Altered specificity of lactococcal proteinase p(i) (lactocepin I) in humectant systems reflecting the water activity and salt content of cheddar cheese

Positive role of cell wall anchored proteinase PrtP in adhesion of lactococci

BACKGROUND

The first step in biofilm formation is bacterial attachment to solid surfaces, which is dependent on the cell surface physico-chemical properties. Cell wall anchored proteins (CWAP) are among the known adhesins that confer the adhesive properties to pathogenic Gram-positive bacteria. To investigate the role of CWAP of non-pathogen Gram-positive bacteria in the initial steps of biofilm formation, we evaluated the physico-chemical properties and adhesion to solid surfaces of Lactococcus lactis. To be able to grow in milk this dairy bacterium expresses a cell wall anchored proteinase PrtP for breakdown of milk caseins.

RESULTS

The influence of the anchored cell wall proteinase PrtP on microbial surface physico-chemical properties, and consequently on adhesion, was evaluated using lactococci carrying different alleles of prtP. The presence of cell wall anchored proteinase on the surface of lactococcal cells resulted in an increased affinity to solvents with different physico-chemical properties (apolar and Lewis acid-base solvents). These properties were observed regardless of whether the PrtP variant was biologically active or not, and were not observed in strains without PrtP. Anchored PrtP displayed a significant increase in cell adhesion to solid glass and tetrafluoroethylene surfaces.

CONCLUSION

Obtained results indicate that exposure of an anchored cell wall proteinase PrtP, and not its proteolytic activity, is responsible for greater cell hydrophobicity and adhesion. The increased bacterial affinity to polar and apolar solvents indicated that exposure of PrtP on lactococcal cell surface could enhance the capacity to exchange attractive van der Waals interactions, and consequently increase their adhesion to different types of solid surfaces and solvents.

Biotechnological methods to accelerate cheddar cheese ripening

Cheese is one of the dairy products that can result from the enzymatic coagulation of milk. The basic steps of the transformation of milk into cheese are coagulation, draining, and ripening. Ripening is the complex process required for the development of a cheese's flavor, texture and aroma. Proteolysis, lipolysis and glycolysis are the three main biochemical reactions that are responsible for the basic changes during the maturation period. As ripening is a relatively expensive process for the cheese industry, reducing maturation time without destroying the quality of the ripened cheese has economic and technological benefits. Elevated ripening temperatures, addition of enzymes, addition of cheese slurry, attenuated starters, adjunct cultures, genetically engineered starters and recombinant enzymes and microencapsulation of ripening enzymes are traditional and modern methods used to accelerate cheese ripening. In this context, an up to date review of Cheddar cheese ripening is presented.

A new approach to monitoring proteolysis phenomena using antibodies specifically directed against the enzyme cleavage site on its substrate

Proteolysis is a generic biochemical process that is central in all biological activities. A new strategy for monitoring this biochemical process is proposed here. This approach is based on the production of rabbit polyclonal anti-peptide antibodies directly against the cleavage site on the substrate of the enzyme responsible for proteolysis. So long as the molecule's cleavage site is intact, the antibody will bind to the protein. However, after cleavage of the peptide bond by the protease, the antibody will no longer be able to recognize the substrate. Thus, the development of an ELISA that uses this specific antibody allows hydrolysis of the substrate protein to be monitored. Hydrolysis of beta-casein by plasmin, the main indigenous protease of milk, during the ripening of Swiss-type cheese, has been chosen as a model for this study.

Contribution of Lactococcus lactis cell envelope proteinase specificity to peptide accumulation and bitterness in reduced-fat Cheddar cheese

Bitterness is a flavor defect in Cheddar cheese that limits consumer acceptance, and specificity of the Lactococcus lactis extracellular proteinase (lactocepin) is widely believed to be a key factor in the development of bitter cheese. To better define the contribution of this enzyme to bitterness, we investigated peptide accumulation and bitterness in 50% reduced-fat Cheddar cheese manufactured with single isogenic strains of Lactococcus lactis as the only starter. Four isogens were developed for the study; one was lactocepin negative, and the others produced a lactocepin with group a, e, or h specificity. Analysis of cheese aqueous extracts by reversed-phase high-pressure liquid chromatography confirmed that accumulation of alpha(S1)-casein (f 1-23)-derived peptides f 1-9, f 1-13, f 1-16, and f 1-17 in cheese was directly influenced by lactocepin specificity. Trained sensory panelists demonstrated that Cheddar cheese made with isogenic starters that produced group a, e, or h lactocepin was significantly more bitter than cheese made with a proteinase-negative isogen and that propensity for bitterness was highest in cells that produced group h lactocepin. These results confirm the role of starter proteinase in bitterness and suggest that the propensity of some industrial strains for production of the bitter flavor defect in cheese could be altered by proteinase gene exchange or gene replacement.

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.

The autoproteolysis of Lactococcus lactis lactocepin III affects its specificity towards beta-casein

The effect of autoproteolysis of Lactococcus lactis lactocepin III on its specificity towards beta-casein was investigated. beta-Casein degradation was performed by using either an autolysin-defective derivative of L. lactis MG1363 carrying the proteinase genes of L. lactis SK11, which was unable to transport oligopeptides, or autoproteolyzed enzyme purified from L. lactis SK11. Comparison of the peptide pools by high-performance liquid chromatography analysis revealed significant differences. To analyze these differences in more detail, the peptides released by the cell-anchored proteinase were identified by on-line coupling of liquid chromatography to mass spectrometry. More than 100 oligopeptides were released from beta-casein by the cell-anchored proteinase. Analysis of the cleavage sites indicated that the specificity of peptide bond cleavage by the cell-anchored proteinase differed significantly from that of the autoproteolyzed enzyme.

Specificity of lactococcus lactis subsp. cremoris SK11 proteinase, lactocepin III, in low-water-activity, high-salt-concentration humectant systems and its stability compared with that of lactocepin I

Marked changes in the specificity of hydrolysis of alphas1-, beta-, and kappa-caseins by lactocepin III from Lactococcus lactis subsp. cremoris SK11 were found in humectant systems giving the equivalent water activity (aw) and salt concentration of cheddar cheese. Correlations were noted between certain peptides produced by the activity of lactocepin III in the humectant systems and peptides found in cheddar cheese. The stability of lactocepin III was compared with that of lactocepin I from L. lactis subsp. cremoris HP in the humectant systems at different pHs. Significant differences between the stability of each of the lactocepin types were evident. The relationship between stability and humectant type, aw, pH, and NaCl concentration was complex. Nevertheless, in those systems where aw, pH, and NaCl concentration were equivalent to those in cheddar cheese, lactocepin I was generally more stable than lactocepin III. It was concluded that differences in the specificity and/or stability of various lactocepin types are likely to persist in cheese itself and therefore potentially contribute to differences in the peptide composition of ripened cheese.