Influence of lactococcal plasmid on the specific growth rate of host cells.

AIMS: To investigate a lactococcal plasmid responsible for a reduction in growth rate of its host cell. METHODS AND RESULTS: Lactococcus lactis subsp. lactis biovar. diacetylactis DRC1 carries a high number of plasmids. The DRC1 wild-type strain was found to grow more slowly than a plasmid-free derivative of DRC1. The plasmids extracted from DRC1 together with an indicator plasmid were cotransformed into the plasmid-free strain DRC1021. A 7.4-kb cryptic plasmid, designated pDR1-1, was found to significantly affect the maximum specific growth rate ( micro max) of the host cell. Polymerase chain reaction (PCR) analysis was carried out in order to detect the presence of pDR1-1 in the other L. lactis strains. The micro max of the single pDR1-1-positive strain was determined to be the same as that of DRC1. SIGNIFICANCE AND IMPACT OF THE STUDY: These results suggest that pDR1-1 (or a pDR1-1-like plasmid) is a critical factor in the reduction of the micro max of DRC1, and that its effect on the micro max is significantly greater than that of any other coexisting plasmid.

Stimulatory and inhibitory effects of protein amino acids on growth rate and efficiency of mixed ruminal bacteria.

Mixed ruminal bacteria were incubated in vitro with glucose, xylose, cellobiose, and various protein amino acids replaced isonitrogenously with 25% (i.e., 25 mg of N/L) of ammonia-N, to determine the growth rate and the amount of sugar consumed in the exponential growth phase. The growth rate and efficiency (grams of bacteria per gram of sugars) increased by 46 and 15%, respectively, when a mixture of 20 amino acids was added. On the other hand, neither growth rate nor efficiency increased when any one of these amino acids was added singly, except for Glu and Gln, each of which produced significant but small improvements. The stimulatory effect of the combined amino acids on bacterial growth declined when each of Leu, Trp, Tyr, Glu, Met, Phe, and Val was removed from the original group of 20. When a mixture of only these seven amino acids was used as a supplement, their stimulatory effects on growth rate and efficiency were only 21 and 25%, respectively, of the effects that the mixture of 20 amino acids showed. The effects increased to 76 and 72% on growth rate and efficiency, respectively, when Gly, Cys, and His were supplied in addition to the seven amino acids. The growth rate and efficiency of the ruminal bacteria were inhibited by an addition of each of Ile, Thr, Cys, Phe, Leu, Lys, or Val to ammonia-N, and the effects of the first five of these amino acids were highly significant. Isoleucine, threonine, and phenylalanine were each inhibitory even at a low concentration (1 mg of NL), while cysteine and leucine showed inhibitory effects at higher concentrations (more than 10 mg of N/L). A higher growth rate of the ruminal bacteria when supplemented with amino acid mixtures was accompanied with a higher growth efficiency, which was attributable to a relatively smaller proportion of energy expended on maintenance according to the Pirt derivation.

Enhancement of bile tolerance in lactococci by Tween 80.

AIMS: The aim of this study is to clarify the effect of Tween 80 on bile tolerance of lactococci. METHODS AND RESULTS: Four out of the six strains of lactococci could grow in broth containing 0.3% bile in the presence of 1% Tween 80, but grew slightly or not at all in the absence of Tween 80. Growing with Tween 80 altered the fatty acid composition of all three strains tested, but it is not clear which fatty acid influences bile tolerance. Material that absorbed light at 260 nm leaked from the cells tested with bile, but the leakage was decreased by addition of 1% Tween 80. Coincidentally, the decrease in the cell count by exposure to bile was suppressed by addition of Tween 80. CONCLUSIONS: Tween 80 enhances bile tolerance of some strains of lactococci. SIGNIFICANCE AND IMPACT OF THE STUDY: It is clarified that Tween 80 in the broth contributes to the bile tolerance of lactococci by reducing the cellular leakage caused by bile.

Cholesterol removal from media by lactococci.

Elevated serum cholesterol in humans is generally a risk factor correlated with the development of coronary heart disease. It has been reported that a culture of Lactobacillus acidophilus actively taking up cholesterol from a laboratory medium would function in vivo to exert a hypocholesterolemic effect. In the present study, seven strains of the genus Lactococcus were examined for their ability to remove cholesterol from laboratory media during growth. All strains of lactococci tested could remove cholesterol from media without degrading cholesterol. The amount of cholesterol removed was strain specific. Among them, Lc. Lactis subsp. lactis biovar diacetylactis N7 could remove as much cholesterol as L. acidophilus ATCC 43121, which had a beneficial influence on serum cholesterol levels in pigs. The manner of cholesterol removal by strain N7 corresponded to the manner of its growth. The growth of strain N7 (growth yield and growth efficiency) was enhanced. The fatty acid composition of the cells of strain N7 was altered by removing cholesterol from the media. The ability to remove cholesterol was also observed in the heat-killed cells of strain N7. However, the amount of cholesterol removed by the cells during growth was significantly higher than that removed by the heat-killed cells. Thus, strain N7 has the ability to remove cholesterol from media independently of whether cells are viable. These results indicate that administration of strain N7 in vivo may well be promising on the hypocholesterolemic effect.

Isolation and characterization of enterocin SE-K4 produced by thermophilic enterococci, Enterococcus faecalis K-4.

Enterococcus sp. K-4, with a bacteriocin-like activity against E. faecium, was isolated from grass silage in Thailand. Morphological, physiological, and phylogenetic studies clearly identified strain K-4 as a strain of E. faecalis. Strain K-4 produced a maximal amount of bacteriocin at 43-45 degrees C. We purified, for the first time, the bacteriocin produced at high temperature by E. faecalis to homogeneity, using adsorption on cells of the producer strain and reversed-phase liquid chromatography. The bacteriocin, designated enterocin SE-K4, is a peptide of about 5 kDa as measured by SDS-PAGE, and Mass spectrometry analysis found the molecular mass of 5356.2, which is in good agreement. The amino acid sequencing of the N-terminal end of enterocin SE-K4 showed apparent sequence similarity to class IIa bacteriocins. Enterocin SE-K4 was active against E. faecium, E. faecalis, Bacillus subtilis, Clostridium beijerinckii, and Listeria monocytogenes. Enterocin SE-K4 is very heat stable.

Efficient protoplast regeneration for some homofermentative lactobacilli and pediococci.

Conditions for protoplast regeneration were examined for several strains of homofermentative lactobacilli and pediococci isolated from silage. Attempts to regenerate protoplasts using previously published agar regeneration media for lactobacilli were unsuccessful for most of the strains. Replacing or increasing colloidal substances in a medium containing raffinose and MgCl(2) as osmotic stabilizers enabled efficient regeneration of the protoplasts at a frequency of 10-99%. A medium containing gelatin, polyvinylpyrrolidone (PVP) and no agar was effective for Lactobacillus plantarum, Lactobacillus pentosus and Lactobacillus rhamnosus protoplasts. An agar medium containing PVP (PVP medium) was effective for Pediococcus sp. protoplasts, and addition of agarose to the PVP medium enabled regeneration of Lactobacillus casei protoplasts. A medium containing calcium alginate gel and no agar was effective for Lactobacillus curvatus protoplasts. The type of colloidal substance required for protoplast regeneration varied from species to species. This result suggested that several kinds of media may be necessary to regenerate protoplasts for all the genera of lactobacilli and pediococci.

Characterization of a phage resistance plasmid, pLKS, of silage-making Lactobacillus plantarum NGRI0101.

Phage contamination has resulted in abnormal fermentation in silage. We isolated a phage-resistant strain, Lactobacillus plantarum NGRI0101 from silage. The strain carried two plasmids, pLKL (6.8 kb) and pLKS (2.0 kb). By curing and retransformation of the plasmids, we clarified that pLKS has phage resistant activity, characterized as no adsorption inhibition. pLKS has 2,025 bp and three orfs, orfl23, orf132, and orf918. The predicted amino acid sequence of the orf918 product showed high similarity to those of Rep proteins of Pediococcus halophilus plasmid pUCL287 and Lactobacillus acidophilus plasmid pLA103. The replication origin (ori) was upstream from orf918. There was no gene similar to typical phage resistant genes encoded by known plasmids. The phage resistance of L. plantarum NGRI0101 may possibly be due to a plasmid-encoded abortive infection.

Inactivation of the glutamate decarboxylase gene in Lactococcus lactis subsp. cremoris.

Lactococcus lactis subsp. lactis strains show glutamate decarboxylase activity, whereas L. lactis subsp. cremoris strains do not. The gadB gene encoding glutamate decarboxylase was detected in the L. lactis subsp. cremoris genome but was poorly expressed. Sequence analysis showed that the gene is inactivated by the frameshift mutation and encoded in a nonfunctional protein.

Purification and some characteristics of enterocin ON-157, a bacteriocin produced by Enterococcus faecium NIAI 157.

Bacteriocin-like activity (BLA) was screened in 690 strains of lactic acid bacteria isolated from plant materials such as silages and fermented vegetables. Among them, a strain identified as Enterococcus faecium NIAI 157 showed a clear BLA against the indicator strain, Ent. faecium IFO 13712. The proteinaceous nature and antimicrobial activity against closely related species strongly indicated that this BLA was a bacteriocin and was designated enterocin ON-157. The bacteriocin activity of this strain was extracellularly produced in the logarithmic growth phase in MRS broth and purified by ultrafiltration, ammonium sulphate precipitation and cation-exchange chromatography. Purified enterocin ON-157 had a molecular weight of approximately 2500 Da in SDS-PAGE analysis and was easily inhibited by treatment with alpha-amylase and proteolytic enzymes. Enterocin ON-157 had a bactericidal mode of action and inhibited the growth of the enterococci, Lactobacillus sake and Listeria monocytogenes. Enterococcus faecium NIAI 157 harboured two plasmids, 49.0 kb and 43.7 kb, and a variant missing a larger plasmid by curing with novobiocin lost the bactriocin activity.

Prevention of aerobic spoilage of maize silage by a genetically modified killer yeast, Kluyveromyces lactis, defective in the ability to grow on lactic acid.

In this study, we propose a new process of adding a genetically modified killer yeast to improve the aerobic stability of silage. Previously constructed Kluyveromyces lactis killer strain PCK27, defective in growth on lactic acid due to disruption of the gene coding for phosphoenolpyruvate carboxykinase, a key enzyme for gluconeogenesis, inhibited the growth of Pichia anomala inoculated as an aerobic spoilage yeast and prevented a rise in pH in a model of silage fermentation. This suppressive effect of PCK27 was not only due to growth competition but also due to the killer protein produced. From these results, we concluded that strain PCK27 can be used as an additive to prolong the aerobic stability of maize silage. In the laboratory-scale experiment of maize silage, the addition of a killer yeast changed the yeast flora and significantly reduced aerobic spoilage.

Lactococci as probiotic strains: adhesion to human enterocyte-like Caco-2 cells and tolerance to low pH and bile.

There have been few studies on the probiotic activity of Lactococcus strains although they are commonly used as starter bacteria in manufacturing many kinds of fermented dairy products. Nine strains of the genus Lactococcus were examined for their probiotic properties, such as adherence to human enterocyte-like Caco-2 cells and tolerance to acid and bile. Six strains were adhesive and the highest adhesion was observed with Lactcoccus lactis ssp. lactis NIAI527. This strain adhered to the microvilli of cells as observed by scanning electron microscopy and also tolerated low pH and bile. These properties should make strain 527 a potential new probiotic strain.

Influence of lactobacillus spp. from An inoculant and of weissella and leuconostoc spp. from forage crops on silage fermentation

Lactobacillus spp. from an inoculant and Weissella and Leuconostoc spp. from forage crops were characterized, and their influence on silage fermentation was studied. Forty-two lactic acid-producing cocci were obtained from forage crops and grasses. All isolates were gram-positive, catalase-negative cocci that produced gas from glucose, and produced more than 90% of their lactate in the D-isomer form. These isolates were divided into groups A and B by sugar fermentation patterns. Two representative strains from the two groups, FG 5 and FG 13, were assigned to the species Weissella paramesenteroides and Leuconostoc pseudomesenteroides, respectively, on the basis of DNA-DNA relatedness. Strains FG 5, FG 13, and SL 1 (Lactobacillus casei), isolated from a commercial inoculant, were used as additives to alfalfa and Italian ryegrass silage preparations. Lactic acid bacterium counts were higher in all additive-treated silages than in the control silage at an early stage of ensiling. During silage fermentation, inoculation with SL 1 more effectively inhibited the growth of aerobic bacteria and clostridia than inoculation with strain FG 5 or FG 13. SL 1-treated silages stored well. However, the control and FG 5- and FG 13-treated silages had a significantly (P < 0.05) higher pH and butyric acid and ammonia nitrogen contents and significantly (P < 0. 05) lower lactate content than SL 1-treated silage. Compared with the control silage, SL 1 treatments reduced the proportion of D-(-)-lactic acid, gas production, and dry matter loss in two kinds of silage, but the FG 5 and FG 13 treatments gave similar values in alfalfa silages and higher values (P < 0.05) in Italian ryegrass silage. The results confirmed that heterofermentative strains of W. paramesenteroides FG 5 and L. pseudomesenteroides FG 13 did not improve silage quality and may cause some fermentation loss.

Isolation and nucleotide sequence of the gene encoding phosphoenolpyruvate carboxykinase from Kluyveromyces lactis.

The KlPCK1 gene encoding phosphoenolpyruvate carboxykinase (PEPCK; ATP-dependent) was cloned from the Kluyveromyces lactis genome using a PCR amplicon from Saccharomyces cerevisiae PCK1 gene as a probe. A DNA fragment of about 4.8 kb containing KlPCK1 complemented PEPCK activity of the mutant of S. cerevisiae defective in PEPCK. The KlPCK1 gene has an open reading frame of 1629 bp (543 amino acids). The KlPCK1 nucleotide sequence and deduced amino acid sequence showed 76% and 84% homologies to those of S. cerevisiae PCK1, respectively. Multiple alignment of ATP-dependent PEPCK genes shows highly conserved regions.

Effect of NaCl-tolerant lactic acid bacteria and NaCl on the fermentation characteristics and aerobic stability of silage.

NaCl-tolerant lactic acid bacteria (LAB) strains LC-10 (Lactobacillus casei) and LP-15 (Lact. plantarum) and NaCl were used as additives to sorghun (Sorghum bicolor). Numbers of LAB were significantly (P < 0.05) higher in all the additive-treated silages than in the control silage at an early stage of ensiling. During the fermentation process, addition of NaCl or LAB effectively inhibited the growth of aerobic bacteria and clostridia, but not yeasts. All the additive-treated silages had significantly (P < 0.05) lower pH, ammonia nitrogen content, dry matter loss and gas production but significantly (P < 0.05) higher lactic acid content and residual water soluble carbohydrates compared with the control silage. The improvement in silage quality was in the order: LAB > NaCl > control. Yeast counts were high in all additive-based silages and they increased during the exposure of the silages to air. As a result, these silages suffered aerobic deterioration, whereas the control silage was stable. The results confirmed that the NaCl or LAB improved fermentation quality but did not prevent aerobic deterioration of the silage.

Selection of killer yeasts (Kluyveromyces lactis) to prevent aerobic deterioration in silage making.

Killer spectra were investigated to select killer yeast strains to inhibit the growth of wild yeasts that cause aerobic deterioration of silage. Kluyveromyces lactis IFO 1267 was characterized by its rapid killing activity and wide spectrum against the target yeasts. Crude killer protein produced by K. lactis IFO 1267 or inoculation of the cell itself inhibited the growth of the target strain, Saccharomyces cerevisiae IFO 0304 (in liquid or in solid culture), particularly when lactose was used as carbon source instead of glucose.