Fate of enterotoxigenic Staphylococcus aureus and staphylococcal enterotoxins in Feta and Galotyri cheeses

Prevalence and Phage-Based Biocontrol of Methicillin-Resistant Staphylococcus aureus Isolated from Raw Milk of Cows with Subclinical Mastitis in Vietnam

S. aureus, particularly methicillin-resistant S. aureus, has been recognized as a main cause of bovine mastitis and food poisoning. This study investigated the prevalence, antibiotic resistance, and phage-based biocontrol of S. aureus and methicillin-resistant S. aureus isolated from raw milk of cows with subclinical mastitis. The results showed that the prevalence of S. aureus and methicillin-resistant S. aureus was 12% (48/400) and 1.5% (6/400), respectively. The S. aureus isolates were highly resistant to penicillin (72.92%), erythromycin (43.75%), and tetracycline (39.58%). Out of 48 S. aureus isolates, 6 were identified as methicillin-resistant strains. Among them, one isolate was found to harbor the sea gene. A total of 5 phages were recovered from 50 pork and 50 chicken meat samples, 1 from pork and 4 from chicken meat samples. Phage PSA2 capable of lysing all 6 methicillin-resistant isolates was selected for characterization. The use of phage PSA2 completely inactivated methicillin-resistant S. aureus SA33 in raw milk at both 24 °C and 4 °C, indicating its potential as a promising antibacterial agent in controlling methicillin-resistant S. aureus in raw milk and treating bovine mastitis.

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.

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.

Genomic analysis, antibiotic resistance, and virulence of Staphylococcus aureus from food and food outbreaks: A potential public concern

Transmission and outbreaks of Staphylococcus aureus among retail food highlights the need to comprehensive analysis the molecular characteristic of Staphylococcus aureus in foods. However, the information about Staphylococcus aureus in north China is limited. In this study, 97 and 28 S. aureus strains were isolated for analysis from 4262 samples of retail food and 61 samples food outbreaks with prevalence rate 2.28 % and 45.9 %, respectively in Jilin, China from 2014 to 2018. This study aimed to investigate the prevalence of S. aureus isolates and characterize by antimicrobial resistance testing, virulence profiles, spa typing, and multilocus sequence typing (MLST) analysis. 60 % (75/125) of the isolates contained at least enterotoxin genes including classic and new SEs genes as following: sea (40/125,32 %), see (36/125,28.8 %), sec (29/125,23.2 %), sell (29/125,23.2 %), seb (25/125,20 %), seh (22/125,17.6 %), sed (6/125,4.8 %), selq (6/125,4.8 %), and selk (6/125,4.8 %). In antimicrobial susceptibility tests, 59.2 % of the isolates (74/125) were considered as multi-drug-resistant isolates and four MRSA strains were all found with high multi-drug-resistance. Phenotype resistance to penicillin (94.4 %), erythromycin (84.2 %), clindamycin (63.9 %), and tetracycline (47.2 %) was observed which was corresponding with genotype resistance. The strains were classified to twenty-two sequence types (STs), fourteen clonal complexes (CCs), and forty-seven spa types. The predominant ST and spa types were ST1(22/125,17.6 %), ST25(20/125,16.00 %), ST398 (14/125,11.2 %) and t127 (20/125,16 %), t078 (14/125,11.2 %), t803 (7/125,5.6 %). The wgSNP analysis of these isolates in food represents showed close relatedness with food outbreaks which pose a potential health risk for consumers and warrants further attention.

Role of milk and milk products in the spread of methicillin-resistant Staphylococcus aureus in the dairy production chain

Milk and milk products can harbor a multiple varieties of microorganisms. Therefore, they can be an important source of foodborne pathogens, including multidrug-resistant bacteria. Methicillin-resistant Staphylococcus aureus (MRSA) causes a wide spectrum of infections both in animals and humans. Over the last two decades, the presence of MRSA in foods and food-producing animals, including milk and milk products, has been frequently reported worldwide, raising public health concerns. In order to monitor and prevent foodborne MRSA contamination, it is necessary to understand their sources, the pheno/genotypic characteristics of the strains, and their transmission dynamics. In this review, studies conducted worldwide were summarized in order to assess the prevalence and diversity of MRSA circulating in milk and milk products. The risk factors for the occurrence of MRSA in milk and milk products were also discussed with preventive and control measures to avoid MRSA contamination in the dairy food chain.

Comparison of the Microbiome of Artisanal Homemade and Industrial Feta Cheese through Amplicon Sequencing and Shotgun Metagenomics

Feta is the most renowned protected designation of origin (PDO) white brined cheese produced in Greece. The fine organoleptic characteristics and the quality of Feta rely on, among other factors, its overall microbial ecosystem. In this study, we employed 16S rDNA and internal transcribed spacer (ITS) amplicon sequencing, as well as shotgun metagenomics, to investigate the microbiome of artisanal homemade and industrial Feta cheese samples from different regions of Greece, which has very rarely been investigated. 16S rDNA data suggested the prevalence of the Lactococcus genus in the homemade samples, while Streptococcus and Lactobacillus genera prevailed in the industrial control samples. Species identification deriving from shotgun metagenomics corroborated these findings, as Lactococcus lactis dominated two homemade samples while Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus were found to be dominating one industrial sample. ITS data revealed a complex diversity of the yeast population among the samples analyzed. Debaryomyces, Kluyveromyces, Cutaneotrichosporon, Pichia, Candida, and Rhodotorula were the major genera identified, which were distributed in a rather arbitrary manner among the different samples. Furthermore, a number of potential metagenome-assembled genomes (MAGs) could be detected among assembled shotgun bins. The overall analysis of the shotgun metagenomics supported the presence of different foodborne pathogens in homemade samples (e.g., Staphylococcus aureus, Listeria monocytogenes, Enterobacter cloacae, and Streptococcus suis), but with low to very low abundances. Concluding, the combination of both amplicon sequencing and shotgun metagenomics allowed us to obtain an in-depth profile of the artisanal homemade Feta cheese microbiome.

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.

Microbial Quality of and Biochemical Changes in Fresh Soft, Acid-Curd Xinotyri Cheese Made from Raw or Pasteurized Goat's Milk

The microbiological quality of and changes in the main physicochemical parameters, together with the evolution of proteolysis, lipolysis and volatile profiles of soft Xinotyri, a traditional Greek acid-curd cheese (pH≈4.4, moisture 65%, salt 1%) made from raw (RMC) or pasteurized (PMC) goat's milk without starters, were evaluated during aerobic storage at 4 ^oC for 60 days. No statistically significant differences between the total nitrogen (TN) and nitrogen fraction (% of TN) contents, the degradation of intact α_s- or β-caseins, total free amino acid (FAA) contents, and the ratio of hydrophilic and hydrophobic peptides in the water-soluble fraction of RMC and PMC were found. Threonine, alanine and lysine were the principal FAAs. Oleic, palmitic, capric and caprylic acids, and ethyl hexonate, ethyl octanoate, ethyl decanoate, ethanol, 3-methyl butanol, phenyl ethyl alcohol and acetone were the most abundant free fatty acids and volatile compounds, respectively. Cheese lipolysis evolved slowly at 4 ^oC, and milk pasteurization had no significant effect on it. Mesophilic lactic acid bacteria (LAB) were predominant in fresh cheese samples. PMC samples had significantly lower levels of enterococci and enterobacteria than RMC samples, while yeasts grew at similar levels during storage at 4 ^oC. All cheese samples (25 g) were free of Salmonella and Listeria monocytogenes. Coagulase-
-positive staphylococci exceeded the 5-log safety threshold in fresh RMC samples, whereas they were suppressed (<100 CFU/g) in all PMC samples. Consequently, pasteurization of raw goat milk's and utilization of commercially defined or natural mesophilic LAB starters are recommended for standardizing the biochemical, microbial and safety qualities of fresh soft Xinotyri cheese.

Nanoshell-Enhanced Raman Spectroscopy on a Microplate for Staphylococcal Enterotoxin B Sensing

A sensitive surface-enhanced Raman scattering (SERS) immunosensor based on the Au nanoparticle (Au NP) shell structure was developed to detect staphylococcal enterotoxin B (SEB) on a microplate. Au NPs modified with 4-nitrothiophenol (4-NTP) and coated with Ag shell of controlled thickness at 6.6 nm exhibited excellent SERS intensity and were used as signal reporters in the detection of SEB. The engaged 4-NTP allowed the significant electromagnetic enhancement between Au NPs and the Ag shell and prevented the dissociation of the Raman reporter. More importantly, 4-NTP-differentiated SERS signals between the sample and microplate. The SERS-based immunosensor had a limit of detection of 1.3 pg/mL SEB. Analysis of SEB-spiked milk samples revealed that the developed method had high accuracy. Therefore, the SERS-encoded Au@Ag core-shell structure-based immunosensor is promising for the detection of biotoxins, pathogens, and environmental pollutants.

Identification of the risk factors associated with cheese production to implement the hazard analysis and critical control points (HACCP) system on cheese farms

The purpose of this paper was to evaluate, by statistical analyses, risk factors on cheese farms that can influence the microbial contamination of their products. Various assessment tools, such as cheese production questionnaires, food handlers' knowledge testing, and hygiene assessment system surveys, were used on 39 cheese farms on the island of Gran Canaria, Spain. The microbiological status of 773 raw milk and cheese samples from the cheese farms was assessed by enumerating total viable counts and 4 pathogens: Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, and Salmonella spp. The results revealed that the highest contamination by Staph. aureus (4.39%, >10(5)cfu/mL) was found in milk, and the highest contamination by E. coli (5.18%, >10(3) cfu/mL) was found in cheese. Very few samples (0.52%) were contaminated by L. monocytogenes or Salmonella spp. The factors associated with any tested microorganism were "handling," "knowledge," and "type of milk." Subsequently, multidimensional logistic analysis for contamination by E. coli showed an independent association for factors "cleaning and disinfection test" and "type of milk." The probability of total aerobic contamination of milk increased with lower hygiene assessment system survey scores. These results emphasize the need to apply and maintain good hygiene practices, and to study risk factors to prevent contamination and bacterial growth. Further research is required in other areas with different cheese farm types to reinforce the validity of these results.

Staphylococcus aureusreservoirs during traditional Austrian raw milk cheese production

Sampling approaches following the dairy chain, including microbiological hygiene status of critical processing steps and physicochemical parameters, contribute to our understanding of howStaphylococcus aureuscontamination risks can be minimised. Such a sampling approach was adopted in this study, together with rapid culture-independent quantification ofStaph. aureusto supplement standard microbiological methods. A regional cheese production chain, involving 18 farms, was sampled on two separate occasions. Overall, 51·4% of bulk milk samples were found to beStaph. aureuspositive, most of them (34·3%) at the limit of culture-based detection.Staph. aureuspositive samples >100 cfu/ml were recorded in 17·1% of bulk milk samples collected mainly during the sampling in November. A higher number ofStaph. aureuspositive bulk milk samples (94·3%) were detected after applying the culture-independent approach. A concentration effect ofStaph. aureuswas observed during curd processing.Staph. aureuswere not consistently detectable with cultural methods during the late ripening phase, but >100Staph. aureuscell equivalents (CE)/ml or g were quantifiable by the culture-independent approach until the end of ripening. Enterotoxin gene PCR and pulsed-field gel electrophoresis (PFGE) typing provided evidence that livestock adapted strains ofStaph. aureusmostly dominate the post processing level and substantiates the belief that animal hygiene plays a pivotal role in minimising the risk ofStaph. aureusassociated contamination in cheese making. Therefore, the actual data strongly support the need for additional sampling activities and recording of physicochemical parameters during semi-hard cheese-making and cheese ripening, to estimate the risk ofStaph. aureuscontamination before consumption.

Following pathogen development and gene expression in a food ecosystem: the case of a Staphylococcus aureus isolate in cheese

Human intoxication or infection due to bacterial food contamination constitutes an economic challenge and a public health problem. Information on the in situ distribution and expression of pathogens responsible for this risk is to date lacking, largely because of technical bottlenecks in detecting signals from minority bacterial populations within a complex microbial and physicochemical ecosystem. We simulated the contamination of a real high-risk cheese with a natural food isolate of Staphylococcus aureus, an enterotoxin-producing pathogen responsible for food poisoning. To overcome the problem of a detection limit in a solid matrix, we chose to work with a fluorescent reporter (superfolder green fluorescent protein) that would allow spatiotemporal monitoring of S. aureus populations and targeted gene expression. The combination of complementary techniques revealed that S. aureus localizes preferentially on the cheese surface during ripening. Immunochemistry and confocal laser scanning microscopy enabled us to visualize, in a single image, dairy bacteria and pathogen populations, virulence gene expression, and the toxin produced. This procedure is readily applicable to other genes of interest, other bacteria, and different types of food matrices.

Enterotoxin-producing Staphylococcus aureus genotype B as a major contaminant in Swiss raw milk cheese

The objective of this study was to characterize Staphylococcus aureus isolates from Swiss raw milk cheeses that had been found to be contaminated with coagulase-positive staphylococci and to estimate the frequency of the various genotypes, in particular the mastitis-associated Staph. aureus genotype B (GTB). The isolates were also tested for staphylococcal enterotoxin (SE) genes and other virulence factors. From 623 coagulase-positive staphylococci isolated from 78 contaminated raw milk cheeses, 609 were found to be Staphylococcus aureus. Genotyping of all Staph. aureus isolates was performed by PCR amplification of the 16S-23S rRNA intergenic spacer region, as this method was used previously to differentiate between mastitis subtypes associated with their clinical outcome. In total, 20 different genotypes were obtained and the 5 most frequently occurring genotypes were distributed in 6.4% or more of the samples. The enterotoxin-producing Staph. aureus GTB, known for its high contagiousness and increased pathogenicity in Swiss mastitis herds, was found to be the most abundant subtype at the sample level (71.8%) as well as among the isolates (62.0%). A subset of 107 isolates of the different genotypes were analyzed for the presence of SE genes and revealed 9 different SE gene patterns, with sed being most frequently detected and 26% being PCR-negative for SE genes. Almost all isolates of the major contaminant GTB contained the SE gene pattern sed, sej, ser, with half of them additionally carrying sea. Production of SE in vitro was consistent with the SE genes detected in most of the cases; however, some isolated GTB did not produce SEA. Staphylococcus aureus Protein A (spa) typing revealed 30 different subtypes and most GTB isolates belonged to the bovine spa type t2953; GTB/t2953 was linked among other subtypes to SE production in cheese and staphylococcal intoxication cases. Furthermore, 1 of the 623 isolates was a methicillin-resistant Staph. aureus, which was an seh-carrying Staph. aureus spa type tbl 0635 (non-GTB). We conclude that control and reduction of enterotoxigenic Staph. aureus GTB in dairy herds in Switzerland will not only prevent economic losses at the farm level but also improve the safety of raw milk cheeses; distribution of methicillin-resistant Staph. aureus via raw milk cheese is of less concern.