Diversity among lactococci isolated from ewes' raw milk and cheese

Comparison of raw cow milk microbiota in two milking systems: a field study

This study investigated the effect of different milking systems on the milk composition and microbial community of raw milk in a commercial dairy farm. Raw milk samples from conventional milking system (CMS) and automatic milking system (AMS) were collected and the microbiota on each was assessed by 16S rRNA gene sequencing. Results showed that the acetone (P = 0.031) and β-hydroxybutyrate (P≤0.001) levels in the raw milk of the AMS group were increased compared with the CMS group. Principal component analysis, unweighted and weighted principal coordinates analysis, and heat map of microbial community composition in the raw milk showed a clear separation between two groups. AMS increased the abundance of the genera Acinetobacter (FDR = 0.004) and Staphylococcus (FDR = 0.004) in the raw milk compared with the CMS group. In contrast, the abundance of the genera Pseudomonas (FDR = 0.028), Lactococcus (FDR = 0.015), Sphingobacterium (FDR = 0.004), Brevundimonas (FDR = 0.005), and Chryseobacterium (FDR = 0.042) in the raw milk was reduced in the AMS group compared with the CMS group. The abundance of the genera Acinetobacter and Staphylococcus in the raw milk was positively correlated with the β-hydroxybutyrate, acetone, free fatty acid, citric acid, and urea nitrogen levels. Furthermore, the abundance of the genus Acinetobacter in the raw milk was negatively correlated with the somatic cell counts. The study demonstrates that the introduction of AMS in the dairy farm can regulate microbiota composition in the raw milk and this modification may exert an effect on reducing the somatic cell counts in the raw milk.

Microbiological Characteristics of Trachanas, a Traditional Fermented Dairy Product from Cyprus

The purpose of this study was to characterize the autochthonous microbiota of Cypriot Trachanas, a traditional fermented ewes’ milk product. For this reason, 12 samples of raw and fermented milk as well as natural starter culture were collected in order to count, isolate, and identify the main species present during Trachanas fermentation. In total, 198 colonies were retrieved and 163 were identified by sequencing analysis at species level. Lactic acid bacteria (LAB) were the predominant group, followed by yeasts. Lactococcus, Lactobacillus, and Enterococcus were frequently isolated from raw milk, and Lactobacillus casei/paracasei predominated in the starter culture. Lactococcus lactis was isolated in high frequency (27.9% of the isolates) at the beginning, while Lactobacillus spp. (20%) and Saccharomyces unisporus (17.9%) were isolated at the end of fermentation. After assessing their technological potential, selected strains could be used as starters to ferment milk for artisanal Trachanas production.

The complex microbiota of raw milk

Here, we review what is known about the microorganisms present in raw milk, including milk from cows, sheep, goats and humans. Milk, due to its high nutritional content, can support a rich microbiota. These microorganisms enter milk from a variety of sources and, once in milk, can play a number of roles, such as facilitating dairy fermentations (e.g. Lactococcus, Lactobacillus, Streptococcus, Propionibacterium and fungal populations), causing spoilage (e.g. Pseudomonas, Clostridium, Bacillus and other spore-forming or thermoduric microorganisms), promoting health (e.g. lactobacilli and bifidobacteria) or causing disease (e.g. Listeria, Salmonella, Escherichia coli, Campylobacter and mycotoxin-producing fungi). There is also concern that the presence of antibiotic residues in milk leads to the development of resistance, particularly among pathogenic bacteria. Here, we comprehensively review these topics, while comparing the approaches, both culture-dependent and culture-independent, which can be taken to investigate the microbial composition of milk.

Characterization of a wild, novel nisin a-producing Lactococcus strain with an L. lactis subsp. cremoris genotype and an L. lactis subsp. lactis phenotype, isolated from Greek raw milk

Several molecular taxonomic studies have revealed that many natural (wild) Lactococcus lactis strains of dairy origin which are phenotypically representative of the L. lactis subspecies lactis cluster genotypically within subspecies cremoris and vice versa. Recently, we isolated two wild nisin-producing (Nis(+)) L. lactis strains, M78 and M104, of the lactis phenotype from Greek raw milk (J. Samelis, A. Lianou, A. Kakouri, C. Delbès, I. Rogelj, B. B. Matijašic, and M. C. Montel, J. Food Prot. 72:783-790, 2009); strain M78 possess a novel nisin A sequence (GenBank accession number HM219853). In this study, the actual subspecies identity of M78 and M104 isolates was elucidated, using 16S rRNA and acmA (encoding lactococcal N-acetylmuramidase) gene and histidine biosynthesis operon polymorphisms and 16S rRNA and ldh (encoding lactate dehydrogenase) gene phylogenies. Except the acmA gene analysis, molecular tools revealed that isolates M78 and M104 clustered with strains of the cremoris genotype, including the LMG 6897(T) strain, while they were distant from strains of the lactis genotype, including the LMG 6890(T) strain. The two wild isolates had identical repetitive sequence-based PCR (rep-PCR), randomly amplified polymorphic DNA (RAPD), plasmid, and whole-cell protein profiles and shared high 16S rRNA (99.9%) and ldh (100%) gene sequence homologies. In contrast, they exhibited identical sugar fermentation and enzymatic patterns which were similar to those of the subspecies lactis LMG 6890(T) strain. To our knowledge, this is the first complete identification report on a wild L. lactis subsp. cremoris genotype of the lactis phenotype which is capable of nisin A production and, thus, has strong potential for use as a novel dairy starter and/or protective culture.

Proteolysis, lipolysis, volatile compounds and sensory characteristics of Hispánico cheeses made using frozen curd from raw and pasteurized ewe milk

Hispánico cheese, manufactured from a mixture of cow and ewe milk, is representative of cheese varieties made using milk from more than one animal species in Mediterranean countries. The shortage of ewe milk production in autumn hinders the uniformity of Hispánico cheese composition throughout the year. To surmount this inconvenience of ewe milk seasonality, curds made in spring from raw and pasteurized ewe milk were stored frozen and used four months later for the manufacture of Hispánico cheese. Experimental cheeses were made by mixing fresh curd from pasteurized cow milk with thawed curd from raw or pasteurized ewe milk, and control cheese from a mixture of pasteurized cow and ewe milk in the same proportion. Characteristics of experimental and control cheeses throughout a 60-d ripening period were investigated. On the one hand, the experimental cheese containing frozen curd from raw ewe milk showed the highest counts of staphylococci, Gram-negative bacteria and coliforms, the highest levels of aminopeptidase and esterase activity, and the highest concentrations of free amino acids, free fatty acids, alcohols and esters. On the other, the experimental cheese containing frozen curd from pasteurized ewe milk had concentrations of free amino acids, free fatty acids and volatile compounds similar to those of control cheese, with the only exception being a higher level of ketones. Flavour intensity reached the highest scores in the experimental cheese containing frozen curd from raw ewe milk, followed by the experimental cheese containing frozen curd from pasteurized ewe milk. Flavour quality scores of both experimental cheeses were similar, and lower than those of control cheese.

Comparative phenotypic and molecular genetic profiling of wild Lactococcus lactis subsp. lactis strains of the L. lactis subsp. lactis and L. lactis subsp. cremoris genotypes, isolated from starter-free cheeses made of raw milk

Twenty Lactococcus lactis strains with an L. lactis subsp. lactis phenotype isolated from five traditional cheeses made of raw milk with no added starters belonging to the L. lactis subsp. lactis and L. lactis subsp. cremoris genotypes (lactis and cremoris genotypes, respectively; 10 strains each) were subjected to a series of phenotypic and genetic typing methods, with the aims of determining their phylogenetic relationships and suitability as starters. Pulsed-field gel electrophoresis (PFGE) analysis of intact genomes digested with SalI and SmaI proved that all strains were different except for three isolates of the cremoris genotype, which showed identical PFGE profiles. Multilocus sequence typing (MLST) analysis using internal sequences of seven loci (namely, atpA, rpoA, pheS, pepN, bcaT, pepX, and 16S rRNA gene) revealed considerable intergenotype nucleotide polymorphism, although deduced amino acid changes were scarce. Analysis of the MLST data for the present strains and others from other dairy and nondairy sources showed that all of them clustered into the cremoris or lactis genotype group, by using both independent and combined gene sequences. These two groups of strains also showed distinctive carbohydrate fermentation and enzyme activity profiles, with the strains in the cremoris group showing broader profiles. However, the profiles of resistance/susceptibility to 16 antibiotics were very similar, showing no atypical resistance, except for tetracycline resistance in three identical cremoris genotype isolates. The numbers and concentrations of volatile compounds produced in milk by the strains belonging to these two groups were clearly different, with the cremoris genotype strains producing higher concentrations of more branched-chain, derived compounds. Together, the present results support the idea that the lactis and cremoris genotypes of phenotypic Lactococcus lactis subsp. lactis actually represent true subspecies. Some strains of the two subspecies in this study appear to be good starter candidates.

Growth stimulation of a proteinase positive Lactococcus lactis strain by a proteinase negative Lactococcus lactis strain

Lactococcus lactis AMP15/pAMP31(D471R) is a proteinase negative, lactose negative strain with a modified oligopeptide transport system, and potential as a debittering agent due to its efficient utilization of hydrophobic peptides. Five wild L. lactis strains of dairy origin, which produced cheeses of high flavour quality, were cocultured with L. lactis AMP15/pAMP31(D471R) in an attempt to select adequate combinations of strains for use as defined cheese starters with potential debittering ability. Four of these strains, L. lactis B6, K16, M21 and P21, inhibited growth of L. lactis AMP15/pAMP31(D471R) at a level of 10(6) to 10(7) cfu mL(-1) after 24 h of incubation, even though production of bacteriocin-like compounds could only be proven for L. lactis M21. When L. lactis AMP15/pAMP31(D471R) was cocultured with the fifth strain, L. lactis N22, its growth was significantly (P<0.001) inhibited whereas growth of L. lactis N22 was significantly stimulated. The nature of the interaction was studied and it was established that L. lactis N22 is auxotrophic for folate, a compound produced and excreted by L. lactis AMP15/pAMP31(D471R).

[Lactococcus lactis: an opportunistic bacterium?]

Lactic bacteria have been used for centuries to ferment food and thus, to better preserve them. Considered as inoffensive for man, its use has largely spread in food industry. Some species are even being considered for the treatment of human diseases. Indeed, the development of new cellular biology technologies opens opportunities for the use of these bacteria as biotherapeutic agents. These species would produce heterogeneous proteins such as enzymes (lipase, lactase, esterase), chemical mediators (hormones and interleukins), and molecules able to stimulate local immune responses. However, rare cases of human infection, sometimes severe, were reported recently. They generally occured in patients with comorbidities with consumption of unpasteurized dairy products reported for some. The scarcity of these cases and their favorable outcome should not hinder the industrial and medical use of these bacteria.

Genotypic and phenotypic diversity of Lactococcus lactis isolates from Batzos, a Greek PDO raw goat milk cheese

The genotypic and phenotypic variability of 40 Lactococcus lactis isolates obtained from three cheese-making trials of Batzos cheese made one in each, winter, spring and summer was investigated. RAPD-PCR, plasmid profiling and PFGE were used to study the genetic variability and distinguish closely related isolates. Results showed a high degree of heterogeneity among strains. According to PFGE data, all strains except one were clustered together (at a similarity level of approximately 50%) with the L. lactis subsp. lactis reference strain and eleven groups of isolates consisting of 2-8 strains each were distinguished. Plasmid profiling results revealed that there were eight isolates lacking plasmids and nine having unique plasmids. Twenty-three isolates were allocated into six groups. There was an interesting similarity between the plasmid profiling groups and those formed according to PFGE. Clustering of strains according to RAPD-PCR was in agreement with results obtained by both plasmid profiling and PFGE for the majority of the strains. In addition, results obtained by molecular methods indicate a grouping of most of the strains according to the season of cheese production. All strains inhibited the growth of Escherichia coli O157:H7. Their ability to affect the growth of Yersinia enterocolitica, Staphylococcus aureus, Bacillus cereus, Listeria monocytogenes and Enterococcus faecalis was strain dependent. In 42.5% of the isolates high acidifying ability in milk after 24 h was recorded and these were isolates, mainly, from fresh cheese. The 75% of the isolates from winter cheese exhibited higher Lys- than Leu-aminopeptidase activity while the approximately 67% of the isolates from summer cheese showed higher Leu- than Lys-aminopeptidase activity. Their caseinolytic activity after growth in milk for 24 h was significant with preference for alpha(s)-casein degradation. The majority (90%) of the strains formed methanethiol from methionine and this ability was strain dependent. These results suggest that among the wild lactococcal population from Batzos cheese there are interesting strains appropriate to be used as starters for the dairy industry.

Phenotypic and genetic diversity of Lactococcus lactis and Enterococcus spp. strains isolated from Northern Spain starter-free farmhouse cheeses

To evaluate a previous phenotypic classification of lactococci, 39 presumed lactococcal strains were classified by molecular techniques. The strains were also subjected to several typing techniques to estimate the phenotypic and genetic diversity present in original populations from starter-free farmhouse cheeses. Partial Amplified rDNA Restriction Analysis (partial ARDRA) with either restriction enzyme MboII or HhaI divided these isolates into four distinctive groups. Sequencing of representative amplicons identified 29 isolates as belonging to Lactococcus lactis subsp. lactis (24) and Lactococcus lactis subsp. cremoris (5). The remaining 10 isolates were shown to be Enterococcus durans (8) and Enterococcus faecalis (2), which were misclassified by the traditional tests. Thus, partial ARDRA was successfully used to classify wild Lactococcus-like strains into Lactococcus and Enterococcus species. The technique also allowed differentiation of L. lactis strains at subspecies level. The 29 strains of L. lactis showed five different fermentation profiles, four distinct Random Amplification of Polymorphic DNA (RAPD) profiles, and 14 unrelated profiles by both Restriction Fragment Length Polymorphism analyzed by Pulsed Field Gel Electrophoresis (RFLP-PFGE) and Sodium Dodecyl Sulphate Polyacrylamide Gel Electrophoresis (SDS-PAGE). Using the same techniques, the 10 enterococcal strains showed four fermentation profiles, four RADP, and six by RFLP-PFGE and SDS-PAGE, respectively. Several typing techniques, especially RFLP-PFGE and SDS-PAGE, revealed wide phenotypic and genetic variability in both the lactococcal and enterococcal isolates. Two simple, rapid and cheap techniques (partial ARDRA and SDS-PAGE) are proposed as reliable tools for the classification and typing of new lactococcal-like isolates.

Caseinolysis in cheese by Enterobacteriaceae strains of dairy origin

AIMS

To study the effect of Enterobacteriaceae strains of dairy origin on caseins under cheese manufacture and ripening conditions.

METHODS AND RESULTS

Strains belonging to the genera Enterobacter, Escherichia, Hafnia and Serratia were isolated from fresh raw milk cheeses. Residual caseins in cheeses made from milk individually inoculated with 10 strains of Enterobacteriaceae were determined by capillary electrophoresis. Hierarchical cluster analysis of strains based on data of residual caseins grouped together strains from the same genus, excepting Hafnia strains, which were separated into two groups. Serratia was the most proteolytic genus in our study. Preferences for degradation of casein fractions differed among the four genera studied.

CONCLUSIONS

Enterobacteriaceae strains posses proteolytic systems active on all casein fractions under cheese manufacture and ripening conditions. The effects on caseins were similar for strains belonging to the same genus.

SIGNIFICANCE AND IMPACT OF THE STUDY

The presence of Enterobacteriaceae in cheeses may affect proteolysis during ripening. Assays of Enterobacteriaceae proteolytic activity on milk agar plates may underestimate their caseinolytic activity in cheese.

Hydrophilic and hydrophobic peptides produced in cheese by wild Lactococcus lactis strains

AIMS

To study the production of hydrophilic and hydrophobic peptides in cheese by 32 wild Lactococcus lactis strains of different RAPD patterns and to compare them with the peptides produced by lactococcal cells incubated with whole casein.

METHOD AND RESULTS

Chromatograms of peptides from cheeses made using each strain as single starter culture were divided into five regions, and strains were classified in three groups by hierarchical cluster analysis of region areas. Thirty out of the 32 wild L. lactis strains produced higher levels of hydrophobic peptides in cheese than on whole casein.

CONCLUSIONS

Cheese was a more favourable substrate than whole casein for hydrophobic peptide formation by L. lactis strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

New strains of lactococci should be screened for bitterness under cheese conditions, as the formation of hydrophobic peptides may be underestimated in assays with casein as substrate.

Effect of wild strains of Lactococcus lactis on the volatile profile and the sensory characteristics of ewes' raw milk cheese

The production of volatile compounds by wild strains of Lactococcus lactis used as starter cultures and their effect on the sensory characteristics of ewes' raw milk cheese were investigated. Sixteen vats of cheese were manufactured and ripened for 120 d in two experiments, each of them duplicated. In the first experiment, milk was inoculated with different ratios of four wild Lactococcus lactis strains, two producing and two not producing branched-chain volatile compounds, and in the second experiment with different ratios of a commercial starter culture and the two strains producing branched-chain volatile compounds. Cheese pH, proteolysis, and aminopeptidase activity increased when the strains producing branched-chain volatile compounds were inoculated at a higher rate. Fifty volatile compounds were identified in cheeses using a purge and trap system coupled to a gas chromatography-mass spectrometry apparatus. The relative abundances of 30 volatile compounds (8 alcohols, 5 aldehydes, 3 ketones, 12 esters, 1 sulfur compound, and 1 benzenic compound) were influenced by starter culture composition. 2-Methylpropanol, 3-methylbutanol, isobutyl acetate, isoamyl acetate, ethyl butyrate, isobutyl butyrate, and isoamyl butyrate were always more abundant in the cheeses made with a higher level of L. lactis strains producing branched-chain volatile compounds. Flavor intensity was enhanced by a high level of L. lactis strains producing branched-chain volatile compounds in the first experiment, in which four wild L. lactis strains were used as starter culture, but not in the second experiment, in which a combination of two wild L. lactis strains and the commercial starter culture were used. Flavor quality, as judged by trained panelists, was impaired in both experiments by a high level of L. lactis strains producing branched-chain volatile compounds.

Technological performance of several Lactococcus and Enterococcus strains of dairy origin in milk

Thirty-nine strains (29 Lactococcus strains and 10 Enterococcus strains) isolated from five different artisanal cheeses were subjected to technological characterization. Several strains of lactococci and enterococci produced lactic acid at a rate and final concentration suitable for large-scale cheesemaking. However, extensive phenotypic differences between strains were encountered. Proteolytic activity correlated quite well with acidification for all strains, with the more proteolytic strains being the best acidifiers. The strains were also assayed for the production of organic acids and volatile components in milk. With few exceptions, enterococcus isolates produced more formic acid and acetic acid than did lactococcus isolates. The volatile-compound profiles obtained were rather simple. The main volatile component produced by most strains was ethanol. Since the inclusion of enterococcus strains in food systems is controversial, tests were also performed to detect recognized determinants of virulence (namely, aggregation, gelatinase and hemolysin production, and antibiotic resistance). Aggregation in both liquid and solid media was observed only for two Enterococcus durans isolates. None of the strains studied produced gelatinase under the conditions of the assay. Beta-hemolysin activity was clearly detected in two Enterococcus faecalis strains, which also produced the biogenic amine tyramine from tyrosine in a laboratory medium. In general, the enterococcus strains were more resistant to the antibiotics assayed than were the lactococcus strains. Both the minimum inhibitory concentration (MIC) modes and the highest MIC values were consistently higher for the enterococci. Nevertheless, particular strains of lactococci were resistant to antibiotics such as bacitracin, cephalothin, clindamycin, streptomycin, and tetracycline.

Application of RAPD analysis for identification of Lactococcus lactis subsp. cremoris strains isolated from artisanal cultures

Randomly amplified polymorphic DNA (RAPD) was used for identification of Lactococcus lactis subsp. cremoris strains isolated 40 years ago from various dairy homemade products. Total genomic DNAs from six randomly chosen isolates and the reference strain Lactococcus lactis subsp. cremoris NIZO B64 were amplified using four different 10-mer primers. Although most RAPD fragments were common to all six isolates, a sufficient number of polymorphic fragments were also detected that allowed clear distinction of the isolates and the reference strain. The results indicate that RAPD analysis could be a useful and efficient method to distinguish Lactococcus lactis subsp. cremoris at the strain level and to detect genetic diversity.

Cross-inhibition among wild strains of Lactococcus lactis isolated from the same ecological niche

The cross-inhibition between 23 Lactococcus lactis subsp. lactis strains and 9 L. lactis subsp. cremoris strains with different randomly amplified polymorphic DNA patterns, all isolated from the same ecological niche--cheese made in the spring at a single factory from raw milk without added lactic starter cultures-was investigated. Cross-inhibition, as determined by the agar well diffusion assay, was recorded in 130 cases (12.7%) out of 1.024 total cases, with 109 cases due to supernatants of L. lactis subsp. lactis strains and 21 cases due to supernatants of L. lactis subsp. cremoris strains. L. lactis strains isolated in April, May, and June showed differences in their inhibitory activities, with cross-inhibition against each other in 34.7, 14.1, and 6.1% of the cases, respectively. Polymerase chain reaction techniques using specific primers for nisin, lacticin 481, and lactococcin A only revealed the presence of the structural gene of lacticin 481 in two L. lactis subsp. lactis strains.

Hydrolysis of caseins and formation of hydrophilic and hydrophobic peptides by wild Lactococcus lactis strains isolated from raw ewes' milk cheese

AIMS

To investigate the hydrolysis of alphaS1-, alphaS0-, betaB-, betaA1- and betaA2-caseins by 32 wild lactococci of different randomly amplified polymorphic DNA (RAPD) patterns, isolated from raw ewes' milk cheese, and the production of hydrophilic and hydrophobic peptides from whole casein by those strains.

METHODS AND RESULTS

Most strains hydrolysed all caseins, and degraded beta-caseins to a larger extent than alphaS-caseins, when the proteolytic activity of whole cells was determined by capillary electrophoresis. Higher levels of hydrophilic than of hydrophobic peptides were produced from whole casein by all strains, according to reverse-phase high performance liquid chromatography analyses.

CONCLUSIONS

Cell envelope proteinases of most lactococci isolated from raw ewes' milk cheese were CEPII, CEPII/III or CEPIII (classification of Exterkate et al. 1993). A negative correlation was found between degraded alphaS- and beta-caseins and a highly positive correlation between hydrophilic and hydrophobic peptides.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fast acid-producing lactococci from raw ewes' milk cheese have considerable and diverse caseinolytic activities. Their peptide production patterns do not reveal serious risks of bitter-flavour defect in cheeses if used as components of dairy starters.

Genotypic and phenotypic heterogeneity among lactococci isolated from traditional Pecorino Sardo cheese

Twenty-nine Lactococcus lactis isolates from one traditional 24 h-old Pecorino Sardo cheese were characterized phenotypically, technologically and genotypically in order to assess the biodiversity within this wild microbial population. Two DNA-based techniques, plasmid profiling and PFGE, were used for the genetic typing of the isolates. All 29 isolates were characterized at strain level and eight different genotypes were recognized. In addition, by combining the results from plasmid profile analysis and PFGE, it was possible to identify closely related isolates probably belonging to the same clonal lineage. The dominant biotype was identified in the 24 h-old cheese, as were the strains believed to act as starters for the curd. Atypical lactococci, able to grow in 6.5% NaCl, were isolated. The results suggest that wild bacterial populations should be preserved in order to protect the traditional raw milk cheeses, and to select new starter strains for the dairy industry.