Survival of Tick-Borne Encephalitis Virus in Goat Cheese and Milk

Survival of tick-borne encephalitis virus was studied from pasteurized and unpasteurized goat milk and from salted/unsalted and spiced/unspiced cheese made from goat milk inoculated with low and high litres of infective virus. Both soft (63 °C, 30 min) and fast (72 °C, 15 s) pasteurization conditions destroyed viable virus particles. A small amount of infective virus could be detected only for 5‒10 days from milk, and from unsalted cheese. From milk inoculated with a higher amount of virus, infectious viral particles were detectable for 20‒25 days and from unsalted cheese samples for 10‒15 days, independently of the use of spices. Pasteurization and salt treatment made goat milk and cheese safely consumable. These two methods must be used when making any human food from goat milk to avoid milk-borne human TBEV infections.

Unprecedented Diversity of Lactococcal Group 936 Bacteriophages Revealed by Amplicon Sequencing of the Portal Protein Gene

Lactococcus lactis is one of the most important bacteria in dairy fermentations, being used in the production of cheese and buttermilk. The processes are vulnerable to phage attacks, and undefined mixtures of lactococcal strains are often used to reduce the risk of bacteriophage caused fermentation failure. Other preventive measures include culture rotation to prevent phage build-up and phage monitoring. Phage diversity, rather than quantity, is the largest threat to fermentations using undefined mixed starter cultures. We have developed a method for culture independent diversity analysis of lytic bacteriophages of the 936 group, the phages most commonly found in dairies. Using, as a target, a highly variable region of the portal protein gene, we demonstrate an unprecedented diversity and the presence of new 936 phages in samples taken from cheese production. The method should be useful to the dairy industry and starter culture manufacturers in their efforts to reduce phage problems.

Complete genomic sequences of Propionibacterium freudenreichii phages from Swiss cheese reveal greater diversity than Cutibacterium (formerly Propionibacterium) acnes phages

BACKGROUND

A remarkable exception to the large genetic diversity often observed for bacteriophages infecting a specific bacterial host was found for the Cutibacterium acnes (formerly Propionibacterium acnes) phages, which are highly homogeneous. Phages infecting the related species, which is also a member of the Propionibacteriaceae family, Propionibacterium freudenreichii, a bacterium used in production of Swiss-type cheeses, have also been described and are common contaminants of the cheese manufacturing process. However, little is known about their genetic composition and diversity.

RESULTS

We obtained seven independently isolated bacteriophages that infect P. freudenreichii from Swiss-type cheese samples, and determined their complete genome sequences. These data revealed that all seven phage isolates are of similar genomic length and GC% content, but their genomes are highly diverse, including genes encoding the capsid, tape measure, and tail proteins. In contrast to C. acnes phages, all P. freudenreichii phage genomes encode a putative integrase protein, suggesting they are capable of lysogenic growth. This is supported by the finding of related prophages in some P. freudenreichii strains. The seven phages could further be distinguished as belonging to two distinct genomic types, or 'clusters', based on nucleotide sequences, and host range analyses conducted on a collection of P. freudenreichii strains show a higher degree of host specificity than is observed for the C. acnes phages.

CONCLUSIONS

Overall, our data demonstrate P. freudenreichii bacteriophages are distinct from C. acnes phages, as evidenced by their higher genetic diversity, potential for lysogenic growth, and more restricted host ranges. This suggests substantial differences in the evolution of these related species from the Propionibacteriaceae family and their phages, which is potentially related to their distinct environmental niches.

A high-throughput qPCR system for simultaneous quantitative detection of dairy Lactococcus lactis and Leuconostoc bacteriophages

Simultaneous quantitative detection of Lactococcus (Lc.) lactis and Leuconostoc species bacteriophages (phages) has not been reported in dairies using undefined mixed-strain DL-starters, probably due to the lack of applicable methods. We optimized a high-throughput qPCR system that allows simultaneous quantitative detection of Lc. lactis 936 (now SK1virus), P335, c2 (now C2virus) and Leuconostoc phage groups. Component assays are designed to have high efficiencies and nearly the same dynamic detection ranges, i.e., from ~1.1 x 10⁵ to ~1.1 x 10¹ phage genomes per reaction, which corresponds to ~9 x 10⁷ to ~9 x 10³ phage particles mL^-1 without any additional up-concentrating steps. The amplification efficiencies of the corresponding assays were 100.1±2.6, 98.7±2.3, 101.0±2.3 and 96.2±6.2. The qPCR system was tested on samples obtained from a dairy plant that employed traditional mother-bulk-cheese vat system. High levels of 936 and P335 phages were detected in the mother culture and the bulk starter, but also in the whey samples. Low levels of phages were detected in the cheese milk samples.

An outbreak of tick-borne encephalitis associated with raw goat milk and cheese consumption, Croatia, 2015

The aim of this report is to emphasize the risk of acquiring TBE by the consumption of raw milk and dairy products. In April-May 2015, we registered the first outbreak of tick-borne encephalitis in Croatia in seven members out of ten exposed persons who consumed raw goat milk or cheese from the same supplier. Infection was confirmed by TBEV enzyme-linked immunosorbent assay (ELISA) in all patients. None had been vaccinated nor had observed a tick bite.

Control of Listeria monocytogenes growth in soft cheeses by bacteriophage P100

The purpose of this study was to determine the effect of bacteriophage P100 on strains of Listeria monocytogenes in artificially inoculated soft cheeses. A mix of L. monocytogenes 1/2a and Scott A was inoculated in Minas Frescal and Coalho cheeses (approximately 10(5) cfu/g) with the bacteriophage added thereafter (8.3 × 10(7) PFU/g). Samples were analyzed immediately, and then stored at 10 °C for seven days. At time zero, 30 min post-infection, the bacteriophage P100 reduced L. monocytogenes counts by 2.3 log units in Minas Frescal cheese and by 2.1 log units in Coalho cheese, compared to controls without bacteriophage. However, in samples stored under refrigeration for seven days, the bacteriophage P100 was only weakly antilisterial, with the lowest decimal reduction (DR) for the cheeses: 1.0 log unit for Minas Frescal and 0.8 log units for Coalho cheese. The treatment produced a statistically significant decrease in the counts of viable cells (p < 0.05) and in all assays performed, we observed an increase of approximately one log cycle in the number of viable cells of L. monocytogenes in the samples under refrigeration for seven days. Moreover, a smaller effect of phages was observed. These results, along with other published data, indicate that the effectiveness of the phage treatment depends on the initial concentration of L. monocytogenes, and that a high concentration of phages per unit area is required to ensure sustained inactivation of target pathogens on food surfaces.

A rapid procedure for detecting noroviruses from cheese and fresh lettuce

Noroviruses (NoVs) are recognized as the most common agents of outbreaks of food-borne viral gastroenteritis and the efficiency of different methods for detection of NoVs from food matrices have been tested in several laboratories worldwide. The aim of this study was to develop a rapid and sensitive method for recovery of NoVs by using a filtration concentration method followed by PCR amplification for detection of NoVs from cheese and fresh lettuce. Experimentally, a fecal suspension containing different number of NoVs copies was spiked in the food surface and extracted by a direct elution using a Stomacher apparatus. An Ozone-Safe solvent Vertrel XF treatment was included for cheese samples for removing particulate matter. The watery phase was collected and the viral concentration was performed by the adsorption-elution method using negatively charged membranes with inorganic solvents in a Stericup and afterwards ultrafiltered using a Centriprep Concentrator 50 to obtain a final volume of 2ml. RNA isolation was carried out with the QIAamp Viral RNA Mini Kit available commercially and reverse transcription was carried out with a Pd(N6) random primer. Real time quantitative PCR (TaqMan) and qualitative PCR were used for molecular detection of NoVs. The recovery rate of NoVs ranged from 5.2 to 72.3% in lettuce and from 6 to 56.3% in cheese. The results indicate that this method is suitable for detection of NoVs contamination in food and will help establish the cause and source of NoVs outbreaks of food-borne illness.

Diversity of Streptococcus thermophilus phages in a large-production cheese factory in Argentina

Phage infections still represent a serious risk to the dairy industry, in which Streptococcus thermophilus is used in starter cultures for the manufacture of yogurt and cheese. The goal of the present study was to analyze the biodiversity of the virulent S. thermophilus phage population in one Argentinean cheese plant. Ten distinct S. thermophilus phages were isolated from cheese whey samples collected in a 2-mo survey. They were then characterized by their morphology, host range, and restriction patterns. These phages were also classified within the 2 main groups of S. thermophilus phages (cos- and pac-type) using a newly adapted multiplex PCR method. Six phages were classified as cos-type phages, whereas the 4 others belonged to the pac-type group. This study illustrates the phage diversity that can be found in one factory that rotates several cultures of S. thermophilus. Limiting the number of starter cultures is likely to reduce phage biodiversity within a fermentation facility.

Spread and variability of the integrase gene in Lactobacillus delbrueckii ssp. lactis strains and phages isolated from whey starter cultures

AIMS

To determine the presence, diffusion and variability of the integrase (int) gene in Lactobacillus delbrueckii ssp. lactis isolated from natural whey starters used for the production of Italian hard cheeses.

METHODS AND RESULTS

A PCR-based protocol aimed to amplify an internal fragment of the int gene was optimized taking into account phage genome sequences available from public databases. Thirty-seven of the 39 strains tested showed the presence of the putative int gene. Southern blot hybridization experiments confirmed data obtained by PCR. The presence of the putative int gene was observed also in 20 of 23 Lact. delbrueckii ssp. lactis lytic phages isolated from the same starter cultures used to isolate strains. Phylogenetic analysis of partial int gene revealed a high similarity both within and between strain- and phage-derived sequences. Sixty per cent of the int-positive strains resulted inducible with mitomycin C, and two of them released active phage particles.

CONCLUSIONS

Our preliminary findings seem to suggest that an important number of Lact. delbrueckii ssp. lactis strains associated with the whey starters are lysogenic.

SIGNIFICANCE AND IMPACT OF THE STUDY

Further contribution to obtain a clearer picture of the complex relationship between thermophilic lactic acid bacteria phage and host in whey starters for Italian, hard-cooked cheeses.

Thermophilic lactic acid bacteria phages isolated from Argentinian dairy industries

Sixty-one natural phages (59 of Streptococcus thermophilus and 2 of Lactobacillus delbrueckii subsp. bulgaricus) were isolated from Argentinian dairy plants from November 1994 to July 2000. Specifically, 17 yogurt samples (18% of all samples) and 26 cheese samples (79%) contained phages lytic to S. thermophilus strains. The number of viral particles found in samples ranged from 10(2) to 10(9) PFU/ml. The phages belonged to Bradley's group B or the Siphoviridae family (morphotype B1). They showed high burst size values and remarkably short latent periods. The results of this study show that phages were found more frequently in cheesemaking processes than in yogurt-making processes. The commercial streptococcus strains appeared to propagate more phages, whereas the natural strains propagated fewer phage strains. These results suggest that the naturally occurring cultures are inherently more phage resistant.

First evidence of lysogeny in Propionibacterium freudenreichii subsp. shermanii

Dairy propionic acid bacteria, particularly the species Propionibacterium freudenreichii, play a major role in the ripening of Swiss type cheese. Isometric and filamentous bacteriophages infecting P. freudenreichii have previously been isolated from cheese. In order to determine the origin of these bacteriophages, lysogeny of P. freudenreichii was determined by isometric bacteriophage type analysis. The genomic DNA of 76 strains were hybridized with the DNA of nine bacteriophages isolated from Swiss type cheeses, and the DNA of 25 strains exhibited strong hybridization. Three of these strains released bacteriophage particules following UV irradiation (254 nm) or treatment with low concentrations of mitomycin C. A prophage-cured derivative of P. freudenreichii was readily isolated and subsequently relysogenized. Lysogeny was therefore formally demonstrated in P. freudenreichii.

Occurrence of Propionibacterium freudenreichii bacteriophages in swiss cheese

We isolated bacteriophages active against Propionibacterium freudenreichii from 16 of 32 swiss cheese samples. Bacteriophage concentrations ranged from 14 to 7 x 10(5) PFU/g, depending on the sample and the sensitive strain used for detection. Only a few strains, 8 of the 44 strains of P. freudenreichii in our collection, were sensitive. We observed that multiplication of bacteriophages occurred in the cheese loaf during multiplication of propionibacteria in a warm curing room, but it seems that these bacteriophages have no adverse effect on the development of the propionic flora. We also found that sensitive cells, originating from either the starter or the cheese-making milk, were present at a high level (10(9) CFU/g) in the cheese.

Detection and classification of Streptococcus thermophilus bacteriophages isolated from industrial milk fermentation

In the last 30 years, 81 Streptococcus thermophilus bacteriophage isolates were collected from industrial yogurt (n = 40) and cheese (n = 41) fermentation. Forty-six distinct restriction patterns of phage DNA (11 in yogurt and 35 in cheese) were observed. The phages were investigated for host range, serological properties, and DNA homology to study whether these three independent techniques can be used to classify the phages into taxonomic groups. Yogurt factory-derived phages were classified into the same two subgroups by serology, host range analysis, and hybridization with subgroup-specific DNA sequences. Cheese factory-derived phages, however, could not be classified: the 35 cheese phage isolates with distinct restriction patterns showed 34 different host ranges. All but one cheese phage isolate showed serological cross-reactivity with yogurt phages. A phage DNA fragment that hybridized with all phage DNA samples was cloned, establishing the genetic relatedness of all S. thermophilus phages from our collection. With the sequence information from an unusually conserved S. thermophilus phage DNA element (H. Brüssow, A. Probst, M. Frémont, and J. Sidoti, Virology 200:854-857, 1994), a PCR-based phage detection method was developed for cheese whey from a factory that produced mozzarella cheese with complex undefined starter mixes. PCR allowed the detection of phages in cheese whey (detection limit, 10(3) PFU/ml) which could not be detected by dot blot hybridization techniques (detection limit, 10(7) PFU/ml).