Compatibility of Commercially Produced Protective Cultures with Common Cheesemaking Cultures and Their Antagonistic Effect on Foodborne Pathogens

A Snapshot, Using a Multi-Omic Approach, of the Metabolic Cross-Talk and the Dynamics of the Resident Microbiota in Ripening Cheese Inoculated with Listeria innocua

Raw milk cheeses harbor complex microbial communities. Some of these microorganisms are technologically essential, but undesirable microorganisms can also be present. While most of the microbial dynamics and cross-talking studies involving interaction between food-derived bacteria have been carried out on agar plates in laboratory-controlled conditions, the present study evaluated the modulation of the resident microbiota and the changes of metabolite production directly in ripening raw milk cheese inoculated with Listeria innocua strains. Using a proxy of the pathogenic Listeria monocytogenes, we aimed to establish the key microbiota players and chemical signals that characterize Latteria raw milk cheese over 60 days of ripening time. The microbiota of both the control and Listeria-inoculated cheeses was analyzed using 16S rRNA targeted amplicon sequencing, while direct analysis in real time mass spectrometry (DART-HRMS) was applied to investigate the differences in the metabolic profiles of the cheeses. The diversity analysis showed the same microbial diversity trend in both the control cheese and the inoculated cheese, while the taxonomic analysis highlighted the most representative genera of bacteria in both the control and inoculated cheese: Lactobacillus and Streptococcus. On the other hand, the metabolic fingerprints revealed that the complex interactions between resident microbiota and L. innocua were governed by continuously changing chemical signals. Changes in the amounts of small organic acids, hydroxyl fatty acids, and antimicrobial compounds, including pyroglutamic acid, hydroxy-isocaproic acid, malic acid, phenyllactic acid, and lactic acid, were observed over time in the L. innocua-inoculated cheese. In cheese that was inoculated with L. innocua, Streptococcus was significantly correlated with the volatile compounds carboxylbenzaldheyde and cyclohexanecarboxylic acid, while Lactobacillus was positively correlated with some volatile and flavor compounds (cyclohexanecarboxylic acid, pyroxidal acid, aminobenzoic acid, and vanillic acid). Therefore, we determined the metabolic markers that characterize a raw milk cheese inoculated with L. innocua, the changes in these markers with the ripening time, and the positive correlation of flavor and volatile compounds with the resident microbiota. This multi-omics approach could suggest innovative food safety strategies based on the enhanced management of undesirable microorganisms by means of strain selection in raw matrices and the addition of specific antimicrobial metabolites to prevent the growth of undesirable microorganisms.

Growth of methicillin-resistant Staphylococcus aureus during raw milk soft cheese-production and the inhibitory effect of starter cultures

The consumption of raw milk or raw milk products might be a potential risk factor for the transmission of methicillin-resistant Staphylococcus aureus (MRSA). Therefore, we studied MRSA growth during raw milk soft cheese-production. Furthermore, we investigated the inhibitory effect of four starter cultures (Lactococcus lactis, Lacticaseibacillus rhamnosus, Lactiplantibacillus plantarum, Lactobacillus helveticus) on the growth of MRSA in a spot-agar-assay and in raw milk co-culture following a cheesemaking temperature profile. During the initial phases of raw milk cheese-production, MRSA counts increased by 2 log units. In the ripening phase, MRSA counts only dropped slightly and remained high up to the end of the storage. Comparable MRSA counts were found in the rind and core and strain-specific differences in survival were observed. In the spot-agar-assay, all four starter cultures showed strong or intermediate inhibition of MRSA growth. In contrast, in raw milk, only Lactococcus lactis strongly inhibited MRSA, whereas all other starter cultures only had minor inhibitory effects on MRSA growth. Our results indicate that MRSA follow a similar growth pattern as described for other S. aureus during raw milk soft cheese-production and illustrate the potential use of appropriate starter cultures to inhibit MRSA growth during the production of raw milk cheese.

Effect of storage temperature and produce type on the survival or growth of Listeria monocytogenes on peeled rinds and fresh-cut produce

Whole and fresh-cut produce are minimally processed and susceptible to microbial contamination. This study evaluated the survival or growth of L. monocytogenes on peeled rinds, and fresh-cut produce at different storage temperatures. Fresh-cut fruits and vegetables, including cantaloupe, watermelon, pear, papaya, pineapple, broccoli, cauliflower, lettuce, bell pepper, and kale (25 g pieces) were spot inoculated with 4 log CFU/g of L. monocytogenes and stored at 4 or 13°C for 6  days. Cantaloupe and bell pepper rind disks (20 cm²), mimicking whole produce were inoculated with low inoculum level (4 log CFU/mL) and high inoculum level (6 log CFU/mL) and stored at 24°C up to 8  days and 4°C up to 14  days, respectively. L. monocytogenes counts on fresh-cut pear samples stored at 4°C increased significantly by 0.27 log CFU/g. However, Listeria levels on kale (day 4), cauliflower (day 6), and broccoli (day 2) were significantly reduced by 0.73, 1.18, and 0.80 log CFU/g, respectively, at 4°C. At 13°C, the bacterial counts increased significantly after a day of storage on fresh-cut watermelons (increasing by 1.10 log CFU/g) and cantaloupes (increasing by 1.52 log CFU/g). Similar increases were observed on pears (1.00 log CFU/g), papayas (1.65 log CFU/g), and green bell peppers (1.72 log CFU/g). Pineapple samples did not support the growth of L. monocytogenes at 13°C with a significant reduction of 1.80 log CFU/g by day 6. L. monocytogenes levels significantly increased in fresh-cut lettuce at 13°C but remained stable on kale, cauliflower, and broccoli after 6  days of storage. Stable population was observed also on cantaloupe rinds up to 8  days at 24°C. While on the outer surface of bell peppers, the population level decreased below the detectable limit of the test (10 CFU/20 cm²) after 14  days of storage at 4°C. The results demonstrated variable survival behavior of L. monocytogenes on fresh-cut produce with produce type and storage temperature.

Isolation and Characterization of Competitive Exclusion Microorganisms from Animal Wastes-Based Composts against Listeria monocytogenes

AIM

To isolate the slow-growing or viable but non-culturable competitive exclusion (CE) microorganisms from composts and then verify the anti-Listeria monocytogenes activities of those CE isolates in compost.

METHODS AND RESULTS

CE strains were isolated from composts using double- or triple-layer agar methods, purified, and then characterized. Both compost extracts and solid compost samples were spiked with a cocktail of 3 L. monocytogenes strains which were co-inoculated with or without CE strain cocktail and incubated at both 22°C and 35°C for 168 h. Results indicated that the addition of resuscitation promoting factor (Rpf) promoted the growth of slow-growing species from composts. About 50% of the isolated CE strains (n=40) were identified as Bacillus spp., 17 strains can inhibit more than ten tested L. monocytogenes strains, and 9 strains were motile and competitive biofilm formers. In compost extracts, the growth potentials of L. monocytogenes were reduced up to 2.2 logs when co-culturing with CE strains. In compost samples, the addition of CE strains reduced L. monocytogenes population by ca. 1.3 log CFU/g at 22°C after 24 to 168 h incubation.

CONCLUSION

Our modified double/triple-layer agar procedure with Rpf as growth supplement coupled with spot-on-lawn testing can be a quick and efficient method for isolating CE candidates from composts. The efficacy of CE strains against L. monocytogenes in compost extracts and compost samples was affected by compost type, nutrient level, and incubation temperature.

SIGNIFICANCE AND IMPACT OF THE STUDY

Compost is a rich source of CE microorganisms and compost-adapted CE microorganisms have the potential as a biological agent to control L. monocytogenes in agricultural environments.

Probiotic potential of commercial dairy-associated protective cultures: In vitro and in vivo protection against Listeria monocytogenes infection

Protective bacterial cultures (PCs) are commercially available to producers to control undesirable microbes in foods, including foodborne pathogens such as Listeria monocytogenes. They are generally recognized as safe for consumption and many are capable of producing bacteriocins. Yet their potential to act as probiotics and confer a health benefit on the host is not known. This study investigated the ability of three commercial PCs to survive human gastrointestinal conditions and exert anti-infective properties against L. monocytogenes. Counts of two PCs of Lactiplantibacillus plantarum remained unchanged after exposure to simulated gastrointestinal conditions, whereas counts of the PC Lactococcus lactis subsp. lactis were reduced by 5.3 log CFU/mL. Cultures of Lactiplantibacillus plantarum and Lactococcus lactis subsp. lactis adhered to human Caco-2 epithelial cells at ∼ 6 log CFU/mL. This pretreatment reduced subsequent L. monocytogenes adhesion and invasion by 1-1.6 log CFU/mL and 3.8-4.9 log CFU/mL, respectively, compared to control. L. monocytogenes-induced cytotoxicity was also reduced from 29.1% in untreated monolayers to ∼ 8% in those treated with PCs. Pretreatment of Caco-2 monolayers with Lactococcus lactis subsp. lactis and one PC of Lactiplantibacillus plantarum reduced L. monocytogenes translocation by ≥ 1.2 log CFU/mL compared to control (≥ 94.5% inhibition). All PCs significantly reduced Dextran^FITC permeability through Caco-2 monolayers to approximately half that of control. Pretreatment with PCs also reduced L. monocytogenes-induced mortality in Caenorhabditis elegans. These findings demonstrate the potential for commercially produced PCs to exert probiotic effects in the host through protection against L. monocytogenes infection, thus providing an additional benefit to food safety beyond inhibiting pathogen growth, survival, and virulence in foods.

Effect of Commercial Protective Cultures and Bacterial Fermentates on Listeria monocytogenes Growth in a Refrigerated High-Moisture Model Cheese

ABSTRACT

Biopreservatives are clean-label ingredients used to control pathogenic and spoilage microorganisms in ready-to-eat foods, including cheese. In a first set of experiments, the efficacies of six commercial biopreservatives in controlling Listeria monocytogenes growth at 4°C were tested in a high-moisture model cheese (pH 6.00, 56% moisture, and 1.25% salt) made of cream, micellar casein, water, salt, lactose, lactic acid, and a single protective culture (PC-1, PC-2, or PC-3 at 106 CFU/g [target]) or bacterial fermentate (CM-1 or CM-2 [cultured milk] or CSV-1 [cultured sugar-vinegar blend], 0.5 or 1.0% target level). Cheeses were inoculated with 3 log CFU/g L. monocytogenes (5-strain cocktail), after which 25-g samples were vacuum sealed and stored at 4°C for 8 weeks. L. monocytogenes populations from triplicate samples were enumerated weekly on modified Oxford agar in duplicate trials. L. monocytogenes growth (≥1-log increase) was observed in approximately 1 week in control cheese and those formulated with 106 CFU of PC-1 or PC-2 per g. Growth was delayed to 2.5 weeks in model cheeses formulated with 106 CFU of PC-3 per g or 0.5% CM-2 and to 3 weeks with 0.5% CM-1 or CSV-1. Growth was further delayed to 6.5 to 7.5 weeks in model cheeses formulated with 1.0% CM-1 or CM-2, while formulation with 1.0% CSV-1 inhibited L. monocytogenes growth for 8 weeks. In a second set of experiments, the combined effects of pH and 0.5% CSV-1 on L. monocytogenes inhibition were investigated. Incorporation of 0.5% CSV-1 delayed L. monocytogenes growth to 3, 6, and >10 weeks in cheeses of pH 6.00, 5.75, and 5.50, respectively, versus growth observed in 1, 1, and 3.5 weeks in control cheeses. These data suggest that certain fermentates have greater antilisterial activity than protective cultures in directly acidified cheeses with direct biopreservative incorporation and refrigerated storage. Further research is needed to optimize the conditions to prevent listerial growth by utilizing protective cultures in fresh, soft cheeses.

HIGHLIGHTS

Monitoring Growth Compatibility and Bacteriocin Gene Transcription of Adjunct and Starter Lactic Acid Bacterial Strains in Milk

ABSTRACT

When developing protective starter cultures for application in cheese technologies, monitoring growth interactions between starter and adjunct lactic acid bacterial (LAB) species and in situ expression of bacteriocin genes in the mixtures is crucial. This study first aimed to monitor the growth of mixed LAB strain populations during milk model fermentations by microbial counts and real-time quantitative PCR. The primary starter strains, Streptococcus thermophilus ST1 and costarter Lactococcus lactis subsp. cremoris M78, served as the basic starter composite coinoculated in all milk treatments. Adjunct bacteriocinogenic Enterococcus faecium strains KE82 and GL31 and the ripening Lactiplantibacillus plantarum H25 strain were added separately to the starter composite, resulting in four LAB combination treatments. The second aim was to quantify gene transcripts of nisin and enterocins B and A synthesized by strains M78, KE82, and GL31, respectively, by reverse transcription-real-time quantitative PCR and to detect the in situ antilisterial effects of the cocultures. Adjunct LAB strains showed growth compatibility with the starter, since all of them exhibited 2- to 3-log-unit increases in their population levels compared to their initial inoculation levels, with ST1 prevailing in all treatments. KE82 grew more competitively than GL31, whereas cocultures with KE82 displayed the strongest in situ antilisterial activity. Nisin gene expression levels were higher at the exponential phase of microbial growth in all treatments. Finally, the expression levels of nisin and enterocin A and B genes were interrelated, indicating an antagonistic activity.

HIGHLIGHTS

Application of Enterococcus faecium KE82, an Enterocin A-B-P-Producing Strain, as an Adjunct Culture Enhances Inactivation of Listeria monocytogenes during Traditional Protected Designation of Origin Galotyri Processing

ABSTRACT

The ability of the enterocin A-B-P-producing Enterococcus faecium KE82 adjunct strain to inactivate Listeria monocytogenes during protected designation of origin Galotyri processing was evaluated. Three trials were conducted with artisan cheeses made from traditionally "boiled" (85°C) ewe's milk. The milk was cooled at 42°C and divided in two treatments. A1 milk was inoculated with Streptococcus thermophilus ST1 and Lactococcus lactis subsp. cremoris M78, and A2 was inoculated with the basic starter ST1+M78 plus KE82 (step 1). All milks were fermented at 20 to 22°C for 24 h (step 2), and the curds were drained at 12°C for 72 h (step 3) and then salted with 1.5 to 1.8% salt to obtain the fresh Galotyri cheeses (step 4). These fresh cheeses were then ripened at 4°C for 30 days (step 5). Because artificial listerial contamination in the dairy plant was prohibited, samples of A1 and A2 cheese milk (200 mL) or curd (200 g) were collected after steps 1 through 5, inoculated with L. monocytogenes 10 (3 to 4 log CFU/mL or g), incubated at 37, 22, 12, and 4°C for predefined periods, and analyzed for microbial levels and pH. L. monocytogenes levels declined in all cheese curd portions contaminated after steps 2 through 5 (pH 4.36 to 4.84) when stored at 4 or 12°C for 15 days. The final net reductions in Listeria populations were 2.00-, 1.07-, 0.54-, and 0.61-log greater in the A2 than in the A1 curd portions after steps 2, 3, 4, and 5, respectively. In step 1, conducted to simulate the whole cheese milk fermentation process, L. monocytogenes levels declined by 1.47 log CFU/mL more in the A2 than in the A1 milk portions after 72 h at 22°C; however, slight growth (0.6 log CFU/mL) occurred during the first 6 h at 37°C. E. faecium KE82 was compatible with the starter culture and enhanced inactivation of L. monocytogenes during all steps of Galotyri cheese processing. The antilisterial effects of the combined acid and enterocin were the weakest in the fermenting milks, the strongest in the unsalted fermented curds, and declined again in the salted fresh cheeses.

HIGHLIGHTS

Application of bioactive glycolipids to control Listeria monocytogenes biofilms and as post-lethality contaminants in milk and cheese

Listeria monocytogenes can form persistent biofilms on food processing surfaces, resulting in cross-contamination of food products, including milk and milk products. Natural glycolipids are a promising intervention to control undesirable microbes due to their antimicrobial activity and low toxicity. This study aimed to determine the antimicrobial activity of glycolipids to control L. monocytogenes biofilms as well as in milk and on Queso Fresco. Application of a natural glycolipid product significantly reduced biofilm-associated L. monocytogenes on both polystyrene and stainless steel at concentrations as low as 45 mg/L. When added to UHT skim milk, a concentration of 1000 mg/L inhibited L. monocytogenes growth through 7 days of storage at 7 °C, and application of 1300 and 1500 mg/L reduced counts to levels below the limit of enumeration at day 21. In contrast, 2000 mg/L were necessary to inhibit growth through 7 days in whole milk. Glycolipid solutions at concentrations ≥10% reduced L. monocytogenes counts on Queso Fresco through 7 days when applied as a dip. Overall, natural glycolipids have potential as a natural alternative for the removal of biofilms and as an antimicrobial to control L. monocytogenes in milk and milk products with short shelf lives.

Lactic Acid Bacteria Adjunct Cultures Exert a Mitigation Effect against Spoilage Microbiota in Fresh Cheese

Lactic acid bacteria (LAB) have a strong mitigation potential as adjunct cultures to inhibit undesirable bacteria in fermented foods. In fresh cheese with low salt concentration, spoilage and pathogenic bacteria can affect the shelf life with smear on the surface and packaging blowing. In this work, we studied the spoilage microbiota of an Italian fresh cheese to find tailor-made protective cultures for its shelf life improvement. On 14-tested LAB, three of them, namely Lacticaseibacillus rhamnosus LRH05, Latilactobacillus sakei LSK04, and Carnobacterium maltaromaticum CNB06 were the most effective in inhibiting Gram-negative bacteria. These cultures were assessed by the cultivation-dependent and DNA metabarcoding approach using in vitro experiments and industrial trials. Soft cheese with and without adjunct cultures were prepared and stored at 8 and 14 °C until the end of the shelf life in modified atmosphere packaging. Data demonstrated that the use of adjunct cultures reduce and/or modulate the growth of spoilage microbiota at both temperatures. Particularly, during industrial experiments, C. maltaromaticum CNB06 and Lcb. rhamnosus RH05 lowered psychrotrophic bacteria of almost 3 Log CFU/g in a 5-week stored cheese. On the contrary, Llb. sakei LSK04 was able to colonize the cheese but it was not a good candidate for its inhibition capacity. The combined approach applied in this work allowed to evaluate the protective potential of LAB strains against Gram-negative communities.

The effect of protective cultures on Staphylococcus aureus growth and enterotoxin production

Staphylococcus aureus is the causative agent of staphylococcal food poisoning and is a common contaminant in milk. Despite efforts to control S. aureus, recalls and outbreaks continue to occur, highlighting the need for additional interventions. This study determined the potential for protective cultures (PC) that are commercially available to producers to control S. aureus growth in raw milk and attenuate virulence by impeding staphylococcal enterotoxin (SE) production in raw milk and laboratory medium. Cultures of Hafnia alvei and Lactococcus lactis effectively inhibited S. aureus growth in raw milk to counts ~5 log CFU/mL lower than control when cocultured following a cheesemaking time and temperature profile; two cultures of Lactobacillus plantarum inhibited growth to ~1.5 log CFU/mL less than control. Cocultures of S. aureus with Lc. lactis, H. alvei and Lb. plantarum in raw milk reduced SE levels by 24.9%, 62.4%, and 76%, respectively. Lc. lactis also decreased SE production in raw milk in the absence of PC-mediated growth inhibition. Significant reductions in SE production in the absence of pathogen growth inhibition were also achieved in laboratory medium. Together, these results demonstrate the potential for PCs to inhibit S. aureus growth and impede SE production in the absence of growth inhibition.