Determinants of Epiphytic Fitness in Bacteria

Plant species-dependent transmission of Escherichia coli O157:H7 from the spermosphere to cotyledons and first leaves

The colonization of six edible plant species: alfalfa, broccoli, coriander, lettuce, parsley and rocket, by the human pathogen Shigatoxigenic Escherichia coli was investigated following two modes of artificial inoculation of seeds, by soaking or watering. The frequency and extent of colonization of cotyledons depended on the mode of inoculation, with three, rapidly germinating species being successfully colonized after overnight soaking, but slower germinating species requiring prolonged exposure to bacteria by watering of the surrounding growth media. Separate analysis of the cotyledons and leaves from individual plants highlighted that successful colonization of the true leaves was also species dependent. For three species, failure of transfer, or lack of nutrients or suitable microhabitat on the leaf surface resulted in infrequent bacterial colonization. Colonization of leaves was lower and generally in proportion to that in cotyledons, if present. The potential risks associated with consumption of leafy produce are discussed.

The RsmA RNA-Binding Proteins in Pseudomonas syringae Exhibit Distinct and Overlapping Roles in Modulating Virulence and Survival Under Different Nutritional Conditions

The post-transcriptional regulator RsmA globally controls gene expression in bacteria. Previous studies showed that RsmA2 and RsmA3 played critical roles in regulating type III secretion system (T3SS), motility, syringafactin, and alginate productions in Pseudomonas syringae pv. tomato strain DC3000 (PstDC3000). In this study, we investigated global gene expression profiles of the wild-type PstDC3000, the rsmA3 mutant, and the rsmA2/A3 double mutant in the hrp-inducing minimum medium (HMM) and King's B (KB) medium. By comparing the rsmA2/A3 and rsmA3 mutants to PstDC3000, a total of 1358 and 1074 differentially expressed genes (DEGs) in HMM, and 870 and 1463 DEGs in KB were uncovered, respectively. When comparing the rsmA2/A3 mutant with the rsmA3 mutant, 277 and 741 DEGs in HMM and KB, respectively, were revealed. Transcriptomic analysis revealed that the rsmY, rsmZ, and rsmX1-5 non-coding small RNAs (ncsRNAs) were positively affected by RsmA2 and RsmA3, while RsmA3 positively regulates the expression of the rsmA2 gene and negatively regulates both rsmA1 and rsmA5 gene expression. Comparative transcriptomic analysis showed that RsmA2 and RsmA3 synergistically influenced the expression of genes involved in T3SS and alginate biosynthesis in HMM and chemotaxis in KB. RsmA2 and RsmA3 inversely affected genes involved in syringafactin production in HMM and ribosomal protein biosynthesis in KB. In addition, RsmA2 played a major role in influencing genes involved in sarcosine and thiamine biosynthesis in HMM and in mannitol and phosphate metabolism in KB. On the other hand, genes involved in fatty acid metabolism, cellulose biosynthesis, signal transduction, and stress responses were mainly impacted by RsmA3 in both HMM and KB; whereas RsmA3 played a major role in controlling genes involved in c-di-GMP, phosphate metabolism, chemotaxis, and capsular polysaccharide in HMM. Furthermore, regulation of syringafactin production and oxidative stress by RsmA2 and RsmA3 was experimentally verified. Our results suggested the potential interplay among the RsmA proteins, which exhibit distinct and overlapping roles in modulating virulence and survival in P. syringae under different nutritional conditions.

Comparative transcriptomic analysis of global gene expression mediated by (p) ppGpp reveals common regulatory networks in Pseudomonas syringae

BACKGROUND

Pseudomonas syringae is an important plant pathogen, which could adapt many different environmental conditions. Under the nutrient-limited and other stress conditions, P. syringae produces nucleotide signal molecules, i.e., guanosine tetra/pentaphosphate ((p)ppGpp), to globally regulate gene expression. Previous studies showed that (p) ppGpp played an important role in regulating virulence factors in P. syringae pv. tomato DC3000 (PstDC3000) and P. syringae pv. syringae B728a (PssB728a). Here we present a comparative transcriptomic analysis to uncover the overall effects of (p)ppGpp-mediated stringent response in P. syringae.

RESULTS

In this study, we investigated global gene expression profiles of PstDC3000 and PssB728a and their corresponding (p)ppGpp⁰ mutants in hrp-inducing minimal medium (HMM) using RNA-seq. A total of 1886 and 1562 differentially expressed genes (DEGs) were uncovered between the (p)ppGpp⁰ mutants and the wild-type in PstDC3000 and PssB728a, respectively. Comparative transcriptomics identified 1613 common DEGs, as well as 444 and 293 unique DEGs in PstDC3000 and PssB728a, respectively. Functional cluster analysis revealed that (p) ppGpp positively regulated a variety of virulence-associated genes, including type III secretion system (T3SS), type VI secretion system (T6SS), cell motility, cell division, and alginate biosynthesis, while negatively regulated multiple basic physiological processes, including DNA replication, RNA processes, nucleotide biosynthesis, fatty acid metabolism, ribosome protein biosynthesis, and amino acid metabolism in both PstDC3000 and PssB728a. Furthermore, (p) ppGpp had divergent effects on other processes in PstDC3000 and PssB728a, including phytotoxin, nitrogen regulation and general secretion pathway (GSP).

CONCLUSION

In this study, comparative transcriptomic analysis reveals common regulatory networks in both PstDC3000 and PssB728a mediated by (p) ppGpp in HMM. In both P. syringae systems, (p) ppGpp re-allocate cellular resources by suppressing multiple basic physiological activities and enhancing virulence gene expression, suggesting a balance between growth, survival and virulence. Our research is important in that due to similar global gene expression mediated by (p) ppGpp in both PstDC3000 and PssB728a, it is reasonable to propose that (p) ppGpp could be used as a target to develop novel control measures to fight against important plant bacterial diseases.

Exploring Rain as Source of Biological Control Agents for Fire Blight on Apple

Poor survival on plants can limit the efficacy of Biological Control Agents (BCAs) in the field. Yet bacteria survive in the atmosphere, despite their exposure to high solar radiation and extreme temperatures. If conditions in the atmosphere are similar to, or more extreme than, the environmental conditions on the plant surface, then precipitation may serve as a reservoir of robust BCAs. To test this hypothesis, two hundred and fifty-four rain-borne isolates were screened for in vitro inhibition of Erwinia amylovora, the causal agent of fire blight, as well as of other plant pathogenic bacteria, fungi and oomycetes. Two isolates showed strong activity against E. amylovora and other plant pathogenic bacteria, while other isolates showed activity against fungal and oomycete pathogens. Survival assays suggested that the two isolates that inhibited E. amylovora were able to survive on apple blossoms and branches similarly to E. amylovora. Pathogen population size and associated fire blight symptoms were significantly reduced when detached apple blossoms were treated with the two isolates before pathogen inoculation, however, disease reduction on attached blossoms within an orchard was inconsistent. Using whole genome sequencing, the isolates were identified as Pantoea agglomerans and P. ananatis, respectively. A UV-mutagenesis screen pointed to a phenazine antibiotic D-alanylgriseoluteic acid synthesis gene cluster as being at the base of the antimicrobial activity of the P. agglomerans isolate. Our work reveals the potential of precipitation as an under-explored source of BCAs, whole genome sequencing as an effective approach to precisely identify BCAs, and UV-mutagenesis as a technically simple screen to investigate the genetic basis of BCAs. More field trials are needed to determine the efficacy of the identified BCAs in fire blight control.

Characterization of medicinal plant-associated biocontrol Bacillus subtilis (SSL2) by liquid chromatography-mass spectrometry and evaluation of compounds by in silico and in vitro methods

This study explores the antimicrobial properties of bioactive secondary metabolites extracted from the medicinal plant (Solanum surattense)-associated Bacillus subtilis strain SSL2. The secondary metabolites were extracted from B. subtilis (SSL2) using ethyl acetate, acetone, butanol, chloroform and methanol solvents. The crude extract was tested against two wilt causing pathogens: Ralstonia solanacearum and Fusarium oxysporum. The results revealed that the ethyl acetate extract has maximum inhibition against both the pathogens tested in this study. Furthermore, liquid chromatography-mass spectrometry (LC-MS) analysis of ethyl acetate extract identified 80 different compounds based on mass-to-charge ratio, database difference, resolution of mass spectrum and so on. Among the 80 compounds, citrulline (m/z = 158.0917), chloramphenicol (m/z = 195.075) and carnitine (m/z 162.11) were further selected based on m/z ratio for in silico and in vitro analyses. The in silico analysis revealed that citrulline, chloramphenicol and carnitine inhibited the virulent genes phcA (R. solanacearum) and ste12 (F. oxysporum). Further, under in vitro condition, citrulline and chloramphenicol were found to inhibit the growth of R. solanacearum and F. oxysporum. On the basis of the biocontrol activity of B. subtilis (SSL2) in in silico and in vitro conditions, the bacteria could be used as a biocontrol agent against both bacterial and fungal wilt-causing pathogens. However, this needs to be tested in pot studies or field conditions before being used as biocontrol agents.Communicated by Ramaswamy H. Sarma.

Thermo-responsive expression and differential secretion of the extracellular enzyme levansucrase in the plant pathogenic bacterium Pseudomonas syringae pv. glycinea

In the plant pathogen Pseudomonas syringae, production of the exopolysaccharide levan is mediated by extracellular levansucrase (Lsc), which is encoded by two functional genes, lscB and lscC. Comparison of extracellular protein profiles of P. syringae pv. glycinea PG4180 grown at 18 and 28 degrees C and Western blots revealed that Lsc was predominantly found in the supernatant at 18 degrees C, a temperature fostering virulence of this pathogen. Northern blot analysis indicated that transcription of lscB and lscC was temperature-dependent. Quantification of Lsc in supernatants and cellular protein samples of mutants defective in either lscB or lscC confirmed that LscB secretion at low temperature was due to a combination of thermo-regulated transcription and secretion. In contrast, LscC accumulated in the periplasmic space. LscB and LscC differ in only five amino acid residues, one of which is a cysteine residue. Temperature shift experiments suggested that de novo synthesized protein(s) at 18 degrees C might be responsible for differential LscB secretion and that the presumed secretory machinery was stable when cells were shifted to 28 degrees C. Our results imply that Lsc export and secretion may occur by yet-to-be identified novel mechanism(s).

Identification of plant-associated enterococci

AIMS

Enterococcus isolates from forage grass were subjected to taxonomical investigations and tested for antibiotic resistance.

METHODS AND RESULTS

The identification procedure included phenotypic characterizations, restriction analyses of polymerase chain reaction-amplified 16S rDNA, whole-cell protein profile analyses and 16S rDNA sequence analyses. Agar diffusion tests were performed to detect antibiotic resistance.

CONCLUSION

The isolates were identified as belonging to the species Enterococcus faecium, Ent. mundtii, Ent. casseliflavus, Ent. faecalis and Ent. sulfureus. However, the majority of isolates differed distinctly in their restriction patterns from those of known species. They formed a group of a homogeneous 16S rDNA genotype (VI). The 16S rDNA sequence of a representative isolate revealed the closest relationship to the species Ent. faecalis (similarity of 97.4%). All isolates were sensitive to vancomycin, but almost all were resistant to gentamycin and streptomycin.

SIGNIFICANCE AND IMPACT OF THE STUDY

The taxonomical investigations suggest that the isolates of the 16S rDNA genotype VI represent a new plant-associated Enterococcus species.

Population dynamics and antagonistic potential of enterococci colonizing the phyllosphere of grasses

AIMS

To investigate the spatial and temporal dynamics of enterococci colonizing forage grass and their ability to produce bacteriocins.

METHODS AND RESULTS

Enterococci could be detected on above-ground plant parts throughout the growing season, with high continuity but low cell numbers (2.60 x 101-6.16 x 104 cfu g-1 fresh matter). A total of 750 strains were isolated and identified by their whole-cell protein patterns as Enterococcus faecalis (7.9%), Ent. mundtii (7.9%), Ent. casseliflavus (5.5%), Ent. faecium (5.2%) and Ent. sulfureus (0.1%). The vast majority of the strains (69.7%) formed a homogeneous 16S rDNA genotype that differed from those of known enterococci. A screening for antagonistic activity using an agar spot test revealed that 18.4% of all isolates were potential antagonists. Partially-purified proteins extracted from cell-free culture supernatant fluids of various species were characterized as pH- and heat-stable bacteriocins active against a wide range of lactic acid bacteria, clostridia and Listeria. The producing strains were antagonistically active even on 'phylloplane agar' at temperatures between 4 and 37 degrees C.

CONCLUSION

Enterococci are a common part of the epiphytic microflora of grasses, displaying probably some antagonistic activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide new information on the distribution, species diversity and antagonistic potential of enterococci in the phyllosphere.

Characterization and mutational analysis of three allelic lsc genes encoding levansucrase in Pseudomonas syringae

In the plant pathogen Pseudomonas syringae pv. glycinea PG4180 and other bacterial species, synthesis of the exopolysaccharide levan is catalyzed by the extracellular enzyme levansucrase. The results of Southern blotting and PCR analysis indicated the presence of three levansucrase-encoding genes in strain PG4180: lscA, lscB, and lscC. In this study, lscB and lscC were cloned from a genomic library of strain PG4180. Sequence analysis of the two lsc genes showed that they were virtually identical to each other and highly similar to the previously characterized lscA gene. lscA and lscC had a chromosomal location, whereas lscB resided on an indigenous plasmid of PG4180. Mutants with impaired expression of individual lsc genes and double mutants were generated by marker exchange mutagenesis. Determination of levansucrase activities in these mutants revealed that the lscB gene product was secreted but not that of lscA or lscC. Our results indicated that lscB and lscC but not lscA contributed to periplasmic levan synthesis of PG4180. The lscB lscC double mutant was completely defective in levan formation and could be complemented by either lscB or lscC. Our data suggested a compartment-specific localization of two lsc gene products, with LscB being the secreted, extracellular enzyme and LscC being the predominantly periplasmic levansucrase. Results of Western blot analyses indicated that lscA was not expressed and that lscA was not associated with levansucrase activities in any particular protein fraction. LscA could be detected in PG4180 only when transcribed from the vector-borne P(lac) promoter. PCR screening in various P. syringae strains with primers derived from the three characterized lsc genes demonstrated the presence of multiple Lsc isoenzymes in other P. syringae pathovars.

Biological Control of Pseudomonas syringae pv. glycinea by Epiphytic Bacteria under Field Conditions

The efficacy of a bacterial strain as a biocontrol agent in the field may be related to the ecological similarity between the biocontrol agent and the target pathogen. Therefore, a number of different Pseudomonas syringae strains were evaluated for their antagonistic activities in vitro (agar-diffusion assay) and in planta (greenhouse assay) against the target pathogen, Pseudomonas syringae pv. glycinea. Six strains of five different pathovars were found to be antagonistic in vitro as well as in planta. The epiphytic fitness of the antagonistic Pseudomonas syringae strain 22d/93 and its two antibiotic-resistant mutants were examined on soybean plants in the fields. After adaptation the parental strain and its mutants had the ability to establish and maintain large epiphytic populations (about 106 cfu/g FW) over the whole growing season after a single spray inoculation. The epiphytic behaviors of the mutants and the parent were not significantly different. The introduced bacteria did not influence the total bacterial population size. When the antagonist was coinoculated with the pathogen, the development of the pathogen was significantly reduced during the whole growing season. When the antagonistic strain was inoculated 4 weeks in advance of the pathogen, this antagonistic effect could be markedly enhanced. The final population size of the pathogen reached just 104 cfu/g FW and was significantly reduced to 0.12% compared to the pathogen alone. This study demonstrates that biological control of foliar pathogens through colonization of the host plants with near isogenic or ecologically similar antagonistical strains seems to be a realistic goal.

The Ecology and Biogeography of Microorganisms on Plant Surfaces

The vast surface of the plant axis, stretching from root tips occasionally buried deeply in anoxic sediment, to apical meristems held far aloft, provides an extraordinarily diverse habitat for microorganisms. Each zone has to a greater or lesser extent its own cohort of microorganisms, in aggregate comprising representatives from all three primary domains of life-Bacteria, Archaea, and Eucarya. While the plant sets the stage for its microbial inhabitants, they, in turn, have established varied relationships with their large partner. These associations range from relatively inconsequential (transient epiphytic saprophytes) to substantial (epiphytic commensals, mutualistic symbionts, endophytes, or pathogens). Through recent technological breakthroughs, a much better perspective is beginning to emerge on the nature of these relationships, but still relatively little is known about the role of epiphytic microbial associations in the life of the plant.

Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae-a pathogen, ice nucleus, and epiphyte

The extremely large number of leaves produced by terrestrial and aquatic plants provide habitats for colonization by a diversity of microorganisms. This review focuses on the bacterial component of leaf microbial communities, with emphasis on Pseudomonas syringae-a species that participates in leaf ecosystems as a pathogen, ice nucleus, and epiphyte. Among the diversity of bacteria that colonize leaves, none has received wider attention than P. syringae, as it gained notoriety for being the first recombinant organism (Ice(-) P. syringae) to be deliberately introduced into the environment. We focus on P. syringae to illustrate the attractiveness and somewhat unique opportunities provided by leaf ecosystems for addressing fundamental questions of microbial population dynamics and mechanisms of plant-bacterium interactions. Leaf ecosystems are dynamic and ephemeral. The physical environment surrounding phyllosphere microbes changes continuously with daily cycles in temperature, radiation, relative humidity, wind velocity, and leaf wetness. Slightly longer-term changes occur as weather systems pass. Seasonal climatic changes impose still a longer cycle. The physical and physiological characteristics of leaves change as they expand, mature, and senesce and as host phenology changes. Many of these factors influence the development of populations of P. syringae upon populations of leaves. P. syringae was first studied for its ability to cause disease on plants. However, disease causation is but one aspect of its life strategy. The bacterium can be found in association with healthy leaves, growing and surviving for many generations on the surfaces of leaves as an epiphyte. A number of genes and traits have been identified that contribute to the fitness of P. syringae in the phyllosphere. While still in their infancy, such research efforts demonstrate that the P. syringae-leaf ecosystem is a particularly attractive system with which to bridge the gap between what is known about the molecular biology of genes linked to pathogenicity and the ecology and epidemiology of associated diseases as they occur in natural settings, the field.

Location and survival of leaf-associated bacteria in relation to pathogenicity and potential for growth within the leaf

The growth and survival of pathogenic and nonpathogenic Pseudomonas syringae strains and of the nonpathogenic species Pantoea agglomerans, Stenotrophomonas maltophilia, and Methylobacterium organophilum were compared in the phyllosphere of bean. In general, the plant pathogens survived better than the nonpathogens on leaves under environmental stress. The sizes of the total leaf-associated populations of the pathogenic P. syringae strains were greater than the sizes of the total leaf-associated populations of the nonpathogens under dry conditions but not under moist conditions. In these studies the surface sterilants hydrogen peroxide and UV irradiation were used to differentiate cells that were fully exposed on the surface from nonexposed cells that were in "protected sites" that were inaccessible to these agents. In general, the population sizes in protected sites increased with time after inoculation of plants. The proportion of bacteria on leaves that were in protected sites was generally greater for pathogens than for nonpathogens and was greater under dry conditions than under moist conditions. When organisms were vacuum infiltrated into leaves, the sizes of the nonexposed "internal" populations were greater for pathogenic P. syringae strains than for nonpathogenic P. syringae strains. The sizes of the populations of the nonpathogenic species failed to increase or even decreased. The sizes of nonexposed populations following spray inoculation were correlated with the sizes of nonexposed, internal populations which developed after vacuum infiltration and incubation. While the sizes of the populations of the pathogenic P. syringae strains increased on leaves under dry conditions, the sizes of the populations of the nonpathogenic strains of P. syringae, P. agglomerans, and S. maltophilia decreased when the organisms were applied to plants. The sizes of the populations on dry leaves were also correlated with the sizes of the nonexposed populations that developed following vacuum infiltration. Although pathogenicity was not required for growth in the phyllosphere under high-relative-humidity conditions, pathogenicity apparently was involved in the ability to access and/or multiply in certain protected sites in the phyllosphere and in growth on dry leaves.

Bacteria as modular organisms

The body plan of modular organisms is based on an indeterminate structure composed of iterated units or modules arrayed at various levels of complexity (such as leaves, twigs, and branches). Examples of modular organisms include plants and many sessile benthic invertebrates. In contrast, the body of unitary organisms is a determinate structure consisting usually of a strictly defined number of parts (such as legs or wings) established only during embryogenesis. Mobile animals are examples. Unlike that of unitary creatures, the form of a modular organism derives from a characteristic pattern of branching or budding of modules, which may remain attached or become separated to live physiologically independent lives as parts of a clone. Modular organisms tend to be sessile or passively mobile and, as genetic individuals, have the capacity for exponential increase in size. They do not necessarily undergo systemic senescence, and do not segregate somatic from germ line cells. It is argued here that bacteria are essentially modular organisms where the bacterial cell, microcolony, and macrocolony are modules of different levels of complexity analogous to modules of macroorganisms. This interpretation provides a broad conceptual basis for understanding the natural history of bacteria, and may illuminate the evolutionary origins and developmental biology of modular creatures.

Cloning, nucleotide sequence, and expression in Escherichia coli of levansucrase genes from the plant pathogens Pseudomonas syringae pv. glycinea and P. syringae pv. phaseolicola

Plant-pathogenic bacteria produce various extracellular polysaccharides (EPSs) which may function as virulence factors in diseases caused by these bacteria. The EPS levan is synthesized by the extracellular enzyme levansucrase in Pseudomonas syringae, Erwinia amylovora, and other bacterial species. The lsc genes encoding levansucrase from P. syringae pv. glycinea PG4180 and P. syringae pv. phaseolicola NCPPB 1321 were cloned, and their nucleotide sequences were determined. Heterologous expression of the lsc gene in Escherichia coli was found in four and two genomic library clones of strains PG4180 and NCPPB 1321, respectively. A 3. 0-kb PstI fragment common to all six clones conferred levan synthesis on E. coli when further subcloned. Nucleotide sequence analysis revealed a 1,248-bp open reading frame (ORF) derived from PG4180 and a 1,296-bp ORF derived from NCPPB 1321, which were both designated lsc. Both ORFs showed high homology to the E. amylovora and Zymomonas mobilis lsc genes at the nucleic acid and deduced amino acid sequence levels. Levansucrase was not secreted into the supernatant but was located in the periplasmic fraction of E. coli harboring the lsc gene. Expression of lsc was found to be dependent on the vector-based Plac promoter, indicating that the native promoter of lsc was not functional in E. coli. Insertion of an antibiotic resistance cassette in the lsc gene abolished levan synthesis in E. coli. A PCR screening with primers derived from lsc of P. syringae pv. glycinea PG4180 allowed the detection of this gene in a number of related bacteria.

Xanthomonas campestris pv. campestris requires a functional pigB for epiphytic survival and host infection

When cauliflower plants (Brassica oleraceae) were misted with bacterial suspensions of Xanthomonas campestris pv. campestris (causal agent of black rot of cruciferous plants), two separate populations of the pathogen were associated with the leaves. Initially, bacteria removable by sonication and sensitive to sodium hypochlorite treatment predominated (easily removable epiphytic bacteria, EREB). However, after 2 weeks, bacteria not removable by sonication and insensitive to sodium hypochlorite treatment were dominant. Although the exact location of this second population of the pathogen was not determined, evidence is presented to support its location in protected sites on the leaf surface, pigB of this pathogen is required for production of extracellular polysaccharide (EPS), xanthomonadin pigments, and the diffusible signal molecule, DF (diffusible factor). DF can extracellularly restore EPS and xanthomonadin production to pigB mutant strains. Parent strain B-24 and pigB mutant strain B24-B2 were identical for in planta growth and symptomatology after artificial infection by injection in leaf mid-veins. Subsequently, X. campestris pv. campestris parent strain B-24, Tn3HoHo1 pigB insertion mutation strain B24-B2, chromosomally restored pigB mutation strain B24-B2R, and strain B24-79 with a Tn3HoHo1 insertion in an unrelated part of the genome were compared for epiphytic survival on, and natural infection of, cauliflower. After application, strains B-24, B24-B2R, and B24-79 all maintained leaf EREB populations of between approximately 3 and 6 (log [1 + CFU per g of fresh weight]) over a 3-week period, whereas B24-B2 populations fell to nearly undetectable levels. Plants sprayed with strains B-24, B24-B2R, and B24-79 averaged between 1.0 and 1.2 lesions, whereas those sprayed with B24-B2 averaged only 0.03 lesions per plant after 3 weeks. Differences in EREB population levels did not explain the observed differences in host infection frequencies, and the results indicated that strain B24-B2 was reduced in its ability to infect the host via the hydathodes, but unaffected in infection via wounds. When strains B-24 and B24-B2 were mixed in equal numbers and sprayed on plants together, B24-B2 epiphytic populations were intermediate between those of B-24 applied alone and B24-B2 applied alone. These results indicate that a functional pigB is required for epiphytic survival and natural host infection under the experimental conditions tested, and suggest that DF, xanthomonadins, and EPS could all be important for survival of this pathogen on the leaf surface, and/or for host infection.

Effect of bacterial distribution and activity on conjugal gene transfer on the phylloplane of the bush bean (Phaseolus vulgaris)

Conjugal plasmid transfer was examined on the phylloplane of bean (Phaseolus vulgaris) and related to the spatial distribution pattern and metabolic activity of the bacteria. The donor (Pseudomonas putida KT2442) harbored a derivative of the TOL plasmid, which conferred kanamycin resistance and had the gfp gene inserted downstream of a lac promoter. A chromosomal insertion of lacIq prevented expression of the gfp gene. The recipient (P. putida KT2440) had a chromosomal tetracycline resistance marker. Thus, transconjugants could be enumerated by plating and visualized in situ as green fluorescent cells. Sterile bean seedlings were inoculated with donors and recipients at densities of approximately 10(5) cells per cm2. To manipulate the density and metabolic activity (measured by incorporation of [3H]leucine) of the inoculated bacteria, plants were grown at various relative humidities (RH). At 100% RH, the transconjugants reached a density of 3 x 10(3) CFU/cm2, corresponding to about one-third of the recipient population. At 25% RH, numbers of transconjugants were below the detection limit. Immediately after inoculation onto the leaves, the per-cell metabolic activity of the inocula increased by up to eight times (100% RH), followed by a decrease to the initial level after 96 h. The metabolic activity of the bacteria was not rate limiting for conjugation, and no correlation between the two parameters was observed. Apparently, leaf exudates insured that the activity of the bacteria was above a threshold value for transfer to occur. Transconjugants were primarily observed in junctures between epidermal cells and in substomatal cavities. The distribution of the transconjugants was similar to the distribution of indigenous bacteria on nonsterile leaves. Compared to polycarbonate filters, with cell densities equal to the overall density on the leaves, transfer ratios on leaves were up to 30 times higher. Thus, aggregation of the bacteria into microhabitats on the phylloplane had a great stimulatory effect on transfer.

Characterization of the alginate biosynthetic gene cluster in Pseudomonas syringae pv. syringae

Alginate, a copolymer of D-mannuronic acid and L-guluronic acid, is produced by a variety of pseudomonads, including Pseudomonas syringae. Alginate biosynthesis has been most extensively studied in P. aeruginosa, and a number of structural and regulatory genes from this species have been cloned and characterized. In the present study, an alginate-defective (Alg-) mutant of P. syringae pv. syringae FF5 was shown to contain a Tn5 insertion in algL, a gene encoding alginate lyase. A cosmid clone designated pSK2 restored alginate production to the algL mutant and was shown to contain homologs of algD, alg8, alg44, algG, algX (alg60), algL, algF, and algA. The order and arrangement of the structural gene cluster were virtually identical to those previously described for P. aeruginosa. Complementation analyses, however, indicated that the structural gene clusters in P. aeruginosa and P. syringae were not functionally interchangeable when expressed from their native promoters. A region upstream of the algD gene in P. syringae pv. syringae was shown to activate the transcription of a promoterless glucuronidase (uidA) gene and indicated that transcription initiated upstream of algD as described for P. aeruginosa. Transcription of the algD promoter from P. syringae FF5 was significantly higher at 32 degrees C than at 18 or 26 degrees C and was stimulated when copper sulfate or sodium chloride was added to the medium. Alginate gene expression was also stimulated by the addition of the nonionic solute sorbitol, indicating that osmolarity is a signal for algD expression in P. syringae FF5.

Microbial population dynamics on leaves

Microbial population dynamics on leaves in time and space are a function of immigration, emigration, growth, and death. Insight into the relative significance of each population process to the generation of specific dynamics for individual microorganisms is necessary to understanding the ecology and life history strategy of the microorganism and to developing effective control strategies. Additionally, information on the significance of within-leaf versus extra-leaf processes to the generation of phyllosphere dynamics is important to determining the range of spatial scales over which a population should be studied. Unfortunately, such information is difficult to obtain due to the lack of effective methodologies for distinguishing these processes within phyllosphere populations. Future research efforts should focus on the quantification of immigration, emigration, growth, and death relative to the population dynamics of phyllosphere microorganisms.

Characteristics of Insertional Mutants of Pseudomonas syringae with Reduced Epiphytic Fitness

Random Tn 5 mutagenesis was used to identify genes ir. Pseudomonas syringae which contribute to epiphytic fitness. Mutants were selected on the basis of deficiencies in epiphytic growth or survival on plants rather than deficiencies in predetermined phenotypes exhibited in culture. A sample freezing procedure was used to measure the population sizes of 5,300 mutants of P. syringae exposed to alternating wet and dry conditions on bean leaves in growth chambers. Eighty-two mutants exhibited reduced population sizes. Of these mutants, over half exhibited a reduced ability to survive the stresses associated with dry leaves, while others grew more slowly or attained reduced stationary-phase population sizes on leaves. While some epiphytic fitness mutants were altered in phenotypes that could be measured in culture, many mutants were not altered in any in vitro phenotype examined. Only three of the epiphytic fitness mutants were auxotrophs, and none had catabolic deficiencies for any of 31 organic compounds tested. Other mutants that exhibited reductions in one or more of the following were identified: motility, osmotolerance, desiccation tolerance, growth rate in batch culture, and extracellular polysaccharide production. All of the mutants retained the abilities to produce disease symptoms on the compatible host plant, bean, to incite a hypersensitive response on the non-host plant, tobacco, and to produce a fluorescent pyoverdine siderophore.

Effect of phenotypic plasticity on epiphytic survival and colonization by Pseudomonas syringae

The bacterial epiphyte Pseudomonas syringae MF714R was cultured on agar or in broth or collected from colonized leaves; it was then inoculated onto greenhouse-grown bean plants incubated in a growth chamber at low relative humidity or in the field or onto field-grown bean plants. Cells cultured in liquid medium survived the least well after inoculation of leaf surfaces under all conditions. Cells cultured in solid medium exhibited the highest percent survival and desiccation tolerance in the growth chamber but generally survived less well in the field than did cells harvested from plants. Cells harvested from plants and inoculated onto plants in the field usually exhibited the highest percent survival, started to increase in population earlier, and reached a higher number than did cells cultured in vitro. Differences in field survival were apparently not attributable to differential UV tolerance. The observed effects of phenotypic plasticity on epiphytic survival and colonization should be considered in risk assessment studies, in studies of bacterial epidemiology, and in the use of microbial antagonists for biological pest control.