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1.
Characterization of salt-tolerant mutant for enhancement of l-threonine production in Escherichia coli 总被引:1,自引:0,他引:1
Escherichia coli strain HS3, metabolically engineered to have Met–, AHVr, IleL and AECr characteristics, produced 58.0 g/l of l-threonine, but it was neither salt-tolerant nor osmotolerant; and the growth and threonine production of the strain were
severely inhibited both by the addition of NaCl with a concentration higher than 2% and by the presence of glucose with a
concentration higher than 10%. Therefore, salt-tolerant mutants were isolated. The salt-tolerant mutants, HS454 and HS528
which were derived from strain HS3, were both tolerant to salt (2%) and hyperproductive. The growth and l-threonine production by the mutant strain HS454 were almost unaffected by a glucose concentration lower than 10%, but gradually
reduced with increasing glucose concentration, up to 15%. However, the mutant strain HS528 showed slightly enhanced growth
and l-threonine production with increasing glucose concentration, up to 10–12.5%. Strains HS454 and HS528 produced 69.8 g/l and
74.0 g/l of l-threonine, respectively in a 5-l jar fermentor.
Received: 21 January 2000 / Received revision: 31 March 2000 / Accepted: 5 May 2000 相似文献
2.
Efficient transformation of pBR322 and its derived
plasmids, which have been widely used as cloning vectors in Escherichia
coli, was observed in Pseudomonas avenae (K1), the pathogen of
leaf blight disease in cereals. Moreover, there was a 10- to 50-fold
transformation efficiency (1.3–3.0 × 106/μg DNA) in the
proline-auxotrophic mutant (Pr47), whose virulence to rice seedlings
decreased. Similar enhancement of the frequency of transfer by mobilization
of RSF1010, a broad host range plasmid, was observed in the recipient Pr47
strain in mating with donor Pseudomonas syringae. The plasmids
harbored in these strains were maintained very stably after subcultures.
Thus, a highly efficient transformation system with pBR322-derived plasmids
used as a vector and Pseudomonas as a host bacterium was developed.
Received: 13 July 1996 / Accepted: 26 August 1996 相似文献
3.
In this study, the effects of inositol addition on expression of the MAL gene encoding maltase and phosphatidylinositol (PI) biosynthesis in Schizosaccharomyces pombe (a naturally inositol-requiring strain) were examined. We found that specific maltase activity was at its maximum when the concentration of added inositol reached 6 μg ml−1 in a synthetic medium containing 2.0% (w/v) glucose. When the concentration of added inositol was 1 μg ml−1 in the medium, repression of MAL gene expression occurred at glucose concentration higher than 0.2% (w/v). However, when S. pombe was cultured in the synthetic medium containing 6 μg ml−1, repression of maltase gene expression occurred only at initial glucose concentration above 1.0% (w/v). More mRNA encoding maltase was detected in the cells grown in the medium with 6 μg ml−1 inositol than in those grown in the same medium with 1 μg ml−1 inositol. These results demonstrate that higher inositol concentrations in the synthetic medium could derepress MAL gene expression in S. pombe. PI content of the yeast cells grown in the synthetic medium with 6 μg ml−1 of inositol was higher than that of the yeast cells grown in the same medium with 1 μg ml−1 of inositol. This means that PI may be involved in the derepression of MAL gene expression in S. pombe. 相似文献
4.
Plant regeneration via somatic embryogenesis, and transient gene expression in sweet potato protoplasts 总被引:1,自引:0,他引:1
A method for regenerating plants from petiole protoplasts of the in vitro-raised sweet potato cultivar Jewel is described.
Protoplast yields of 3.0–5.0×106 were obtained following 4–6 h digestion of 1- to 2-cm petioles (1 g fresh weight) with 1% Cellulase-R10, 2% Macerozyme-R10,
and 0.3% Pectolyase Y-23 in a washing solution with 9% mannitol. A plating density of 105 protoplasts/ml was optimal for subsequent division. An initial division frequency of 12–15% was obtained in liquid or agarose-solidified
KP8 culture medium supplemented with 2,4-dichlorophenoxyacetic acid (2,4-D) (0.9 μm), and zeatin (2.3 μm). Colonies consisting of 100–200 cells were formed after 4 weeks in the dark at 24±2°C. The frequency of colony formation
was improved by the gradual addition of fresh liquid KP8 medium of lower osmoticum. Protocalli (1–2 mm in diameter) were formed
after an additional 4–6 weeks under continuous illumination and regular dilution with fresh culture medium. Morphogenic callus
formed globular and heart-shaped embryos that developed into cotyledon stage embryos, following transfer of calli onto medium
containing 2,4-D (11.3 μm) and benzylaminopurine (2.2 μm). Subsequently, embryo conversion to plantlets was obtained on basal medium with 2% sucrose and 3.5 μm gibberellic acid. Regenerated plantlets were successfully transplanted in soil. Mature plants appeared phenotypically normal.
The same petiole protoplast populations showed transient expression of the gusA gene introduced using electroporation.
Received: 10 October 1997 / Revision received: 10 February 1998 / Accepted: 2 March 1998 相似文献
5.
Abstract
Grazing behavior of both individual cells and populations of the marine hypotrich Euplotes mutabilis, a largely benthic ciliate that feeds on suspended particles, was studied using fluorescent latex microspheres. Microspheres
of sizes 0.57-, 1.90-, 3.06-, 5.66-, and 10.0-μm diam were offered at concentrations from 102 to 106 ml−1. Their uptake in a ten-min period was determined. Food particles within such ranges of size and concentration are found under
natural conditions. The ciliates ingested particles of all sizes offered. Uptake rates at all concentrations were dependent
upon particle size, with 1.90- and 3.06-μm diam microspheres having the highest uptake rate in all cases. For all sizes, there
was an increase in the percentage of feeding cells and in the uptake rate (the number of particles ingested cell−1 h−1), with increasing particle concentration. When uptake was expressed as the volume ingested, maximum values were obtained
for 5.85-μm diam microspheres at a concentration of 106 ml−1. By taking a small number of large particles, present at a low concentration in the medium, a ciliate can ingest a greater
biovolume than by taking a high number of small particles which are abundant in the medium. These results demonstrate that
some ciliates can graze particles of a wide range of sizes. Also, its nutrition can be affected by changing ambient concentrations
of different prey, both through effects on the proportion of feeding cells and through the relative food content of the particles.
The data can also add to the understanding of food niche partitioning between species. At low particle concentrations, particularly,
it is important to consider the behavior of individual ciliates as well as of the whole population.
Received: 11 February 1997; Accepted: 21 October 1997 相似文献
6.
Microbial Responses to Environmentally Toxic Cadmium 总被引:15,自引:0,他引:15
Abstract
We analyzed the soil microbial communities from one uncontaminated and two metal-impacted soils and found that while cadmium
adversely affected the numbers of culturable bacteria in all soils, cadmium-resistant isolates were found from each of the
soils. With exposure to 24 and 48 μg ml-1 soluble cadmium, the metal-contaminated soil communities were more resistant than the uncontaminated soil community. In addition,
in one metal-stressed soil, the resistant population became more resistant with increased cadmium levels. Ribosomal 16S DNA
sequencing identified the isolates as Arthrobacter,
Bacillus, or Pseudomonas spp. Further characterization demonstrated that two of the isolates were highly resistant to soluble cadmium with maximum
resistance at 275 μg ml-1 cadmium. These isolates were also resistant to a variety of antibiotics, namely ampicillin, gentamicin, penicillin, and streptomycin,
but no overall correlation was found between enhanced antibiotic resistance and cadmium resistance. One Pseudomonas isolate H1 did become more resistant with increasing cadmium levels, suggesting a different resistance mechanism at high
cadmium concentrations.
Received: 29 April 1999; Accepted: 7 July 1999; Online Publication: 30 November 1999 相似文献
7.
J R Kastner W J Jones R S Roberts 《Journal of industrial microbiology & biotechnology》1999,22(2):65-70
Candida shehatae cells pre-grown on D-xylose simultaneously consumed mixtures of D-xylose and D-glucose, under both non-growing (anoxic) and actively growing conditions (aerobic), to produce ethanol. The rate of D-glucose consumption was independent of the D-xylose concentration for cells induced on D-xylose. However, the D-xylose consumption rate was approximately three times lower than the D-glucose consumption rate at a 50% D-glucose: 50% D-xylose mixture. Repression was not observed (substrate utilization rates were approximately equal) when the percentage of
D-glucose and D-xylose was changed to 22% and 78%, respectively. In fermentations with actively growing cells (50% glucose and D-xylose), ethanol yields from D-xylose increased, the % D-xylose utilized increased, and the xylitol yield was significantly reduced in the presence of D-glucose, compared to anoxic fermentations (YETOH,xylose = 0.2–0.40 g g−1, 75–100%, and Yxylitol = 0–0.2 g g−1 compared to YETOH,xylose = 0.15 g g−1, 56%, Yxylitol = 0.51 g g−1, respectively). To increase ethanol levels and reduce process time, fed-batch fermentations were performed in a single stage
reactor employing two phases: (1) rapid aerobic growth on D-xylose (μ = 0.32 h−1) to high cell densities; (2) D-glucose addition and anaerobic conditions to produce ethanol (YETOH,xylose = 0.23 g g−1). The process generated high cell densities, 2 × 109 cells ml−1, and produced 45–50 g L−1 ethanol within 50 h from a mixture of D-glucose and D-xylose (compared to 30 g L−1 in 80 h in the best batch process). The two-phase process minimized loss of cell viability, increased D-xylose utilization, reduced process time, and increased final ethanol levels compared to the batch process.
Received 23 February 1998/ Accepted in revised form 15 July 1998 相似文献
8.
Fabiana R. X. Batista Carlos A. Pereira Ronaldo Z. Mendon?a Angela M. Moraes 《Cytotechnology》2005,49(1):1-9
Animal cells can be cultured both in basal media supplemented with fetal bovine serum (FBS) and in serum-free media. In this
work, the supplementation of Grace’s medium with a set of nutrients to reduce FBS requirements in Spodoptera frugiperda (Sf9) cell culture was evaluated, aiming the production of Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) at a cost lower than those for the production using Sf900 II medium. In Grace’s medium supplemented
with glucose, Pluronic F68 (PF68) and yeast extract (YE), the effects of FBS and milk whey ultrafiltrate (MWU) on cell concentration
and viability during midexponential and stationary growth phase were evaluated. In spite of the fact that FBS presented higher
statistical effects than MWU on all dependent variables in the first cell passage studies, after cell adaptation, AgMNPV polyhedra
production was comparable to that in Sf900 II. Batch cultivation in Grace’s medium with 2.7 g l−1 glucose, 8 g l−1 YE and 0.1% (w/v) PF68 supplemented with 1% (w/v) MWU and 3% (v/v) FBS increased viable cell concentration to about 5-fold
(4.7×106 cells ml−1) when compared to Grace’s containing 10% (v/v) FBS (9.5×105 cells ml−1). AgMNPV polyhedra (PIBs) production was around 3-fold higher in the MWU supplemented medium (1.6×107 PIBs ml−1) than in Grace’s medium with 10% FBS (0.6×107 PIBs ml−1). This study therefore shows a promising achievement to significantly reduce FBS concentration in Sf9 insect cell media,
keeping high productivity in terms of cell concentration and final virus production at a cost almost 50% lower than that observed
for Sf900 II medium.
C.A. Pereira is recipient of a CNPq fellowship. 相似文献
9.
J R Kastner W J Jones R S Roberts 《Journal of industrial microbiology & biotechnology》1998,20(6):339-343
The kinetics of biomass formation, D-xylose utilization, and mixed substrate utilization were determined in a chemostat using the yeast Candida shehatae. The maximum growth rate of C. shehatae grown aerobically on D-xylose was 0.42 h−1 and the Monod constant, K
s, was 0.06 g L−1. The biomass yield, Y
{X/S}, ranged from 0.40 to 0.50 g g−1 over a dilution rate range of 0.2–0.3 h−1, when C. shehatae was grown on pure D-xylose. Mixtures of D-xylose and glucose (∼1 : 1) were simultaneously utilized over a dilution rate from 0.15 to 0.35 h−1 at pH 3.5 and 4.5, but pH 3.5 reduced μmax and reduced the dilution rate range over which D-xylose was utilized in the presence of glucose. At pH 4.5, μmax was not reduced with the mixed sugar feed and the overall or lumped K
s value was not significantly increased (0.058 g L−1
vs 0.06 g L−1), when compared to a pure D-xylose feed. Kinetic data indicate that C. shehatae is an excellent candidate for chemostat production of value added products from renewable carbon sources, since simultaneous
mixed substrate utilization was observed over a wide range of growth rates on a 1 : 1 mixture of glucose and D-xylose.
Received 21 August 1997/ Accepted in revised form 28 May 1998 相似文献
10.
Inhibition of galectin-3 mediated cellular interactions by pectic polysaccharides from dietary sources 总被引:1,自引:0,他引:1
Pectic polysaccharides from dietary sources such as Decalepis hamiltonii—swallow root (SRPP), Hemidesmus indicus (HPP), Nigella sativa—black cumin (BCPP), Andrographis serpyllifolia—(APP), Zingiber officinale—ginger (GRPP) and, citrus pectin (CPP) were examined for galectin inhibitory activity. Inhibition of (a) galectin-3 of MDA-MB-231
cells induced hemagglutination of red blood cells; (b) galectin-3 mediated interaction between normal/metastatic human buccal
cells (NBC)/(MBC) and; (c) invasion of MDA-MB-231 and MBC in the invasive chamber was assessed. Results indicated that SRPP
inhibited hemagglutination at Minimum Inhibitory Concentration (MIC) of 1.86 μg ml−1 equivalent of carbohydrate as apposed to those of BCPP (130 μg ml−1), APP (40 μg ml−1), HPP (40 μg ml−1) and CPP (25 μg ml−1). GRPP even at concentration >1–6 mg ml−1 did not inhibit agglutination. Also SRPP showed ∼15 and 2 fold potent anti hemagglutination activity relative to that of
galectin-3 specific sugars—galactose (MIC-27.1 μg ml−1) and lactose (MIC-4.16 μg ml−1) respectively. Further, SRPP at 10 μg ml−1 inhibited agglutination of NBC by galectin-3 of MDA-MB-231 cells. Modified swallow root pectic polysaccharide (MSRPP) of
50 kDa retained anti hemagglutination activity (MIC of 1.03 μg ml−1) and inhibited MDA-MB-231 and MBC invasion by 73 and 50% with an IC50 of 136 and 200 μg ml−1 respectively. Both SRPP and MSRPP induced apoptosis up to 80% at 100 μg ml−1 concentration by activating ∼2 and 8 folds of Caspase-3 activity. Sugar composition analysis and its correlation with the
galectin inhibitory property indicated that pectic polysaccharides with higher arabinose and galactose content—arabinogalactan
inhibited hemagglutination significantly. 相似文献
11.
A Bacillus sp. RE was resistant to chromium and reduced Cr(VI) without accumulating chromium inside the cell. When Cr(VI) was 10 and
40 μg ml−1, >95% of the total Cr(VI) was reduced in 24 and 72 h of growth, respectively, whereas at 80 μg Cr(VI) ml−1 only 50% of Cr(VI) was reduced. However growth was not affected; the cell mass was 0.7–0.8 mg ml−1 in all cases. The cell-free extract showed Cr(VI) reducing enzyme activity which was enhanced (>5 fold) by NADH and NADPH.
Like whole cells the enzyme also reduced Cr(VI) with decreasing efficiency on increasing Cr(VI) concentration. The enzyme
activity was optimal at pH 6.0 and 30 °C. The enzyme was stable up to 30 °C and from pH 5.5 to 8, but from pH 4 to 5 the enzyme
was severely destabilized. Its Km and Vmax were 14 μm and 3.8 nmol min−1 mg−1 respectively. The enzyme activity was enhanced by Cu2+ and Ni2+ and inhibited by Hg2+.
Received 21 September 2005; Revisions requested 5 October 2005; Revisions received 16 November 2005; Accepted 16 November
2005 相似文献
12.
The present study demonstrates that B-type Ca2+ channels observed in rat ventricular myocytes markedly reacted to agents known to affect the ion-motive plasma membrane Ca2+-ATPase (PMCA) pump. Chlorpromazine (CPZ)-activated B-type Ca2+ channels were completely blocked by internal application of PMCA pump inhibitors, namely La3+ (100 μm), eosin (10 μm) and AIF3 (100 μm). Calmodulin (50 U/ml), the main endogenous positive regulator of PMCA, was unable to activate but significantly reduced
CPZ-activated B-type channel activity. In the same manner, ATP (1 and 4 mm), the main energizing substrate of PMCA, was able to reversibly and significantly reduce this activity in a dose-dependent
manner. Interestingly, anti-PMCA antibody 5F10, but not anti-Na/K ATPase antibody (used as a negative control) induced a marked
Ba2+-conducting channel activity that shared the same characteristics with that of CPZ-activated B-type channels. 5F10-Activated
channels were mostly selective towards Ba2+, mainly had three observed conductance levels (23, 47 and 85 pS), were observed with a frequency of about 1 out of 5 membrane
patches and were completely blocked by 10 μm eosin. These results suggest that B-type Ca2+ channels are some form of the PMCA pump.
Received: 24 July 2000/Revised: 5 October 2000 相似文献
13.
A naked plasmid with human pre-pro-insulin gene was transferred into skeletal muscle of diabetic rats by electric pulses and gene expression was detected. Blood glucose concentration was decreased from 24 mmol l–1 to 8.5 mmol l–1. Circulating insulin-like protein was increased significantly to 15–20 U ml–1, while that of the control group injected with the empty vector remained at less than 10 U ml–1. The low blood glucose concentration lasted for more than two months. These studies indicate that electroporational transfer of plasmid with human pre-pro-insulin gene into skeletal muscle could be a potential method of gene therapy for human diabetes mellitus. 相似文献
14.
The filamentous fungus Stachybotrys sp has been shown to possess a rich β-glucosidase system composed of five β-glucosidases. One of them was already purified
to homogeneity and characterized. In this work, a second β-glucosidase was purified and characterized. The filamentous fungal
A19 strain was fed-batch cultivated on cellulose, and its extracellular cellulases (mainly β-glucosidases) were analyzed.
The purified enzyme is a monomeric protein of 78 kDa molecular weight and exhibits optimal activity at pH 6.0 and at 50°C.
The kinetic parameters, K
m and V
max, on para-nitro-phenyl-β-d-glucopyranosid (p-NPG) as a substrate were, respectively, 1.846 ± 0.11 mM and 211 ± 0.08 μmol min−1 ml−1. One interesting feature of this enzyme is its high stability in a wide range of pH from 4 to 10. Besides its aryl β-glucosidase
activity towards salicin, methylumbellypheryl-β-d-glucoside (MU-Glc), and p-NPG, it showed a true β-glucosidase activity because it splits cellobiose into two glucose monomers. This enzyme has the
capacity to synthesize short oligosaccharides from cellobiose as the substrate concentration reaches 30% with a recovery of
40%. We give evidences for the involvement of a transglucosylation to synthesize cellotetraose by a sequential addition of
glucose to cellotriose. 相似文献
15.
Sugar substrates which depress the intracellular level of inorganic phosphate in baker's yeast (d-glucose,d-fructose,d-mannose, sucrose, as well as maltose andd-galactose after appropriate induction) also make transmembrane flux of phosphate anions possible. Acetate and ethanol, although
readily oxidized, as well as nonmetabolized sugars, do not produce the effect. Phosphate uptake in whole cells (but not in
protoplasts) is accelerated by preincubation with substrate either aerobically or anaerobically but the actual presence of
substrate in the incubation medium is required for transport to take place. Starved cells take up phosphate from the medium
with aK
m of 3mm, the half-activation concentration by glucose being 18mm, the amount taken up being constant under given conditions (40 μmol/g dry wt. here). Phosphate-rich cells lose phosphate
to the medium in the presence of a suitable substrate. The uptake process is characterized by an activation energy of 13400
cal/mol at 10−6
m phosphate and of 9400 cal/mol at 10−3
m phosphate. The process shows two optima at pH 5.0 and 7.0. A short-lived intermediate of fermentative sugar metabolism is
postulated as essential for the translocation of phosphate across the yeast membrane. 相似文献
16.
Boiero L Perrig D Masciarelli O Penna C Cassán F Luna V 《Applied microbiology and biotechnology》2007,74(4):874-880
The aim of this work was to evaluate phytohormone biosynthesis, siderophores production, and phosphate solubilization in three
strains (E109, USDA110, and SEMIA5080) of Bradyrhizobium japonicum, most commonly used for inoculation of soybean and nonlegumes in USA, Canada, and South America. Siderophore production and
phosphate solubilization were evaluated in selective culture conditions, which had negative results. Indole-3-acetic acid
(IAA), gibberellic acid (GA3), and abscisic acid (ABA) production were analyzed by gas chromatography–mass spectrometry (GC-MS). Ethylene and zeatin biosynthesis
were determined by GS–flame ionization detection and high-performance liquid chromatography (HPLC-UV), respectively. IAA,
zeatin, and GA3 were found in all three strains; however, their levels were significantly higher (p < 0.01) in SEMIA5080 (3.8 μg ml−1), USDA110 (2.5 μg ml−1), and E109 (0.87 μg ml−1), respectively. ABA biosynthesis was detected only in USDA110 (0.019 μg ml−1). Ethylene was found in all three strains, with highest production rate (18.1 ng ml−1 h−1) in E109 cultured in yeast extract mannitol medium plus l-methionine. This is the first report of IAA, GA3, zeatin, ethylene, and ABA production by B. japonicum in pure cultures, using quantitative physicochemical methodology. The three strains have differential capability to produce
the five major phytohormones and this fact may have an important technological implication for inoculant formulation. 相似文献
17.
The house fly (Musca domestica L.) alimentary canal was evaluated for the potential of horizontal transfer of tetM on plasmid pCF10 among Enterococcus faecalis. Two sets of experiments were conducted: (1) house flies without surface sterilization and (2) surface-sterilized flies.
Both sets of flies were exposed to E. faecalis OG1RF:pCF10 as donor for 12 h and then E. faecalis OG1SSp as recipient for 1 h. Another group of flies received the recipient first for 12 h followed by exposure to the donor
strain for 1 h. House flies were screened daily to determine the donor, recipient, and transconjugant bacterial load for up
to 5 days. In addition, the sponge-like mouth parts used for food uptake (labellum) of surface-sterilized house flies were
removed and analyzed for donors, recipients, and transconjugants, separately. In both groups of flies (n = 90 flies/group), transfer occurred within 24 h after exposure with a transconjugant/donor rate from 8.6 × 10−5 to 4.5 × 101. Transconjugants were also isolated from the house fly labellum. Our data suggest that the house fly digestive tract provides
a suitable environment for horizontal transfer of conjugative plasmids and antibiotic resistance genes among enterococci.
Our results emphasize the importance of this insect as a potential vector of antibiotic-resistant bacterial strains. 相似文献
18.
Abstract
The investigation of the bacterial community in the Kühw?rter Wasser, a macrophyte-dominated arm of the River Danube backwater
system near Vienna, revealed that variation in microbial densities and biomass could be related to a characteristic sequence
in morphotype composition over the seasons. Maximal bacterial cell numbers and biomass occured in early summer, with values
of up to 9 × 109 cells l−1 and 122 μg C l−1, respectively, caused by a massive increase of vibrio-shaped cells. On the other hand, in early spring, filamentous bacteria
were responsible for a marked increase in bacterial biomass, making up 40% of the total bacterial biomass. Over the year,
rod-shaped cells were the dominating morphotype, while the biomass of cocci was rather negligible. In winter, cell numbers
and biomass showed minimal values with 2.0 × 109 cells l−1 and 28 μg C l−1, respectively, and bacteria were considered to be substrate and temperature limited during this period. Saturation values
of the incorporation of 3H-thymidine into DNA, for the estimation of bacterial secondary production, varied seasonally, ranging from 5 nm to 40 nm. Thus, saturation experiments needed to be conducted on a regular basis. Also, the amount of labeled thymidine in the DNA,
as a percentage of labeled thymidine in the TCA precipitate, varied over the year. Minimum values of 45% were recorded during
the cold season, while maximum values of 75–80% at the beginning of June coincided with high chlorophyll a values and minimal K
m-values derived from saturation experiments. The potential role of the nitrogen-rich nucleoside thymidine as a readily utilizable
substrate for bacteria during labeling experiments, under varying conditions of substrate availability, is discussed. Bacterial
secondary production rates ranged from 0.3 μg C l−1 h−1 in winter to values of 10 μg C l−1 h−1 in August, where phytoplanktonic biomass reached the summer maximum, and bacterial biomass was calculated to be renewed 3
times per day. An estimation of the bacterial carbon demand showed that for the major part of the year, with the exception
of early spring, the bacterioplankton community in the Kühw?rter Wasser was dependent on carbon sources other than phytoplanktonic
primary production.
Received: 22 March 1996; Revised: 1 August 1996 相似文献
19.
Mobilization frequencies of the nonconjugative plasmid pMON5003 were quantified using Escherichia coli TB1(pRK2013) as donor of a helper plasmid, E. coli M182 (pMON5003) as donor of the nonconjugative plasmid, and Pseudomonas fluorescens as recipient. Initial mating experiments were conducted in nutrient and minimal salts media and pea seed exudates. Mobilization
rates were higher during early stationary growth of donors, helpers, and recipients. Numbers of transconjugants were higher
in biparental matings when donors contained both conjugative and nonconjugative plasmids, versus tri-parental matings. A mathematical
model was developed to predict a nonconjugative plasmid transfer rate parameter (δ), estimating the proportion of conjugative
matings in which a plasmid is mobilized. Values of δ ranged from 8 × 10−3 to 7.9 × 10−1. Transfer frequencies for pMON5003 from E. coli to P. fluorescens on pea seeds and roots were determined. Transconjugants (P. fluorescens 2-79 (pMON5003)) were isolated from seeds, roots, and soil, but mobilization frequencies were lower than in liquid media. 相似文献
20.
Transferable lincosamide-macrolide resistance in Bacteroides. 总被引:27,自引:0,他引:27
Inter- and intraspecies transfer of resistance to clindamycin, lincomycin, and erythromycin in the strict anaerobe, Bacteroides, is described. This lincosamide-macrolide resistance was found to be specified by a 27 × 106-dalton plasmid, designated pBF4, originally identified in a clinical Bacteroides fragilis isolate. Transfer of this plasmid to a strain of Bacteroides uniformis was demonstrated to occur by a deoxyribonuclease insensitive process which required cell-to-cell contact. Chloroform sterilized donor cell supernatants or filtrates of donor cells did not mediate resistance transfer. Transfer of the antibiotic resistance and pBF4 plasmid deoxyribonucleic acid (DNA) were always coincident. Drug resistant progeny recovered from such matings were able to transfer the pBF4 plasmid and its associated resistance markers to a suitable B.fragilis recipient strain. Compared to interspecies matings, resistance transfer was 100- to 1000-fold greater between isogenic donor and recipient strains, suggesting the possibility of a host controlled restriction-modification system. 相似文献