Pantoea Agglomerans Strain Research Articles

Solubilization of organic and inorganic phosphates by three highly efficient soil bacterial isolates

Screening soil samples collected from a diverse range of slightly alkaline soil types, we have isolated 22 competent phosphate solubilizing bacteria (PSB). Three isolates identified as Pantoea agglomerans strain P5, Microbacterium laevaniformans strain P7 and Pseudomonas putida strain P13 hydrolyzed inorganic and organic phosphate compounds effectively. Bacterial growth rates and phosphate solubilization activities were measured quantitatively under various environmental conditions. In general, a close association was evident between phosphate solubilizing ability and growth rate which is an indicator of active metabolism. All three PSB were able to withstand temperature as high as 42°C, high concentration of NaCl upto 5% and a wide range of initial pH from 5 to 11 while hydrolyzing phosphate compounds actively. Such criteria make these isolates superior candidates for biofertilizers that are capable of utilizing both organic and mineral phosphate substrates to release absorbable phosphate ion for plants.

Temporal Analysis of Cotton Boll Symptoms Resulting From Southern Green Stink Bug Feeding and Transmission of a Bacterial Pathogen

The southern green stink bug, Nezara viridula (L.), is a significant pest of cotton, Gossypium hirsutum L., and is becoming an increasing challenge due to the decrease in use of broad-spectrum insecticides on the crop. The southern green stink bug can vector an opportunistic Pantoea agglomerans strain (designated Sc 1-R) into cotton bolls, resulting in infection. The appearance of stink bug damage varies, and pest managers cannot readily identify its source. This research reports a systematic depiction of green, immature boll responses at various stages of maturity (1, 2, and 3 wk post-anthesis [WPA]) to stink bug injury and to infection by the vectored cotton pathogen by demonstrating the progression of effects 1, 2, and 3 wk after exposure (WAE). When laboratory-reared adult southern green stink bug not harboring Sc 1-R deposited bacteria into greenhouse-grown bolls at 1, 2, or 3 WPA during feeding/probing, bacteria reached concentrations of 10(9), 10(9), and 10(3) colony-forming units (CFUs)/g tissue, respectively, at 3 WAE, yet caused minimal seed and lint damage regardless of the age of the bolls that were penetrated. Bolls at a maturity of 1 or 2 WPA showed similar susceptibility when exposed to stink bugs that vectored Sc 1-R. After a week of infection, seeds were salmon-pink with normal white lint and up to 10(4) CFUs/g tissue when Sc 1-R was detected. Necrosis of the entire inoculated locule(s) with a maximum Sc 1-R concentration detected at 10(8) CFUs/g tissue occurred in samples harvested 2 or 3 WAE. Conversely, seed and lint deterioration due to the transmitted opportunist into bolls exposed 3 WPA was confined to the puncture site. In summary, after a week of development, bolls were tolerant to southern green stink bug feeding/ probing damage and to nonpathogenic bacteria, but they were severely damaged when the opportunistic pathogen Sc 1-R was transmitted. At 3 WPA, the fruit was immune to the spread of the pathogen with infections confined to the puncture site.

Effect of γ-irradiation on membrane fatty acids and peptidoglycan’s muropeptides ofPantoea agglomerans, a plant pathogen

The aim of this study was to evaluate the effect of gamma-irradiation on the fatty acids (FA) and muropeptides content of two strains of an Enterobacteriacea: Pantoea agglomerans. Pantoea agglomerans strains ATCC 49174 and RL1 isolated from irradiated carrots were used for this study. Radiation treatments (1 and 3.5 kGy) were performed to study the radiotolerance. Total lipids were obtained by multiple extractions using methanol/chloroform (2 : 1) and were quantified by GC. Muropeptides were purified by successive enzymatic digestions and analysed using a reverse phase C(18) column in high performance liquid chromatography. A significant (P <or= 0.05) modification of the bacterial wall was noticed for the membrane FA composition and the muropeptides. Effects of irradiation on the bacterial membrane are noticeable and could play an important role on the cellular response and ability to survive this harsh environment. To our knowledge, it is the first study to demonstrate the effects of ionizing irradiation on the modification of the FA and one of the few to confirm its effects on the muropeptides of the peptidoglycan.

Characterization of the Mineral Phosphate-Solubilizing Activity of Pantoea aglomerans MMB051 Isolated from an Iron-Rich Soil in Southeastern Venezuela (Bolívar State)

The mineral phosphate-solubilizing (MPS) activity of a Pantoea agglomerans strain, namely MMB051, isolated from an iron-rich, acidic soil near Ciudad Piar (Bolívar State, Venezuela), was characterized on a chemically defined medium (NBRIP). Various insoluble inorganic phosphates, including tri-calcium phosphate [Ca(3)(PO(4))(2)], iron phosphate (FePO(4)), aluminum phosphate (AlPO(4)), and Rock Phosphate (RP) were tested as sole sources of P for bacterial growth. Solubilization of Ca(3)(PO(4))(2) was very efficient and depended on acidification of the external milieu when MMB051 cells were grown in the presence of glucose. This was also the case when RP was used as the sole P source. On the other hand, the solubilization efficiency toward more insoluble mineral phosphates (FePO(4) and AlPO(4)) was shown to be very low. Even though gluconic acid (GA) was detected on culture supernatants of strain MMB051, a consequence of the direct oxidation pathway of glucose, inorganic-P solubilization seemed also to be related to other processes dependent on active cell growth. Among these, proton release by ammonium (NH(4)(+) ) fixation appeared to be of paramount importance to explain inorganic-P solubilization mediated by strain MMB051. On the contrary, the presence of nitrate (NO(3)(-) ) salts as the sole N source affected negatively the ability of MMB051 cells to solubilize inorganic P.

Antibiosis and Acidification byPantoea agglomeransStrain E325 May Contribute to Suppression ofErwinia amylovora

Pantoea agglomerans strain E325, a commercially available antagonist for fire blight of apple and pear, was originally selected through screening based on suppression of Erwinia amylovora on flower stigmas, but specific mechanisms of antagonism were unknown. Bacterial modification of pH was evaluated as a possible mechanism by analyzing stigma exudates extracted from 'Gala' apple stigmas. The pH values for field samples were only slightly lower than controls, but indicated a range (pH 5 to 6) conducive for antibiotic activity according to subsequent assays. Under low-phosphate and low-pH conditions, an antibacterial product of E325 with high specificity to E. amylovora was effective at low concentrations. A minimum of 20 to 40 ng of a ninhydrin-reactive compound purified using RP-HPLC caused visible inhibition in assays. Activity was heat stable and unaffected by amino acids, iron, or enzymes known to affect antibiotics of P. agglomerans. Antibiosis was diminished, however, under basic conditions, and with increasing phosphate concentrations at pH 6 and 7. Inhibition was not observed in media containing phosphate concentrations commonly used in antibiosis assays. We propose that E325 suppresses the fire blight pathogen not only by competing for nutrients on the stigma, but by producing an antibiotic specific to E. amylovora. Further work is necessary to substantiate that the compound is produced and active on flower stigmas.

Carotenoids’ influence on radiotolerance ofPantoea agglomerans, a plant pathogen

The aim of this study was to evaluate the effect of gamma radiation on the carotenoid content of two strains of the Enterobacteriaceae: Pantoea agglomerans. Pantoea agglomerans strains ATCC 49174 and RL1 were used for this study. Successive radiation treatments were performed to study the radiotolerance. Total carotenoids were obtained by multiple extraction using chloroform/methanol (2 : 1), quantified by measuring the optical density at 453 nm and their antioxidant activity measured by a colorimetric method. The D(10) studies were conducted using a UC-15A irradiator loaded with 60Co. Bacterial counts from various dilutions were carried out after irradiation. Strain ATCC 49174 irradiated at 1 kGy produced 4.3 times more carotenoids than the control, whereas carotenoid synthesis increased by 2.9-fold in the strain RL1. However, there was no significant difference in the D(10) values. Carotenoid increased production is influenced by gamma radiation but does not modify the tolerance to radiations. To our knowledge, this is the first study to demonstrate the effects of gamma radiation on carotenoid production levels.

MECHANISMS OF PANTOEA AGGLOMERANS STRAIN E325 AS ANTAGONIST OF ERWINIA AMYLOVORA

Pantoea agglomerans strain E325, the active ingredient in Bloomtime BiologicalTM, was originally isolated from apple blossoms and selected based on broad screening with detached crab apple flowers. To evaluate pH modification on 'Gala' apple stigmas as a possible mode of antagonism, exudates were extracted from flower stigmas and tested with a pH electrode. Measurement of pH in field samples resulted in only slight differences but indicated a low pH range (between 5 and 6) conducive for antibiotic activity based on laboratory assays. An extracellular product of E325 was shown under low-phosphate and low-pH conditions to inhibit Erwinia amylovora but not strains of 17 other microbial species tested so far. A minimum of 20 to 40 ng of the compound, purified using RP-HPLC, caused visible inhibition of the pathogen in plate assays. Inhibition was relatively heat stable and unaffected by amino acids, ferric ions or enzymes previously used to characterize antibiotics from other strains of P. agglomerans. Conversely, activity was critically affected by pH and phosphate buffering capacity. It was deactivated under basic conditions, and at pH 6 and 7, it diminished or disappeared at a rate that increased with the phosphate buffer concentration. The inhibitory activity was often undetectable at phosphate concentrations commonly used in tests for antibiosis. Work is in progress to develop methodology for direct evaluation of pH on flower stigmas and to further characterize the inhibitory compound produced by strain E325.

The structure of the O-specific polysaccharide of the lipopolysaccharide from Pantoea agglomerans strain FL1

A neutral O-specific polysaccharide consisting of d-rhamnose was obtained by mild acid hydrolysis of the lipopolysaccharide of the plant pathogenic bacterium Pantoea agglomerans strain FL1, a common epiphyte of many plant species, and associated with Pseudomonas savastanoi pv. savastanoi in young and apparently intact olive knots. By means of compositional and methylation analyses, and NMR spectroscopy, the chemical repeating unit of the polymer was identified as a linear tetrasaccharide of the structure: [Display omitted]

Management of Xanthomonas Leaf Blight of Onion with a Plant Activator, Biological Control Agents, and Copper Bactericides.

Xanthomonas leaf blight (Xanthomonas axonopodis pv. allii) is a yield-limiting disease of onion (Allium cepa) in the western United States. Frequent applications of copper-based bactericides amended with an ethylenebisdithiocarbamate fungicide (e.g., maneb or mancozeb, class B2 carcinogens) provide some disease suppression, but strategies to reduce conventional bactericide use are needed to minimize grower costs, environmental impact, and public exposure to class B2 pesticides. Applications of acibenzolar-S-methyl reduced in planta and epiphytic populations of X. axonopodis pv. allii as effectively as applications of copper hydroxide-mancozeb in growth chamber studies. Under field conditions, four weekly applications of acibenzolar-S-methyl reduced severity of Xanthomonas leaf blight as or more effectively than 9 to 12 weekly applications of copper hydroxide or copper hydroxide-mancozeb. Acibenzolar-S-methyl applications did not increase bulb yield or grade compared with copper bactericide treatments. However, bulb yield was reduced 22 to 27% when 10 weekly applications of acibenzolar-S-methyl were made in the absence of disease. Application of a commercial formulation of both Pantoea agglomerans strain C9-1 and Pseudomonas fluorescens strain A506 reduced severity of Xanthomonas leaf blight in field experiments. Weekly copper hydroxide applications starting 1 to 2 weeks before bulb initiation were as effective as weekly applications started 3 to 4 weeks before bulb initiation, irrespective of the maneb rate used. Integration of acibenzolar-S-methyl and biological control agents with copper hydroxide in a carefully timed spray program may eliminate the use of the class B2 carcinogens maneb and mancozeb on onion without compromising efficacy for management of Xanthomonas leaf blight.

An Extracellular Protease of Pseudomonas fluorescens Inactivates Antibiotics of Pantoea agglomerans

ABSTRACT Pseudomonas fluorescens A506 and Pantoea agglomerans strains Eh252 and C9-1 are biological control agents that suppress fire blight, an important disease of pear and apple caused by the bacterium Erwinia amylovora. Pseudomonas fluorescens strain A506 suppresses disease largely through competitive exclusion of E. amylovora on surfaces of blossoms, the primary infection court, whereas Pantoea agglomerans strains Eh252 and C9-1 produce antibiotics that are toxic to E. amylovora. In this study, an extracellular protease produced by A506 is characterized and evaluated for its capacity to inactivate the antibiotics produced by the strains of Pantoea agglomerans. Activity of the extracellular protease was optimal at pH 9 and inhibited by zinc- or calcium-chelators, indicating that the protease is an alkaline metalloprotease. In an agar plate bioassay, partially purified extracellular protease inactivated the antibiotics mccEh252 and herbicolin O, which are produced by Pantoea agglomerans strains Eh252 and C9-1, respectively. Derivatives of A506 deficient in extracellular protease production were obtained by transposon mutagenesis, and the aprX gene encoding the protease was cloned and sequenced. Strain A506 inactivated mccEh252 and herbicolin O in agar plate bioassays, whereas the aprX mutant did not inactivate the antibiotics. Both A506 and the aprX mutant were insensitive to antibiosis by C9-1 and Eh252; thus, the protease was not required to protect A506 from antibiosis. These data highlight a previously unknown role of the extracellular protease produced by Pseudomonas fluorescens A506 in interactions among plant-associated microbes.

Anatomy of Cranberry Stem Gall and Localization of Bacteria in Galls

ABSTRACT Cranberry stem gall is characterized by tumors that girdle stems, thereby killing all distal leaves, flowers, and fruit. Bacteria that produce high levels of the plant growth hormone indole-3-acetic acid (IAA) are associated with and believed to cause cranberry stem gall. The anatomy of naturally occurring galls on woody cranberry plants and galls caused by inoculation of micropropagated cranberry plants with Pantoea agglomerans strain 4/99 was consistent with elevated levels of IAA in plants. Field galls exhibited hypertrophy and hyperplasia of tissue external to the vascular cambium, resulting in extensive stem swelling and splitting of the periderm. Similarly, galls on micropropagated plants contained enlarged cortical parenchyma cells. The current year's xylem vessels in field galls were narrow and dense compared with xylem vessels of healthy stems. Curved xylem elements apparently developed de novo within field galls and galls on inoculated plants. Cavities and fissures in both types of galls contained dense aggregates of bacteria. Treatment of micropropagated plants with synthetic IAA caused hypertrophy of cortical parenchyma and formation of adventitious roots. The results support the hypothesis that IAA-producing bacteria cause cranberry stem gall.

Role of glutamine synthetase in phenazine antibiotic production byPantoea agglomeransEh1087

Pantoea agglomerans strain Eh1087 produces the phenazine antibiotic D-alanylgriseoluteic acid. A glutamine auxotroph harboring an insertion in a putative glnA gene was obtained by transposon-mutagenesis of Eh1087 that produced less D-alanylgriseoluteic acid than the parental strain (strain Eh7.1). Cosmids encoding the Eh1087 glnA were isolated by their ability to complement the mutant for prototrophy. The role of the Eh1087 glnA locus was functionally confirmed by complementation of an Escherichia coli glnA mutant. Analysis of the nucleotide and deduced amino acid sequences of the Eh1087 glnA gene indicated a high degree of similarity to the glnA genes and glutamine synthetase enzymes of other Enterobacteriaceae. Isotopic labelling experiments with 15N-labelled ammonium sulfate demonstrated that wild-type Eh1087 incorporated 15N into griseoluteic acid more readily than the glnA mutant Eh7.1. We conclude that the 2 nitrogens in the phenazine nucleus originate from glutamine and the intracellular glutamine synthesized by Eh1087 is a source of the phenazine nucleus nitrogens even in glutamine-rich environments.

Biological control of fire blight in pear orchards with a formulation of Pantoea agglomerans strain Eh 24

Biological control by using epiphytic bacteria against Erwinia amylovora has been considered as an alternative method for controlling the disease. Talc-based formulation of Pantoea agglomerans strain Eh-24 was applied at 30% and 100% bloom on two pear orchards which were selected from different locations in the Aegean Region in Turkey. Pear orchard trials were replicated for two years (1999 and 2000) in each place. Talc-based formulation of P. agglomerans strain Eh-24 was sprayed on pear trees which were naturally infected with E. amylovora. In the orchard trials conducted in 1999 and 2000, talc-based formulation of P. agglomerans strain Eh-24 reduced the percentage of blighted blossoms on pear orchards by 63% to 76%, approximately. Copper oxychloride+maneb was less effective in reducing the incidence of blossom infection by E. amylovora in each pear orchard than the bioformulation treatment. P. agglomerans strain Eh-24 labelled with StrR+ was applied at 30% and 100% bloom to monitor the colonization and population dynamics of P. agglomerans on pear blossoms. The population size of P. agglomerans strain Eh-24 strR+ on pear blossoms increased from 2x104 to 1.3x106 cfu per blossom over 18 days.

Temperature and Pomaceous Flower Age Related to Colonization byErwinia amylovoraand Antagonists

ABSTRACT Fire blight of apple and pear is initiated by epiphytic populations of Erwinia amylovora on flower stigmas. Predicting this disease and managing it with microbial antagonists depends on an understanding of bacterial colonization on stigmas. Detached 'Manchurian' crab apple flowers were inoculated with E. amylovora and subjected to a range of constant temperatures or various fluctuating temperature regimes. Results may have application to disease risk assessment systems such as the Cougarblight model, which now are based on in vitro growth of the pathogen. In other experiments, detached crab apple flowers and attached 'Gala' apple flowers were maintained at different temperatures for various periods before inoculation with E. amylovora or antagonists (Pseudomonas fluorescens strain A506 and Pantoea agglomerans strains C9-1 and E325). Maximum stigma age supporting bacterial multiplication decreased as temperature increased, and was reduced by pollination. Stigmas were receptive to bacteria at ages older than previously reported, probably due to less interference from indigenous organisms. The study revealed antagonist limitations that possibly affect field performance (e.g., the inability of strain A506 to grow on relatively old stigmas conducive to the pathogen). Such deficiencies could be overcome by selecting other antagonists or using antagonist mixtures in the orchard.

The influence of antibiotic production and pre-emptive colonization on the population dynamics of Pantoea agglomerans (Erwinia herbicola) Eh1087 and Erwinia amylovora in planta.

Stigma colonization by Erwinia amylovora is the crucial first step in the development of most fire blight infections in apple and pear trees. Suppression at this point of the disease process by antagonists of E. amylovora, such as Pantoea agglomerans (Erwinia herbicola) strain Eh1087, is a rational approach to control fire blight. We tested the hypothesis that the ability of E. amylovora to compete with Eh1087 for colonization of a stigma is reduced by the potential for Eh1087 to produce the phenazine antibiotic, d-alanylgriseoluteic acid (AGA). In competition experiments on the stigmas of apple flowers, E. amylovora was significantly less successful against Eh1087 (AGA+) than against EhDeltaAGA (AGA-). Further experiments to test the importance of pre-emptive colonization of the stigma by either the pathogen or the antagonist suggested that AGA production significantly enhanced the competitiveness of Eh1087 when it was applied at the same time or 24 h before the pathogen. We also found that pre-emptive stigma colonization by either the pathogen or the antagonist resulted in a population that was resilient to subsequent invasion by a second species suggesting that niche exclusion has a dominant influence on the dynamics of bacterial populations on stigmas.

Antibiosis Contributes to Biological Control of Fire Blight by Pantoea agglomerans Strain Eh252 in Orchards

ABSTRACT Fire blight, caused by Erwinia amylovora, is the most serious bacterial disease of pear and apple trees. Biological control with strains of Pantoea agglomerans (syn. Erwinia herbicola) may provide an effective disease management strategy for fire blight. Most strains of P. agglomerans evaluated for suppression of fire blight produce compounds that inhibit the growth of E. amylovora in culture. The role of these inhibitory compounds in fire blight suppression in orchard environments has not been studied. In seven field trials in Oregon, we compared the population dynamics and disease suppression with P. agglomerans Eh252, a strain that produces a single antibiotic, with its near-isogenic antibiotic-deficient derivative, strain 10:12. Water or suspensions of Eh252 or 10:12 (1 x 10(8) CFU/ml) were applied at 30 and 70% bloom to pear or apple trees. Aqueous suspensions of freeze-dried cells of E. amylovora (3 x 10(5) CFU/ml) were applied at full bloom. Additional trees were treated with streptomycin or oxytetracycline at 30 and 70% bloom and in some experiments, 1 day after application of the pathogen. Population sizes of Eh252 or 10:12 on pear blossoms were estimated by spreading dilutions of blossom washes on culture media. Average population sizes of Eh252 and 10:12 on blossoms ranged from 10(5) to 10(7) CFU, and in five of six trials, the relative area under the population curve of Eh252 was not significantly different than that of its derivative 10:12. Both Eh252 and 10:12 reduced the growth of the pathogen on blossoms compared with inoculated water-treated controls. Eh252 significantly decreased the incidence of fire blight in six of seven field trials compared with the incidence on water-treated trees, and 10:12 similarly reduced the incidence of fire blight in four of seven trials. In three of seven field trials, trees treated with Eh252 had a significantly lower incidence of fire blight compared with trees treated 3 with 10:12. Overall,3 Eh252 reduced the incidence of fire blight by 55 +/- 8%, 10:12 by 30 +/- 6%, streptomycin by 75 +/- 4%, and oxytetracycline by 16 +/- 14%. The effectiveness of strain 10:12 compared with water treatment indicates that other mechanisms (e.g., competitive exclusion or habitat modification) also contribute to disease suppression by P. agglomerans. The increased suppression of fire blight by the parental strain Eh252 compared with the antibiotic-deficient mutant 10:12 indicates that antibiosis is an important mechanism of biological control of fire blight.

GENES FOR BIOSYNTHESIS OF PANTOCIN A AND B BY PANTOEA AGGLOMERANS EH318

Pantoea agglomerans strain Eh318 has been effective in biological control tests against fire blight and it inhibits the growth of Erwinia amylovora on minimal media. A double zone of inhibition on a plate containing a soft-agar overlay seeded with E. amylovora suggests the action of two distinct antibiotics, pantocin A and pantocin B. The structural characterization of pantocin A is underway, whereas that of pantocin B has been determined to be (R)-N-[((S)-2-Amino-propanoylamino)methyl]-2-methanesulfonyl-succinami c acid. Subcloning and transposon mutagenesis data suggested that the genetic region involved in the biosynthesis of pantocin A is 2.7 kb at most, while that of pantocin B is at least 18.5 kb. The open reading frames (ORF) found within the pantocin A biosynthetic region were designated paaA, paaB and paaC There is also a small ORF present upstream of the paaABC operon. The genes of the operon are similar in organization and, in the case of paaA, in DNA sequence to those involved in microcin C7 biosynthesis in Escherichia coli. Sequencing of the 18.5 kb biosynthetic region for pantocin B has commenced. Results thus far indicate that the region contains genes with putative amino acid sequence similarity to enzymes involved in the biosynthesis of and resistance to other antibiotics. Marker-exchange mutants of Eh318 deficient in the production of pantocin A (Eh421; PanA-), pantocin B (Eh439; PanB-) or both antibiotics (Eh440; PanAB-) were created. Strain Eh440 (PanAB-) was only marginally less effective than Eh318 in several immature pear fruit tests.

Biological control of Monilinia laxa and Rhizopus stolonifer in postharvest of stone fruit by Pantoea agglomerans EPS125 and putative mechanisms of antagonism

Treatment of stone fruits (apricot, peach and nectarine) with Pantoea agglomerans strain EPS125 decreased the incidence and diameter of lesions of brown rot caused by Monilinia laxa and soft rot caused by Rhizopus stolonifer. Root control was achieved on fruits either wounded and subsequently inoculated with the pathogens or non-wounded and naturally infected from orchards. The efficacy of biocontrol was dependent on the concentration of the biocontrol agent and pathogen. At medium to low pathogen dose, optimal EPS125 concentrations were above 10 7 CFU ml −1. The median effective dose (ED 50) of EPS125 was 4.5×10 4 in M. laxa and 2.2×10 5 CFU ml −1 in R. stolonifer. However, EPS125 was more effective in M. laxa than in R. stolonifer as indicated by the ratio between ED 50 of the biocontrol agent and pathogen ( K z / K x ) which was 166 and 1263, respectively. Interactions between the strain EPS125 and the fruit surface, and M. laxa and R. stolonifer, were studied to determine the mechanisms of protection from postharvest rots. The strain EPS125 colonizes, grows and survives on stone fruit wounds. Significant inhibition of conidial germination and hyphal growth of R. stolonifer and M. laxa was achieved when the fungal and EPS125 cells were cocultivated on peel leachate or nectarine juice. However, no effect was observed when the antagonist and the pathogen cells were physically separated by a membrane filter which permits nutrient and metabolite interchange. Therefore, a direct interaction between the strain and the pathogen cells is necessary for antagonism, without a significant contribution of the production of antibiotic substances or nutrient competition. Preemptive exclusion by wound colonization and direct interaction with the pathogen is proposed as the mechanism of biocontrol.

Comparative Dynamics of Adherent and Nonadherent Bacterial Populations on Maize Leaves

ABSTRACT The dynamics of the adherent and nonadherent populations of three bacterial species on maize leaves were examined to identify the extent to which bacteria adhere to leaves and the importance of this adhesion to leaf colonization. Pantoea agglomerans strain BRT98, Clavibacter michiganensis subsp. nebraskensis strain GH2390, and Pseudomonas syringae pv. syringae strain HS191R all rapidly adhered to maize leaves following inoculation, but differed in the percentage of cells that adhered to the leaves. Immediately following inoculation, the percentage of adherent cells was highest for the saprophyte P. agglomerans (8 to 10%) and was much lower for the pathogens C. michiganensis subsp. nebras-kensis and P. syringae pv. syringae (2 to 3 and <1%, respectively), although the results for P. syringae pv. syringae HS191R were based on only one experiment. In the 4 days following inoculation, the percentage of the P. agglomerans populations that adhered to the leaves increased to approximately 70%. Similarly, the percentage of C. michiganensis subsp. nebraskensis and P. syringae pv. syringae cells that resisted removal steadily increased in the days following inoculation, although these increases probably reflected both adherence and localization to endophytic sites. Based on differences in the percentage of cells adhering to several cuticular wax mutants of maize, the rapid adherence of C. michiganensis subsp. nebraskensis cells to maize leaves was influenced by the cuticular wax properties, while the rapid adherence of P. agglomerans was not. Finally, bacterial adherence to leaves was advantageous to P. agglomerans survival and growth on leaves based on the finding that the nonadherent populations of the P. agglomerans strain decreased significantly more than did the adherent populations in the 24 h following inoculation, and increased much less than did the adherent populations over the next 3 days. Similar results with the C. michiganensis subsp. nebraskensis and P. syringae pv. syringae strains indicate that bacterial adherence to leaves, bacterial movement to endophytic sites, or both were advantageous to the survival and growth of these strains on leaves.

The effect of nitrogen and carbon sources on growth of the biocontrol agent Pantoea agglomerans strain CPA-2.

The effect of several nitrogen and carbon sources on the growth of Pantoea agglomerans (strain CPA-2) was studied for the first time. Synthetic nitrogen and carbon sources were tested to obtain a suitable medium. Synthetic yeast extract provided maximum growth and disaccharides such as sucrose, lactose and trehalose improved this growth significantly from 3.2 x 10(9) to 5.5 x 10(9) cfu ml-1. Pantoea agglomerans can be produced in a combination of nitrogen sources such as yeast extract with carbohydrates such as sucrose in shake flask and a laboratory fermenter (5 l). Results suggest good production of this biocontrol agent on a laboratory scale and the potential of scaling up the process.