeria indicator strains did not grow in TGCM+SC cheeses at any stage; they declined 10-fold in fresh cheeses within 5 days and then survived with little death thereafter. Thus, growth inhibition but not inactivation of potent natural Listeria contaminants at levels below 100 CFU/g occurs in the core of traditional Greek Graviera cheese during fermentation, ripening, and storage.