Susan K. Harlander

Comparative effects of exogenous lactase (β-galactosidase) preparations on in vivo lactose digestion

Microbial-derived β-galactosidase (β-gal) enzyme preparations improvein vivo lactose digestion and tolerance through enhanced gastrointestinal digestion of lactose. Three different β-gal preparations, Lactogest (soft gel capsule), Lactaid (caplet), and DairyEase (chewable tablet) and placebo were fed to lactose maldigesters with either 20 g or 50 g of lactose to compare the efficacy of these products and to further establish a dose-response relationship for use. All enzyme preparations dramatically reduced both the peak and total breath hydrogen production when fed with milk containing 20 g of lactose. Four capsules of Lactogest, two caplets of Lactaid, or two tablets of DairyEase (each treatment containing approx 6000 IU) reduced total hydrogen production significantly (P<0.05) below that observed with two capsules of Lactogest (containing approx 3000 IU) in a stoichiometric manner. Symptoms were significantly (P<0.05) less severe with all the β-gal products. In contrast, with 50 g of lactose in water, peak and total hydrogen production was modestly, but not significantly reduced by the enzyme treatment. Furthermore, symptom scores for bloating, cramping, nausea, pain, diarrhea, and flatus were not different between treatments and the control. The 50-g lactose dose appeared to overwhelm the ability of either 3000 or 6000 IU of β-gal to assist significantly with lactose digestion. Results from these studies demonstrate the relative equivalency of chewable, caplet, and soft-get β-gal products, based on IUs of enzyme fed.

WHO-sponsored international collaborative study to evaluate methods for subtyping Listeria monocytogenes : restriction fragment length polymorphism (RFLP) analysis using ribotyping and Southern hybridization with two probes derived from L. monocytogenes chromosome

Seven laboratories participated in a WHO-sponsored international collaborative study, to evaluate methods for subtyping Listeria monocytogenes, by performing restriction fragment length polymorph sm (RFLP) analysis-based subtyping of an international study set of 80 strains of L. monocytogenes that included 22 epidemiologically related groups. The RFLP analysis was done by Southern hybridization with one of two types of probes found in multiple copies on the chromosome of L. monocytogenes. Six laboratories performed ribotyping. These laboratories used EcoRI enzyme to restrict the L. monocytogenes DNA and ribosomal RNA or DNA as the probe for Southern hybridizations. The seventh laboratory used Ncil to restrict the DNA, and two probes, one randomly cloned and the other containing repeat sequences cloned from L. monocytogenes DNA. The overall discriminating power of ribotyping, as estimated by calculation of Simpsons index of diversity, ranged from 0.83 to 0.88 for the six laboratories. The discriminating power of the combination of two probes used by Laboratory 7 was 0.91. Ribotyping and the cloned probes used by Laboratory 7 discriminated poorly between serotype 4b strains. Neither method identified three atypical strains (identified by other subtyping methods) included in three apparently epidemiologically related groups. Ribotyping did not discriminate between strains of serotypes 4b and 4b(X) in one epidemiologically related group of strains; one cloned probe used by Laboratory 7 discriminated between these strains. Intra-laboratory reproducibilities for the seven laboratories ranged from 80.0 to 100%. as determined by their abilities to correctly identify 11 pairs of duplicate strains included in the study set. Inter-laboratory reproducibilities were generally very good considering that no attempt was made to standardize protocols used by the participants.

DNA fingerprinting of lactococci and streptococci used in dairy fermentations

SummaryA DNA fingerprinting procedure was developed for strains of Lactococcus lactis subsps. lactis and cremoris, biovar. diacetylactis, and Streptococcus salivarius subsp. thermophilus, used in dairy fermentations. Total cellular DNA was extracted and digested with restriction endonucleases, HindIII or HaeIII, followed by separation of the fragments using agarose gel electrophoresis. L. lactis C2 was used as a representative strain for examining the effect of growth phase and cell concentration, cell washing conditions prior to lysis, type and concentration of the enzyme used to digest the cell wall, composition of the lysis buffer, and gel electrophoresis conditions. Following optimization of the fingerprinting procedure, electrophoretic migration of fragments from 23 strains produced reproducible gel patterns. L. lactis subsp. lactis strains ML3 and C2 appeared to be identical when restrricted with either Hind III or HaeIII. Similarly, S. salivarius subsp. thermophilus strains 19987 and 19258, and L. lactis subsp. cremoris strains 134 and C3, appeared to have identical DNA fingerprints following digestion with HindIII. To determine the usefulness of this technique for monitoring population changes during fermentation, various ratios of two closely related strains were inoculated into milk and allowed to grow for 16 h at 32° C. The initial inoculum ratios were determined by standard plate counts, and the final ratio was deterimined by DNA fingerprinting. DNA fingerprinting will be useful in the identification, characterization, and comparison of food fermentation microorganisms.

Construction of an integrative food-grade cloning vector for Lactobacillus acidophilus

An integrative cloning vector was constructed using a randomly clonedHindIII-digested chromosomal fragment fromLactobacillus acidophilus ADH inserted into anEscherichia coli vector, pBluescript II SK +. Southern hybridization studies demonstrated homology of the inserted fragment with one otherL. acidophilus strain and oneBifidobacterium strain. Identification of aSauI site located near the middle of the 1.9-kb ADH chromosomal fragment made it possible to clone theLactobacillus bulgaricus β-galactosidase (EC 3.2.1.23) gene into this vector. The vector was unable to replicate in the homologous host,L. acidophilus ADH, following electroporation. The chromosomal fragment allowed the integration of the β-galactosidase gene (β-gal) into the host chromosome via homologous recombination. The size of the two flankingL. acidophilus ADH chromosomal fragments, approxiamately 0.95 kb each, was sufficient to allow the double cross-over to take place. Southern hybridization demonstrated that onlyL. acidophilus andL. bulgaricus DNA has been integrated into the chromosome of the host strain. The β-galactosidase activity of the transformat was increased approximately 200-fold when compared to the enzyme activity of the wild-type strain. The βgal gene remained stable in the transformant strain after 30 transfers in growth media without selection pressure. This first-generation integrative cloning vector is constructed solely of DNA from organisms consumed by humans and could be considered a food-grade vector system.

Characterization of a bacteriocin from Pediococcus acidilactici PC and comparison of bacteriocin-producing strains using molecular typing procedures

SummarySixteen pediococcal strains, including eleven Pediococcus acidilactici and five P. pentosaceus strains were screened for inhibitory potential using a deferred overlay spot method against a limited collection of foodborne pathogens. Of those screened, P. acidilactici PC, an organism isolated from fermented sausage, was effective and subsequently screened for inhibitory potential against 46 foodborne pathogens and 28 other lactic acid bacteria. Strain PC produced an antimicrobial agent capable of inhibiting members of the genera Listeria, Clostridium, Leuconostoc and Pediococcus. Gram-negative microorganisms from seven genera, Lactococcus, Streptococcus and Lactobacillus strains were unaffected by the inhibitory substance. The inhibitory agent was sensitive to proteolytic enzymes and exhibited a bactericidal mode of action, confirming the identity as a bacteriocin. In addition, the partially purified bacteriocin was thermally stable up to 100°C for 60 min and maintained inhibitory potential over a wide range of pH values. Plasmid curing studies suggested linkage of bacteriocin production to a 5.5-MDa plasmid. Plasmid profiles were identical for P. acidilactici PC, PAC1.0 and PO2. Genetic analysis of total genomic DNA via DNA fingerprinting and ribosomal RNA (rRNA) typing provided further evidence that these strains were identical. DNA fingerprinting and rRNA typing also showed utility in discrimination between and within other species of pediococci.

Production of aroma compounds from strawberry cell suspension cultures by addition of precursors

The potential of using short-chain fatty acids and α-keto-acid as precursors for the production of typical fruit-type aroma compounds by strawberry cell suspension cultures was investigated. Analysis of the headspace by gas chromatography revealed that supplemented strawberry cell suspension cultures were capable of producing low concentrations of ethyl butyrate and butyl butyrate, and converting α-ketovalerate to butanal and butanol. No aroma compounds were produced in unsupplemented or heat-treated cell suspension cultures. The results indicated that esterase, decarboxylase, and alcohol dehydrogenase might exist in strawberry cell cultures. Increasing temperature, illumination and addition of mannitol favoured the production of butyl butyrate. No difference was found between one- and two-week-old cultures in the ability to convert precursors to corresponding aroma compounds.

Construction of first-generation lactococcal integrative cloning vectors.

Using a randomly-cloned, HindIII-digested, chromosomal fragment from Lactococcus lactis subsp. lactis LM0230, first-generation lactococcal integrative cloning vectors were developed. Through dideoxy DNA sequence analysis, the cloned chromosomal DNA fragment was determined to be 1026 base pairs. Southern hybridization studies demonstrated applicability of the integrative vector to other strains of L. lactis and L. lactis subsp. cremoris. Identification of a single NruI site near the middle of the chromosomal fragment allowed insertion of the erythromycin (Em)-resistance (eryr) gene obtained from L. lactis IL1837. Integration of the eryr gene into the L. lactis LM0230 chromosome was achieved by a Campbell-like recombination. The nisin (Nis)-resistance (nisr) gene from L. lactis IL1904 was inserted into the NruI site in a separate clone and integration into the L. lactis LM0230 chromosome was achieved via a replacement recombination event following electroporation of the linearized nisr fragment flanked by the cloned chromosomal DNA. Transformants grown in the absence of either Em or Nis for >200 generations and subsequently transferred to various concentrations of the selectable agent confirmed the stability of the integrated genes. Further studies involving the Nis-resistant (Nisr) transformant suggested that the integrated nisr gene may be amplifying within the host chromosome.

Streptococcus Mutans Dextransucrase: Phosphoglycerides and the Detection of Inhibitory Antibodies in Sera

1) Rabbit and human sera contain high concentrations of phospholipids which can interact with dextransucrase causing enhanced glucan production. 2) Antibody in sera capable of blocking glucan synthesis can be accurately detected by adding excess LPC and dextran to the enzyme prior to incubating with antiserum and assaying with sucrose.