A qualitative screening procedure for the detection of casein hydrolysis by bacteria, using sodium dodecyl sulphate polyacrylamide gel electrophoresis

Characterization of lactococci and lactobacilli isolated from semihard goats' cheese

Several strains ofLactococcus lactissubsp.lactis, Lactobacillus caseiandLactobacillus plantarumisolated from traditional goats' cheese have been studied for titratable acidity, proteolysis in milk and enzymic activities. Aminopeptidasc activities were measured with whole cells and cells permeabilized with Triton X-100. Caseinolytic activity was investigated using electrophoresis in polyacrylamide gel with sodium dodecyl sulphate.Lc. lactissubsp.lactishad a level of proteolytic activity in skim milk greater than that ofLb. casei, while this activity inLb. plantarumwas very low. Alanine aminopeptidase activity was almost non-existent for all strains tested, while lysine aminopeptidase activity appeared to be of fundamentally intracellular origin. Leucine aminopeptidase activity was also greater in cells that had been permeabilized than in whole cells forLb. caseiandLb. plantarum. Lc. lactissubsp.lactisleucine aminopeptidase activity was greater in whole cells. No significant hydrolysis of casein was found withLb. caseiI FPL 725 andLb. plantarumIFPL 722 permeabilized with Triton X-100 after 24 h incubation with whole bovine casein.

Characterization of lactococci isolated from homemade kefir

Five bacteriocin-producing lactococci isolates from traditionally prepared kefir were determined as Lactococcus lactis subsp. lactis. The analyzed isolates showed different plasmid profiles and no cross inhibition between them was detected. Moreover, natural isolate BGKF26 was resistant to the antimicrobial activity of nisin producing strain NP45. Plasmid curing experiments revealed that the genes encoding bacteriocin and proteinase production are located on separate genetic elements, except in BGKF26. Production of the tested bacteriocins depends on the concentration of casitone or triptone in the medium. Higher concentrations of casitone or triptone induce bacteriocin activity. Our DNA-DNA hybridization analyses suggest that the analyzed antimicrobial compounds probably are lactococcin-like bacteriocins.

Plasmid content and bacteriocin production by five strains ofLactococcus lactisisolated from semi-hard homemade cheese

In this study, the plasmid content and bacteriocin production of natural isolates of lactococci were investigated. Five bacteriocin producing lactococcal strains (Lactococcus lactis subsp. lactis BGMN1-2, BGMN1-3, BGMN1-5, BGMN1-6, and BGMN2-7) were isolated as nonstarter microflora of semi-hard homemade cheese and characterized. All isolates contained a number of plasmids. It was shown that lcnB structural genes for bacteriocin lactococcin B were located on large plasmids in all isolates. In the strains BGMN1-3 and BGMN1-5 proteinase prtP genes collocated with lcnB. Furthermore, these strains produced two additional bacteriocins (LsbA and LsbB) with genes responsible for their production and immunity located on the small rolling circle-replicating plasmid pMN5. Using deletion experiments of pMN5, minimal replicon of the plasmid and involvement of a bacteriocin locus in plasmid maintenance were identified. In addition, plasmid curing experiments showed that genes for catabolism or transport of 10 carbohydrates in the strain BGMN1-5 were plasmid located.Key words: lactococci, natural isolates, bacteriocin, plasmid curing.

Partial Identification of Water-Soluble Peptides Released at Early Stages of Proteolysis in Sterilized Ovine Cheese-Like Systems: Influence of Type of Coagulant and Starter

Cheese-like systems were manufactured from sterilized ovine milk, using crude aqueous extracts of Cynara cardunculus or cardosin A isolated therefrom as clotting agent. The effect of adding a commercial starter culture was also assessed. The impact of the type of coagulant used during the initial 24 h of proteolysis was evaluated via separation of peptides in the water-soluble extracts by reverse-phase HPLC, followed by partial sequencing via Edman degradation. Cardosin A accounted for most events of primary proteolysis. The major cleavage sites were Phe105-Met106 in kappa-casein, and Leu127-Thr128, Ser142-Trp143, Leu165-Ser166, and Leu190-Tyr191 in beta-casein. The starter culture did not play an active role during the initial stages of ripening.

Proteinase PI and lactococcin A genes are located on the largest plasmid inLactococcus lactissubsp.lactisbv. diacetylactis S50

Lactococcus lactis subsp. lactis bv. diacetylactis S50 produces a lactococcin A-like bacteriocin named bacteriocin S50, and cell envelope-associated PI-type proteinase activity. This strain harbours 3 small size plasmids: pS6 (6.3 kb), pS7a (7.31 kb), and pS7b (7.27 kb). Plasmid curing using a combination of novobiocin treatment (10 µg·mL–1) and sublethal temperature (40 °C) resulted in a very low yield (0.17%) of Prt–, Bac–, Bacsderivatives, which retained all 3 small size resident plasmids. Pulsed-field gel electrophoresis of DNA isolated from the strain S50 and cured derivatives in combination with restriction enzyme analysis and DNA–DNA hybridization revealed that S50 contains 2 additional large plasmids: pS140 (140 kb) and pS80 (80 kb). Conjugation experiments using strain S50 as a donor and various lactococcal recipients resulted in Prt+, Bac+, Bacrtransconjugants. Analysis of these transconjugants strongly indicated that plasmid pS140 harbours the prt and bac genes encoding proteinase and bacteriocin production, and immunity to bacteriocin, since each Prt+, Bac+, Bacrtranconjugant contained pS140. Accordingly, none of the Prt–, Bac–, Bacstransconjugants contained this plasmid. pS140 was a self-transmissible conjugative plasmid regardless of the host lactococcal recipient used in the test. Frequency of conjugation of plasmid pS140 did not depend on either the donor or recipient strain.Key words: Lactococcus, plasmids, conjugation, bacteriocin, proteinase.

Determination of the domain of the Lactobacillus delbrueckii subsp. bulgaricus cell surface proteinase PrtB involved in attachment to the cell wall after heterologous expression of the prtB gene in Lactococcus lactis

Belonging to the subtilase family, the cell surface proteinase (CSP) PrtB of Lactobacillus delbrueckii subsp. bulgaricus differs from other CSPs synthesized by lactic acid bacteria. Expression of the prtB gene under its own promoter was shown to complement the proteinase-deficient strain MG1363 (PrtP(-) PrtM(-)) of Lactococcus lactis subsp. cremoris. Surprisingly, the maturation process of PrtB, unlike that of lactococcal CSP PrtPs, does not require a specific PrtM-like chaperone. The carboxy end of PrtB was previously shown to be different from the consensus anchoring region of other CSPs and exhibits an imperfect duplication of 59 amino acids with a high lysine content. By using a deletion strategy, the removal of the last 99 amino acids, including the degenerated anchoring signal (LPKKT), was found to be sufficient to release a part of the truncated PrtB into the culture medium and led to an increase in PrtB activity. This truncated PrtB is still active and enables L. lactis MG1363 to grow in milk supplemented with glucose. By contrast, deletion of the last 806 amino acids of PrtB led to the secretion of an inactive proteinase. Thus, the utmost carboxy end of PrtB is involved in attachment to the bacterial cell wall. Proteinase PrtB constitutes a powerful tool for cell surface display of heterologous proteins like antigens.

A new cell surface proteinase: sequencing and analysis of the prtB gene from Lactobacillus delbruekii subsp. bulgaricus

Investigation of the chromosomal region downstream of the lacZ gene from Lactobacillus delbrueckii subsp. bulgaricus revealed the presence of a gene (prtB) encoding a proteinase of 1,946 residues with a predicted molecular mass of 212 kDa. The deduced amino acid sequence showed that PrtB proteinase displays significant homology with the N termini and catalytic domains of lactococcal PrtP cell surface proteinases and is probably synthesized as a preproprotein. However, the presence of a cysteine near the histidine of the PrtB active site suggests that PrtB belongs to the subfamily of cysteine subtilisins. The C-terminal region strongly differs from those of PrtP proteinases by having a high lysine content, an imperfect duplication of 41 residues, and a degenerated sequence compared with the consensus sequence for proteins anchoring in the cell walls of gram-positive bacteria. Finally, the product of the truncated prtM-like gene located immediately upstream of the prtB gene seems too short to be involved in the maturation of PrtB.

Characterization of a Cell Envelope-Associated Proteinase Activity from Streptococcus thermophilus H-Strains

The production and biochemical properties of cell envelope-associated proteinases from two strains of Streptococcus thermophilus (strains CNRZ 385 and CNRZ 703) were compared. No significant difference in proteinase activity was found for strain CNRZ 385 when cells were grown in skim milk medium and M17 broth. Strain CNRZ 703 exhibited a threefold-higher proteinase activity when cells were grown in low-heat skim milk medium than when grown in M17 broth. Forty-one percent of the total activity of CNRZ 385 was localized on the cell wall. The optimum pH for enzymatic activity at 37°C was around 7.0. Serine proteinase inhibitors, such as phenylmethylsulfonyl fluoride and diisopropylfluorophosphate, inhibited the enzyme activity in both strains. The divalents cations Ca 2+ , Mg 2+ , and Mn 2+ were activators, while Zn 2+ and Cu 2+ were inhibitors. β-Casein was hydrolyzed more rapidly than α s1 -casein. The results of DNA hybridization and immunoblot studies suggested that the S. thermophilus cell wall proteinase and the lactococcal proteinase are not closely related.

Chromatographic and electrophoretic methods used for analysis of milk proteins

Current knowledge of milk proteins and their behavior in dairy foods is based on early applications of chromatography and electrophoresis. Electrophoretic identification of the number and genetic variety of milk proteins inaugurated a research effort in which chromatographic techniques were successfully applied to the isolation of each milk protein, thus facilitating the characterization and further study of milk and dairy products. This review focuses on recent applications of chromatography for separations and analysis and on analytical applications of electrophoresis.

Cell-wall-associated proteinase of Lactobacillus delbrueckii subsp. bulgaricus CNRZ 397: differential extraction, purification and properties of the enzyme

Whole cells of Lactobacillus delbrueckii subsp. bulgaricus CNRZ 397 were able to hydrolyse alpha- and beta-caseins. Irrespective of the growth medium used, milk or De Man-Rogosa-Sharpe (MRS) broth, identical patterns of alpha- and beta-casein hydrolytic products, respectively, were visualized by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. A soluble proteinase present in cell-wall extracts was active on caseins and displayed the same hydrolytic patterns as whole cells. It was purified from cell-wall extract to homogeneity by ultrafiltration and ion exchange chromatography. The enzyme is a monomer with a molecular mass of 170 kDa, an optimum temperature of 42 degrees C and an optimum pH of 5.5. It was strongly activated by dithiothreitol and partially inhibited by E-64. These properties indicate that cysteine residues play an important role in the enzyme mechanism. The purified proteinase was not able to hydrolyse di- or tripeptides.

Characterization of the Cell Wall-Bound Proteinase of Lactobacillus casei HN14

Lactobacillus casei HN14, which was isolated from homemade cheese, produces an extracellular, cell wall-bound proteinase. The HN14 proteinase can be removed from the cell envelope by washing the cells in a Ca 2+ -free buffer. The activity of the crude proteinase extract is inhibited by phenylmethylsulfonyl fluoride, showing that the enzyme is a serine-type proteinase. Considering the substrate specificity, the HN14 proteinase is similar to the lactococcal PI-type enzyme, since it hydrolyzes β-casein only. Lactobacillus casei HN14 appeared to be plasmid free, which suggests that the proteinase gene is chromosomally located. Chromosomal DNA of this strain hybridizes with DNA probes Q1 (which contains a fragment of the prtM gene) and Q6 and Q92 (which contain fragments of the prtP gene); all three probes originated from the proteinase gene region of Lactococcus lactis subsp. cremoris Wg2. A restriction enzyme map of the proteinase region of Lactobacillus casei HN14 was constructed on the basis of hybridization experiments. Comparison of the restriction enzyme maps of the Lactobacillus casei HN14 proteinase gene region and those of lactococcal proteinase gene regions studied so far indicates that they are highly similar.