黄土高原苹果园土壤水分及水分生产力模拟

以长武地区为例,采用WinEPIC模型模拟1980—2018年间黄土高原旱作苹果园地深剖面土壤水分和水分生产力变化动态,以期为该区苹果产业的可持续发展提供科学依据。结果表明: 长武地区苹果园年均产量为26.37 t·hm^-2,年均蒸散量为673.66 mm,年均水分生产力为4.07 kg·m^-3,成龄果树水分胁迫天数主要受降雨量影响,果树生长后期年均胁迫天数为46.46 d,深层土壤含水量最早于9龄果树开始接近凋萎湿度。长武地区苹果整个生长周期内供水量是对果园产量影响最大的因素,深层土壤有效水含量降低是制约果树生长中后期产量提高的最主要因素,在降水不足的年份果树会利用更深层土壤水分。当深层土壤可利用水分较少时,过多的降水并未被果树利用,而是转化为浅层土壤水分蒸发。对于成龄果树在年供水量低于500 mm或高于700 mm时都会造成产量的下降。针对不同生长时期的果园,在不同的降雨年份应该调整果园水分管理策略,可以通过补充灌溉、拦蓄集聚雨水、覆盖、修剪枝条等管理措施,降低果树非生产性耗水及自身奢侈性耗水,延缓深层土壤干层的出现时间,在保证果树生长的同时避免水资源的浪费。     

Removal of bacteria from water by adhesion to cross-linked poly(vinylpyridinium halide).

Cross-linked poly(vinylpyridinium halide) was found to have a novel and remarkable ability to remove bacteria from water. For example, when 10 g (wet weight) of cross-linked poly(N-benzyl-4-vinylpyridinium bromide) was contacted with 20 ml of suspensions of Escherichia coli (9.7 X 10(4) to 9.7 X 10(7)/ml), Salmonella typhimurium (8.0 X 10(6) to 1.1 X 10(7)/ml), Streptococcus faecalis (5.0 X 10(7)/ml), Staphylococcus aureus (8.1 X 10(7)/ml), and Pseudomonas aeruginosa (3.2 X 10(5)/ml) under stirring in sterilized physiological saline at 37 degrees C, 99% of the viable cells of these bacteria were removed in 2 to 6 h. When suspensions of these bacteria (10(5) to 10(8) cells per ml) were passed through a column (20 mm by 100 cm) of cross-linked poly(N-benzyl-4-vinylpyridinium bromide) at 37 degrees C with a flow rate of 0.8 to 1.4 bed volumes per h, 97 to 100% of the viable cells were eliminated from the suspensions during the treatment. Mechanistic studies demonstrated that cross-linked poly(vinylpyridinium halide) irreversibly captured these bacteria alive during the treatment. That is, total organic carbon was removed during the treatment, and the bacteria which adhered to the resin proliferated on the bacterial medium. The adhesion capacity was estimated to be 10(10) cells per g (dry weight). Total organic carbon was also removed even when the bacteria were killed by heat treatment before the column studies.     

黄土高原半干旱区苜蓿草地土壤干燥化特征与粮草轮作土壤水分恢复效应

实地测定了黄土高原半干旱区固原不同生长年限苜蓿草地和连作8a苜蓿草地翻耕轮作不同年限粮食作物后深层土壤水分特征,分析了苜蓿草地土壤干燥化特征和粮草轮作对土壤水分的恢复效应.结果表明:(1)苜蓿连作1a、5a、8a和12a等4类苜蓿草地0～1000cm土层平均土壤湿度值为6.6%,平均土壤水分过耗量702.8mm,平均土壤干燥化速率147.1mm/a,达到强烈干燥化程度,苜蓿连作5a土壤干层深度超过1000cm,苜蓿连作8a土壤干层深度超过1360cm,苜蓿草地合理利用年限为7a.(2)连作8a苜蓿草地翻耕并轮作4～7a和25a粮食作物等5类粮田0～1000cm土层土壤湿度介于6.74%～11.95%,土壤贮水量恢复值介于210.6～887.3mm,平均土壤水分恢复速率为80.8mm/a.轮作6a后粮田土壤干层轻度恢复程度以上深度达到1000cm.通过粮草轮作使苜蓿草地土壤湿度恢复到当地土壤稳定湿度需要13a以上.黄土高原半干旱区适宜的粮草轮作模式为:7a苜蓿→13a粮食作物.     

New Water Disinfectant: an Insoluble Quaternary Ammonium Resin-Triiodide Combination that Releases Bactericide on Demand

Strongly basic anion-exchange resins form stable, water-insoluble combinations with triiodide ions. The combinations have remarkable antibacterial properties: 3.0 x 10(5)Escherichia coli cells per ml were killed when passed through a 3.8-g column of commercially available resin treated with triiodide (volume 4 ml after treatment). In an attempt to deplete the resin-triiodide complex, 1.14 x 10(9)E. coli cells in 15 liters were passed through the column with no significant loss of effectiveness. The antibacterial capabilities of the resin-triiodide columns ranged from 10(6)Salmonella typhimurium per ml to 1.1 x 10(4)Streptococcus faecalis per ml. Staphylococcus aureus and Pseudomonas aeruginosa were also tested and killed at concentrations of 1.8 x 10(4) and 1.3 x 10(5) per ml, respectively. The cells were not filtered from the water. They emerged from the column in nonviable form. This was demonstrated by using (14)C-labeled bacteria. The irreversible nature of the antibacterial action was revealed when attempts to wash the damaged cells did not restore viability.     

Lethal and mutagenic effect of the XeCl laser on Escherichia coli

The survival rate and reversions to tryptophan-independence of Escherichia coli after XeCl laser irradiation (lambda = 308 nm) within the dose range from 10(3) to 10(5) J/m2 have been studied to show that LD37 is 10(4) J/m2, the survival rate at a maximum dose of 10(5)J/m2 is 1 per cent, and the number of mutants per 10(6) cells survived is 100.     

渭北旱塬苹果园地产量和深层土壤水分效应模拟

为了研究实时气象条件下渭北旱塬不同生长年限苹果园地产量变化趋势和深层土壤水分变化规律,在模型适用性与模拟精度验证基础上,应用WinEPIC模型模拟研究了1962—2001年期间洛川旱塬苹果园地产量演变动态和深层土壤水分效应。结果表明：(1) 在模拟研究期间,洛川旱塬4—40年生苹果园产量整体上呈波动性下降趋势,初期产量逐渐增加,11—23年生达到最大值(平均为28.8 t/hm2),之后随降水量年际波动呈现出明显的波动性降低趋势。(2) 40年间苹果园地遭受的干旱胁迫日数呈波动性上升趋势,与年降水量波动趋势相反。(3) 1—15年生期间苹果园地平均年耗水量高于同期年降水量,导致苹果园地0—10 m土层土壤强烈干燥化,逐月土壤有效含水量波动性降低,1—10年生、11—20年生和21—40年生期间发生土壤干燥化并且程度逐渐加剧,但干燥化速率逐渐减缓,土壤干燥化速率分别为95.4 mm/a、12 mm/a和1.5 mm/a。(4) 随生长年限的延长,苹果园地0—10 m土层土壤湿度逐渐降低、土壤干层分布深度逐渐加大,在14年生时超过了10 m,20年生以后2—10 m 土层形成稳定的土壤干层。因此,基于土壤水分利用的苹果生长与果园利用的合理年限为20 a,最长不宜超过23 a。     

Death of the Escherichia coli K-12 strain W3110 in soil and water.

Whether Escherichia coli K-12 strain W3110 can enter the "viable but nonculturable" state was studied with sterile and nonsterile water and soil at various temperatures. In nonsterile river water, the plate counts of added E. coli cells dropped to less than 10 CFU/ml in less than 10 days. Acridine orange direct counts, direct viable counts, most-probable-number estimates, and PCR analyses indicated that the added E. coli cells were disappearing from the water in parallel with the number of CFU. Similar results were obtained with nonsterile soil, although the decline of the added E. coli was slower. In sterile water or soil, the added E. coli persisted for much longer, often without any decline in the plate counts even after 50 days. In sterile river water at 37 degrees C and sterile artificial seawater at 20 and 37 degrees C, the plate counts declined by 3 to 5 orders of magnitude, while the acridine orange direct counts remained unchanged. However, direct viable counts and various resuscitation studies all indicated that the nonculturable cells were nonviable. Thus, in either sterile or nonsterile water and soil, the decline in plate counts of E. coli K-12 strain W3110 is not due to the cells entering the viable but nonculturable state, but is simply due to their death.     

Effect of sub-lethal concentrations of permethrin on ovary activation in the predator Supputius cincticeps (Heteroptera: Pentatomidae).

Insecticides may cause mortality and deleterious effects on predatory stinkbugs. For this reason, the effect of five concentrations of permethrin applied on third instar nymphs of Supputius cincticeps (St?l) (Heteroptera: Pentatomidae) was investigated on ovary activation in this predator. The nymphs received topical application of permethrin in the following concentrations (mg a.i./ml): 10(-7), 10(-6), 10(-5), 10(-4), and 10(-3). Ovary lengths and oocyte numbers were quantified following first egg mass. Ovary length of S. cincticeps varied from 5.7 mm with 10(-4) mg a.i./ml, to 6.4 mm with 10(-7) mg a.i./ml, with similar values for the other permethrin concentrations and for the control. The number of oocytes per female varied from 13.5 with 10(-3) mg a.i./ml, to 29.2 for the control, with significant differences. The number of oocytes per female of nymphs exposed to a permethrin concentration of 10(-5) mg a.i./ml was similar to that of the control. However, the lower number of oocytes per female from nymphs exposed to other concentrations of permethrin suggests that this insecticide may affect the reproductive capacity of this predator. The results obtained are discussed in relation to tolerance of Heteroptera predators to insecticides and possible hormesis occurrence.     

Immunosensor for the differentiation and detection of Salmonella species based on a quartz crystal microbalance

Immunosensors based on the microgravimetric quartz crystal microbalance (QCM) technique have been developed for the detection of Salmonella species from serogroups A, B and D. Salmonella serogroup-specific murine monoclonal antibodies, respectively, raised against these serogroups were immobilized onto the silver electrodes of piezoelectric (PZ) crystals by cross-linkage via glutaraldehyde (GA) to the electrode surfaces pre-coated with thin polyethyleneimine (PEI) layer. The specific immunosensors developed gave responses in linear ranges from 10(5) to 5x10(8) cells per ml with no significant interference from other strains of Salmonella and Escherichia coli up to 10(8) cells per ml. They showed good repeatability and excellent linear range, achieving detection limits down to 10(4) cells per ml with ability to distinguish different strains of Salmonella. These biosensors exhibited an exquisite specificity evidenced by their ability to discriminate antigens, the structures of which differ only by the isomeric form of di-deoxyhexose. The antibody-modified crystals showed no loss in activity over 4 days under storage at 4 degrees C.     

Cultural and environmental factors affecting the longevity of Escherichia coli in Histosols.

The survival of Escherichia coli in organic soils (Histosols) was examined. The death rate of this organism in Pahokee muck was less than that observed in Pompano fine sand. The number of viable E. coli cells found in the muck was approximately threefold greater than that found in the sand following 8 days of incubation. The initial population of the coliform affected the death rate. The rate of loss of viability varied 100-fold when the population size decreased from 2.5 x 10(7) to 3.4 x 10(4). Other factors affecting the viability of E. coli in muck were aerobic versus anaerobic growth of the organism and moist versus flooded conditions in the soil. The greatest survival of the coliform was noted with anaerobically grown cells amended to flooded soil. That the observed decrease in E. coli viability in soil was the result of biotic factors was demonstrated with amendment of sterile soil with E. coli. When 1.1 x 10(5) bacteria per g of soil were added to sterile muck, a population of 3.0 x 10(7) organisms per g of soil developed over a 10-day period. The role of the protozoa in eradication of the coliform from the muck was indicated by a sixfold increase in the protozoan population in natural soil amended with E. coli. Higher organic matter content in a Histosol compared with a mineral soil resulted in an increased survival of the fecal coliforms. Biotic factors are instrumental in the decline in coliform populations, but the potential for growth of the coliform in the organic soil could extend the survival of the organism.     

Factors influencing the replication of somatic coliphages in the water environment

The potential replication of somatic coliphages in the environment has been considered a drawback for their use as viral indicators, although the extent to which this affects their numbers in environmental samples has not been assessed. In this study, the replication of somatic coliphages in various conditions was assayed using suspensions containing naturally occurring somatic coliphages and Escherichia coli WG5, which is a host strain recommended for detecting somatic coliphages. The effects on phage replication of exposing strain WG5 and phages to a range of physiological conditions and the effects of the presence of suspended particles or other bacteria were also assayed. Phage replication was further tested using a strain of Klebsiella terrigena and naturally occurring E. coli cells as hosts. Our results indicate that threshold densities of both host bacterium and phages should occur simultaneously to ensure appreciable phage replication. Host cells originating from a culture in the exponential growth phase and incubation at 37 degrees C were the best conditions for phage replication in E. coli WG5. In these conditions the threshold densities required to ensure phage replication were about 10(4) host cells/ml and 10(3) phages/ml, or 10(3) host cells/ml and 10(4) phages/ml, or intermediate values of both. The threshold densities needed for phage replication were higher when the cells proceeded from a culture in the stationary growth phase or when suspended particles or other bacteria were present. Furthermore E. coli WG5 was more efficient in supporting phage replication than either K. terrigenae or E. coli cells naturally occurring in sewage. Our results indicate that the phage and bacterium densities and the bacterial physiological conditions needed for phage replication are rarely expected to be found in the natural water environments.     

The fate of rainfall in a mallee and heath vegetated catchment in southern Western Australia

Mallee, a shrub-eucalypt association, once covered large areas of the cereal growing land in Western Australia. The hydrologic consequences of land development have been more than a doubling of water yield and increased deep drainage beyond the plant-root zone. The latter has led to large areas of soil salinization. The uncleared catchment studied was covered with mallee vegetation (65%) and a heath association (35%). Over 12 years of measurement, runoff from the catchment has averaged 0.025 mm per annum with a mean annual rainfall of 376 mm. Half of the runoff resulted from two major events totalling 0.15 mm. Since there was no evidence of groundwater accession in the catchment, the rainfall was balanced by evapotranspiration. Despite virtually no runoff from the whole catchment, considerable internal redistribution of surface water occurred, with localized overland flow in some areas as high as 7.7 mm from 30.9 mm of rainfall in one day. Rainfall penetrated rapidly under the mallee vegetation and the rate of penetration observed could not be achieved solely through the soil even if there was saturated flow. A typical stand of native mallee trees (Eucalyptus pileata and E. eremophila) in the catchment effectively redistributed 8% of the annual rain falling on the stand with 3% lost as interception and 5% going to stemflow. However, on an individual tree basis some 15% of the rain falling on the canopy was lost as intercepted water and 25% ran down the stem. The stemflow caused saturated conditions around the bole of the mallee and dye tracing showed that the water penetrated the soil via the annular pathways of the soil-root interface. Roots of mallee trees were found at 28 m depth and it is postulated that the mallees are adapted to the semi-arid environment by virtue of their ability to store water deep in the soil profile for use during the dry summer months.     

Effects of coumarin, thiopurines, and pyronin Y on amplification of phleomycin-induced death and deoxyribonucleic acid breakdown in Escherichia coli

Phleomycin (/=10 mug of phleomycin per ml were observed among 10(11)E. coli B cells screened, such mutants occurred with a frequency of 10(-6) to 10(-7) among cultures resistant to 1 to 2 mug of phleomycin per ml. These double mutants were cross-resistant to phleomycin plus caffeine. The amplifying compounds, though structurally dissimilar, shared the common characteristic of binding selectively to denatured DNA as measured by equilibrium dialysis methods. The implications of these observations in supporting a model of phleomycin amplification proposed previously (6) and their utility in providing a logic for developing a new class of antibiotics are discussed.     

Induction of viable but nonculturable Escherichia coli O157:H7 in the phyllosphere of lettuce: a food safety risk factor

Escherichia coli O157:H7 continues to be an important human pathogen and has been increasingly linked to food-borne illness associated with fresh produce, particularly leafy greens. The aim of this work was to investigate the fate of E. coli O157:H7 on the phyllosphere of lettuce under low temperature and to evaluate the potential hazard of viable but nonculturable (VBNC) cells induced under such stressful conditions. First, we studied the survival of six bacterial strains following prolonged storage in water at low temperature (4°C) and selected two strains with different nonculturable responses for the construction of E. coli O157:H7 Tn7gfp transformants in order to quantitatively assess the occurrence of human pathogens on the plant surface. Under a suboptimal growth temperature (16°C), both E. coli O157:H7 strains maintained culturability on lettuce leaves, but under more stressful conditions (8°C), the bacterial populations evolved toward the VBNC state. The strain-dependent nonculturable response was more evident in the experiments with different inoculum doses (10(9) and 10(6) E. coli O157:H7 bacteria per g of leaf) when strain BRMSID 188 lost culturability after 15 days and strain ATCC 43895 lost culturability within 7 days, regardless of the inoculum dose. However, the number of cells entering the VBNC state in high-cell-density inoculum (approximately 55%) was lower than in low-cell-density inoculum (approximately 70%). We recorded the presence of verotoxin for 3 days in samples that contained a VBNC population of 4 to 5 log(10) cells but did not detect culturable cells. These findings indicate that E. coli O157:H7 VBNC cells are induced on lettuce plants, and this may have implications regarding food safety.     

Disinfecting properties of performic acid against bacteriophage phi X 174 as a model of small envelope--free viruses

Performic acid HCOOH (PFA) is a wide-spectrum disinfectant. It inactivates viruses, bacteria and bacterial spores, mycobacteria as well as microscopic fungi. Its main drawback is its instability, which makes it a logical necessity that it is to be prepared prior to use from its components HCOOH and H2O2. The mixing of 8 ml HCOOH of the concentration 850 ml/l and 17 ml H2O2 of the concentration 300 ml/l in a 100 ml-volume reagent bottle with a ground-in glass stopper gives, after an 1-hour rest at room temperature and after another 1 hour in a refrigerator, a stock solution that contains about 50 ml/l of PFA the actual concentration of which is determined iodometrically. Bacteriophage phi X 174 (host E. coli C) is characterized by cubic ikosahedral-type symmetry of particles free of envelope, has 27 mm in diameter and contains single-strand cyclic DNA; formerly was classed among Parvoviridae. The possibility of plaque assay-based quantitative determination of the number of infectious particles makes if it a feasible model for assessing disinfectant action on small hydrophilic viruses under conditions close to those of practical disinfection procedures. PFA stock solution diluted to 1 X 10(-3) (0.05 ml/l of effective component) inactivates the model virus of a concentration 10(8) pfu/ml aqueous suspension within 5 min so that no virus is detectable; the drop in the number of pfu amounts to 7 log orders of magnitude. In the presence of 400 ml/l of serum, the identical effect is achieved within 5 min by PFA stock solution diluted to 5 X 10(-3). The lowest PFA concentration that reliably inactivates bacteriophage phi X 174 in aqueous suspension is identical with the lowest concentration inactivating Coxsackie B 1 virus in tissue cultures. On textile, glass, plastic, rubber and metal carriers contaminated by swabbing or by a dried drop of bacteriophage suspension containing about 1 X 10(9) pfu/ml, the lowest reliably effective concentrations of PFA range within 0.25-0.025 ml/l, i.e. PFA stock solutions diluted to 5 X 10(-3)-5 X 10(-4), depending on the type of carrier and the type of contamination.     

Quantitative detection of Escherichia coli O157 in surface waters by using immunomagnetic electrochemiluminescence.

A protocol for the quantitative detection of Escherichia coli O157 in raw and concentrated surface waters using immunomagnetic electrochemiluminescence (IM-ECL) was developed and optimized. Three antibody sandwich formats were tested: commercial anti-O157:H7 IM beads, IM beads made in-house with a polyclonal anti-O157:H7 immunoglobulin G (IgG), or IM beads made in-house with a monoclonal anti-O157:H7 IgG coupled with a polyclonal anti-O157:H7 IgG to which an electrochemiluminescent label (TAG) was attached. The monoclonal IM bead-polyclonal TAG format was chosen for optimization because it gave lower background levels and linear regression slopes of ca. 1.0, indicative of a constant ECL signal per cell. The dynamic range was ca. 10(1) to 10(5) cells ml(-1) in phosphate-buffered saline and in raw water samples. The monoclonal IM beads selectively captured E. coli O157 cells in the presence of ca. 10(8) cells of a non-O157 strain of E. coli ml(-1). Background ECL signals from concentrated (100-fold) water samples were substantially higher and more variable than raw water samples. The background signal was partially eliminated by the addition of polyvinylpolypyrrolidone. Successive cell capture incubations, termed sequential bead capture (SBC), were optimized for establishing baseline ECL values for individual water samples. The linear dynamic range with SBC was ca. 10(2) to 10(5) E. coli O157 cells ml of concentrated water(-1). To validate the protocol, 10-liter surface water samples were spiked with ca. 5,000 E. coli O157 (Odwalla) cells and concentrated by vortex filtration, and 1- or 3-ml aliquots were analyzed by IM-ECL. Differential ECL signals (SBC) from 1- and 3-ml samples were statistically significant and were generally consistent with standard curves for these cell concentrations. Enrichments were conducted with aliquots of spiked raw water and concentrated water using EC broth and minimal lactose broth (MLB). All tubes with concentrated water became turbid and gave a positive ECL response for E. coli O157 (>10,000 ECL units); MLB gave a somewhat higher detection rate with spiked raw water. The potential sensitivity of the IM-ECL assay is ca. 25 E. coli O157 cells ml of raw water(-1), 25 cells 100 ml of 100-fold concentrated water(-1), or 1 to 2 viable cells liter(-1) with concentration and enrichment. The IM-ECL assay appears suitable for routine analysis and screening of water samples.     

柞蚕蛹抗菌肽D对大肠杆菌K_(12)D_(31)的作用

本文报道了不同浓度的柞蚕蛹抗菌肽D,对大肠杆菌K_(12)D_(31)的杀灭作用动力学。在LEG培养液中,抗菌从D的浓度在5微克/毫升时显效。浓度在10微克/毫升以上时,其杀菌速度大于细菌的增殖速度。固定抗菌肽浓度为10微克/毫升,细菌浓度在3×10~7个细菌/毫升,培养在磷酸钾盐缓冲液中,4小时后能全部杀灭。同样浓度细菌在LEG培养液中,4小时后细菌数下降到约为10~2个细菌/毫升,但不能全部杀灭。同时还提供了柞蚕蛹抗菌肽D和B对大肠杆菌K_(12)D_(31)作用不同时间的电镜照片。     

Development of a process for biodetoxification of metal cyanides from waste waters

A bacterial consortium capable of utilising metal cyanides as a source of nitrogen was used to develop a microbiological process for the detoxification of metal cyanides (viz. copper cyanide and zinc cyanide) from electroplating waste water. Optimal conditions biodegradation of both the metal-cyanide compounds were pH 7.5, temperature 35°C, inoculum size 10⁹ cells per ml and glucose or sugarcane molasses requirement of 5 mM or 0.6 ml/l, respectively. Metal precipitates obtained during metal-cyanide biodegradation were identified as metal-hydroxides. When the treatment was carried out in a 27 l rotating biological contactor (RBC) in continuous mode, the system could achieve >99.9% removal of 0.5 mM metal cyanide (ca. 52 mg/l cyanide and 30–40 mg/l copper/zinc) in 15 h with sugarcane molasses as carbon source. The RBC treated effluent was found to be safe for discharge in the environment as confirmed by chemical analysis and fish bioassay studies.     

Studies on the rates of water use of dwarf wheat and their relationship with potential values based on the climatological approach

Summary Field studies were conducted to assess the field evapotranspiration of dwarf wheat under semi-arid conditions of Hissar (India), and to compare the same with the values obtained by various formulae using meteorological parameters. First irrigation to the crop was given three weeks after sowing and subsequent irrigations when about 50% of the available soil moisture was depleted from the root zone. Measured quantity of water was applied according to the soil moisture deficit in the root zone.The daily rate of water use by wheat was quite low during the early part of the growth (about 2 mm per day). It increased gradually up to ear emergence and grain development (about 5 mm per day) and declined towards maturity. Potential evapotranspiration calculated by Thornthwaite formula was most unreliable for estimating evapotranspiration. Evaporation from the U.S. open pan evaporimeter gave quite closer values to the actual rates, except during maturity. Ramdas values did not show much closeness with the actual water use rates. Penman's formula gave values quite closer to the field water use rates. Consumptive use calculated by Blaney-Criddle method did not give appropriate estimate during January and February, but it exhibited much closeness during December and March. re]19750305     

Characterization of a CaCl2-dependent transfection system of Escherichia coli and T3 phage DNA

Transfection by DNA isolated from bacteriophage T3 was studied using Escherichia coli 921/0 as host. The following conditions were found optimal: Competent E. coli 921/0 were obtained by harvesting the bacteria at the onset of late exponential growth (5 X 10(8) cells/ml) and treating the latter with 0.05 M CaCl2. Hereafter, the microbes were suspended in 50 mM Tris-HCl buffer (pH 7.2) and the concentration adjusted to 7 X 10(9) cells/ml. T3 DNA was added and the suspension kept at 0 degrees C for 15 min. Determination of the number of infectious centers was then carried out in the usual way. The efficiency of transfection under these conditions amounted to 10(4) p. f. u./microgram DNA. Preincubation of competent bacteria with T4 DNA at 0 degrees C before the addition of T3 DNA reduced the number of infectious centers. However, if T3- and T4 DNA were added simultaneously no decrease of the transfection efficiency occurred. Calf thymus DNA was without influence on transfection.