Production of yoghurt with mild taste by a Lactobacillus delbrueckii subsp. bulgaricus mutant with altered proteolytic properties

In this communication, we describe the isolation of a Lactobacillus delbrueckii subsp. bulgaricus 92063 mutant strain named pH-P11, which differed from the parent strain by low proteolytic activity and altered regulation of expression of lacZ in the presence of glucose or lactose. In the presence of lactose, beta-galactosidase activity was approximately twice as high in pH-P11 than in the wild type. pH-P11 exhibited protosymbiosis together with Streptococcus thermophilus. Yoghurt produced with pH-P11 was characterized by low acidity and little post-acidification during storage. The organoleptic properties (absence of bitterness and other off-flavors, weak sourness, and clear yoghurt taste) were those of a typical "yoghurt mild". This mild flavor was achieved at rather high cell counts of lactobacilli even at the end of shelf-life. High cell counts in conjunction with high beta-galactosidase activity make pH-P11 an interesting strain for application in yoghurt especially designed for consumers with lactose malabsorption. In contrast to "yoghurt mild", which is predominantly produced with Lactobacillus acidophilus together with Streptococcus thermophilus, the product obtained by fermentation with pH-P11 and Streptococcus thermophilus concurs with international standards for yoghurt. During frequent sub-culturing, strain pH-P11, which is supposed to differ from the wild type by one or a few so-far-not-characterized mutations, showed sufficient stability for application in industrial production.     

Electrophoretic mobility of DNA in gels. III. Experimental study on band inversion

A series of experiments has been undertaken to study the phenomenon of band inversion that can occur during the separation of linear double-stranded DNA in agarose gels under constant electric field. We found that there can be a considerable band inversion when the DNA fragments are moving as a single species. When separating mixtures of fragments, as it is usually done in routine experiments, the band inversion effect is strongly reduced. Our data support the assumption that DNA–DNA interactions can play an important role in electrophoretic separations. © 1995 John Wiley & Sons, Inc.     

Suppression of the btuB451 mutation by mutations in the tonB gene suggests a direct interaction between TonB and TonB-dependent receptor proteins in the outer membrane of Escherichia coli

In cells of Escherichia coli, the function of the tonB gene is needed for energy-dependent transport processes mediated by the outer-membrane receptors for iron siderophore complexes and vitamin B12. The btuB451 mutation has the same effect on vitamin B12 transport as does a tonB mutation. When a btuB451 strain carried a plasmid with the intact tonB gene, partial revertant strains were isolated which had acquired the ability to grow on 5 nM vitamin B12. This suppression activity was associated with the plasmid, suggesting that a mutation within the tonB gene on the plasmid allowed the mutant BtuB receptor to function in the transport of the vitamin. The nucleotide sequence of the entire tonB gene of ten independently isolated suppressing plasmids was determined. Only a single nucleotide change had occurred in each of the cases. The same codon was always affected resulting in the conversion of glutamine-165 to a leucine in seven of the ten isolates and to a lysine in the other three. The phenotype of strains carrying both types of altered tonB genes showed the retention of their function for other TonB-dependent processes in addition to their suppressor properties with respect to the btuB451 mutation. The fact that mutations suppressing the btuB451 mutation occurred in the tonB gene suggests that there is a direct interaction between TonB and TonB-dependent receptors in the outer membrane of E. coli.     

Mice lacking the poly(ADP-ribose) polymerase gene are resistant to pancreatic beta-cell destruction and diabetes development induced by streptozocin

Human type 1 diabetes results from the selective destruction of insulin-producing pancreatic beta cells during islet inflammation. Cytokines and reactive radicals released during this process contribute to beta-cell death. Here we show that mice with a disrupted gene coding for poly (ADP-ribose) polymerase (PARP-/- mice) are completely resistant to the development of diabetes induced by the beta-cell toxin streptozocin. The mice remained normoglycemic and maintained normal levels of total pancreatic insulin content and normal islet ultrastructure. Cultivated PARP-/- islet cells resisted streptozocin-induced lysis and maintained intracellular NAD+ levels. Our results identify NAD+ depletion caused by PARP activation as the dominant metabolic event in islet-cell destruction, and provide information for the development of strategies to prevent the progression or manifestation of the disease in individuals at risk of developing type 1 diabetes.     

Strength of coupling between phyto- and zooplankton in Lake Lucerne (Switzerland) during phosphorus abatement subsequent to a weak eutrophication

The strength of coupling between phyto- and zooplankton was measured from 1961 to 1995 by comparing the grazing effect of zooplankton (visible as clear-water phase only in 1968-1994) and also by excluding zooplankton in limnocorral experiments (1980-1984). Although long-term (1961-1995) measurements show little evidence of temporal changes in total biomass of phytoplankton or zooplankton, there is strong evidence of changes in the strength of coupling due to top-down effects. The ratio of change in biomass caused by cladocerans in the intensive grazing period of each year (May/June) and the recovery of netplankton after this period seems to be strongly influenced by the trophic state of the lake. When Lake Lucerne was mesotrophic (1971-1982), the annual mean of monthly changes in phytoplankton biomass was in the range of 1-2, indicating that the biomass more than doubled (or halved) from month to month (no change = 0). Under oligotrophic conditions, the annual average of monthly changes in biomass was below 0.5. Grazing measurements in limnocorrals at 2 m depth with labelled food (Rhodomonas lacustris) showed distinct diel rhythms, with maximum community grazing rate at dusk and dawn. These diel changes were caused by vertical migration of the zooplankton. Grazing rate and zooplankton biomass were strongly coupled, with a maximum rate of 100-200 ml day^-1 mg^-1 (zooplankton biomass) when daphnids were dominant. The decrease in biomass caused by excessive grazing shows parallel trends in nanoplankton and netplankton. However, the increase in biomass after the clear-water phase was largely caused by netplankton.      

Light pulses do not induce c-fos or per1 in the SCN of hamsters that fail to reentrain to the photocycle

Circadian activity rhythms of most Siberian hamsters (Phodopus sungorus sungorus) fail to reentrain to a 5-h phase shift of the light-dark (LD) cycle. Instead, their rhythms free-run at periods close to 25 h despite the continued presence of the LD cycle. This lack of behavioral reentrainment necessarily means that molecular oscillators in the master circadian pacemaker, the SCN, were unable to reentrain as well. The authors tested the hypothesis that a phase shift of the LD cycle rendered the SCN incapable of responding to photic input. Animals were exposed to a 5-h phase delay of the photocycle, and activity rhythms were monitored until a lack of reentrainment was confirmed. Hamsters were then housed in constant darkness for 24 h and administered a 30-min light pulse 2 circadian hours after activity onset. Brains were then removed, and tissue sections containing the SCN were processed for in situ hybridization. Sections were probed with Siberian hamster c-fos and per1 mRNA probes because light rapidly induces these 2 genes in the SCN during subjective night but not at other circadian phases. Light pulses induced robust expression of both genes in all animals that reentrained to the LD cycle, but no expression was observed in any animal that failed to reentrain. None of the animals exhibited an intermediate response. This finding is the first report of acute shift in a photocycle eliminating photosensitivity in the SCN and suggests that a specific pattern of light exposure may desensitize the SCN to subsequent photic input.