Biological and molecular characterization of a bacteriophage infecting Xanthomonas campestris pv. campestris, isolated from brassica fields.

Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot of crucifers. Here, we report a virus that infects Xcc isolated from brassica fields in Brazil. Morphological, molecular and phylogenetic analysis indicated that the isolated virus is a new member of the genus Pbunavirus, family Myoviridae, and we propose the name "Xanthomonas virus XC 2" for this virus. The isolated virus has a narrow host range, infecting only Xcc isolates, and it did not infect unrelated bacteria. These results indicate that the isolated bacteriophage is highly specific for Xcc and may be a potential agent for biological control.

Pan Proteome of Xanthomonas campestris pv. campestris Isolates Contrasting in Virulence.

Fully sequenced genomes of Xanthomonas campestris pv. campestris (Xcc) strains are reported. However, intra-pathovar differences are still intriguing and far from clear. In this work, the contrasting virulence between two isolates of Xcc - Xcc51 (more virulent) and XccY21 (less virulent) is evaluated by determining their pan proteome profiles. The bacteria are grown in NYG and XVM1 (optimal for induction of hrp regulon) broths and collected at the max-exponential growth phase. Shotgun proteomics reveals a total of 329 proteins when Xcc isolates are grown in XVM1. A comparison of both profiles reveals 47 proteins with significant abundance fluctuations, out of which, 39 show an increased abundance in Xcc51 and are mainly involved in virulence/adaptation mechanisms, genetic information processing, and membrane receptor/iron transport systems, such as BfeA, BtuB, Cap, Clp, Dcp, FyuA, GroEs, HpaG, Tig, and OmpP6. Several differential proteins are further analyzed by qRT-PCR, which reveals a similar expression pattern to the protein abundance. The data shed light on the complex Xcc pathogenicity mechanisms and point out a set of proteins related to the higher virulence of Xcc51. This information is essential for the development of more efficient strategies aiming at the control of black rot disease.

Rapid Colorimetric Detection of Bacterial Species through the Capture of Gold Nanoparticles by Chimeric Phages.

Rapid, inexpensive, and sensitive detection of bacterial pathogens is an important goal for several aspects of human health and safety. We present a simple strategy for detecting a variety of bacterial species based on the interaction between bacterial cells and the viruses that infect them (phages). We engineer phage M13 to display the receptor-binding protein from a phage that naturally targets the desired bacteria. Thiolation of the engineered phages allows the binding of gold nanoparticles, which aggregate on the phages and act as a signal amplifier, resulting in a visible color change due to alteration of surface plasmon resonance properties. We demonstrate the detection of two strains of Escherichia coli, the human pathogens Pseudomonas aeruginosa and Vibrio cholerae, and two strains of the plant pathogen Xanthomonas campestris. The assay can detect ∼100 cells with no cross-reactivity found among the Gram-negative bacterial species tested here. The assay can be performed in less than an hour and is robust to different media, including seawater and human serum. This strategy combines highly evolved biological materials with the optical properties of gold nanoparticles to achieve the simple, sensitive, and specific detection of bacterial species.

Multilocus Sequence Typing of Strains of Bacterial Spot of Lettuce Collected in the United States.

Studies on genetic diversity and recombination in bacterial pathogens are providing a better understanding of the mechanisms shaping bacterial diversity, which can affect disease control. Xanthomonas campestris pv. vitians, causal agent of bacterial leaf spot of lettuce, is a threat to the worldwide lettuce industry. We examined the genetic variation within a sample of 83 strains from California, Florida, and Ohio using multilocus sequence typing of six housekeeping genes, totaling 2.7 kb. Additionally, polymorphism in two virulence-related genes, hrpB2 and a putative glycosyl hydrolase, were examined. Based on housekeeping genes, we found three genetic groups of strains that were all able to induce the disease. These included strains collected from weeds and irrigation water that had haplotypes identical to strains from diseased lettuce. High linkage disequilibrium across the sequenced loci indicates that the pathogen is predominantly clonal but recombination has contributed to the observed sequence variation. Although there was significant genetic variation in X. campestris pv. vitians within and among sampled states, identical haplotypes were observed across all three states. This finding suggests that seedborne inoculum may contribute to the diversity of X. campestris pv. vitians in the United States. Knowledge of the genetic structure of the pathogen may be used for developing resistant lettuce varieties.

Decontamination by Persteril 36 may affect the reliability of DNA-based detection of biological warfare agents-short communication.

Persteril 36 is a disinfectant with a broad spectrum of antimicrobial activity. Because of its bactericidal, virucidal, fungicidal, and sporicidal effectiveness, it is used as a disinfectant against biological warfare agents in the emergency and army services. In case of an attack with potentially harmful biological agents, a person's gear or afflicted skin is sprayed with a diluted solution of Persteril 36 as a precaution. Subsequently, the remains of the biological agents are analyzed. However, the question remains concerning whether DNA can be successfully analyzed from Persteril 36-treated dead bacterial cells. Spore-forming Bacillus subtilis and Gram-negative Pseudomonas aeruginosa and Xanthomonas campestris were splattered on a camouflage suit and treated with 2 or 0.2 % Persteril 36. After the disinfectant vaporized, the bacterial DNA was extracted and quantified by real-time PCR. A sufficient amount of DNA was recovered for downstream analysis only in the case of spore-forming B. subtilis treated with a 0.2 % solution of Persteril 36. The bacterial DNA was almost completely destroyed in Gram-negative bacteria or after treatment with the more concentrated solution in B. subtilis. This phenomenon can lead to false-negative results during the identification of harmful microorganisms.

Rapid, specific, simple, in-field detection of Xanthomonas campestris pathovar musacearum by loop-mediated isothermal amplification.

AIMS: To develop and evaluate a loop-mediated isothermal amplification (LAMP) assay for Xanthomonas campestris pathovar musacearum (Xcm), the causal agent of banana Xanthomonas wilt, a major disease of banana in Africa. METHODS AND RESULTS: LAMP primers were designed to the general secretion pathway protein D gene and tested against 17 isolates of Xcm encompassing the known genetic and geographic diversity of the bacterium and all isolates were detected. Seventeen other Xanthomonas isolates, including closely related Xanthomonas vasicola, other bacterial pathogens/endophytes of Musa and two healthy Musa varieties gave negative results with the LAMP assay. The assay showed good sensitivity, detecting as little as 51 fg of Xcm DNA, a greater level of sensitivity than that of an Xcm PCR assay. Amplification with the LAMP assay was very rapid, typically within 9 min from bacterial cultures. Symptomatic field samples of Musa from Uganda were tested and all produced amplification in less than 13 min. CONCLUSIONS: The LAMP assay provides rapid, sensitive detection of the pathogen that is ideally suited for deployment in laboratories with basic facilities and in-field situations. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first LAMP assay for Xcm which provides a significant improvement compared to existing diagnostics.

[Identification of a new type III effector XC3176 in Xanthomonas campestris pv. campestris].

OBJECTIVE: Type III secretion system (T3SS) is essential for many phytopathogenic bacteria to cause disease in susceptible host plants and to elicit a hypersensitive response in resistant host and non-host plants. Xanthomonas campestris pv. campestris (Xcc) uses T3SS to deliver T3SS effectors (T3SEs) directly into host cells, where they play important roles in pathogenesis. The aim of this study was to identify a new T3SE in Xcc. METHODS: To validate if XC3176 is a T3S effector translocated into plant cells, the promoter and signal region of XC3176 were fused to the plasmid pLJB of harboring HR-inducing AvrBs1 C-terminal domain lack of 58 N-terminal amino acid residues. The recombinant plasmid pLJB3176 was introduced by triparental conjugation into ΔavrBs1 and ΔhrcV. Hypersensitive response induced by the obtained strains ΔavrBs1/pLJB3176 and ΔhrcV/pLJB3176 were examined on the pepper ECW-10R. To determine transcription of XC3176, GUS fusion report strains were constructed. The virulence of Xcc strains was investigated on the Chinese radish by the leaf-clipping method. RESULTS: Hypersensitive response was elicited on the pepper ECW-10R by the strain ΔavrBs1/pLJB3176, but not ΔhrcV/pLJB3176. The GUS activities in the mutant strains ΔhrpX and ΔhrpG were significantly lower than that in the wild type Xcc strain. The mutant of XC3176 reduced virulence significantly and the complementary strain C3176 could restore the virulence as the wild-type strain. CONCLUSION: XC3176 is a T3SS-dependent effector of Xanthomonas campestris pv. campestris. The expression of XC3176 is regulated by hrpG and hrpX. XC3176 is required for the full virulence of Xcc 8004.

[Identification of causative agent of Brassica rapa bacterial diseases based on fatty acid composition of cellular lipids].

The fatty acid composition of cell lipids of 15 strains isolated from the affected plants of rape and five collection strains has been studied. According to the results of chemotaxonomic analysis it has been found that 9 isolated strains are similar to representatives of species Pseudomonas marginalis and Pseudomonas fluorescens, and 6 - to those of Xanthomonas campestris pv. campestris. The authors have established the efficiency of certain methods for the extraction of fatty acids used for the identification of bacteria pathogenic for rape which belong to the genera Pseudomonas and Xanthomonas.

Natural genetic variation of Xanthomonas campestris pv. campestris pathogenicity on arabidopsis revealed by association and reverse genetics.

ABSTRACT The pathogenic bacterium Xanthomonas campestris pv. campestris, the causal agent of black rot of Brassicaceae, manipulates the physiology and the innate immunity of its hosts. Association genetic and reverse-genetic analyses of a world panel of 45 X. campestris pv. campestris strains were used to gain understanding of the genetic basis of the bacterium's pathogenicity to Arabidopsis thaliana. We found that the compositions of the minimal predicted type III secretome varied extensively, with 18 to 28 proteins per strain. There were clear differences in aggressiveness of those X. campestris pv. campestris strains on two Arabidopsis natural accessions. We identified 3 effector genes (xopAC, xopJ5, and xopAL2) and 67 amplified fragment length polymorphism (AFLP) markers that were associated with variations in disease symptoms. The nature and distribution of the AFLP markers remain to be determined, but we observed a low linkage disequilibrium level between predicted effectors and other significant markers, suggesting that additional genetic factors make a meaningful contribution to pathogenicity. Mutagenesis of type III effectors in X. campestris pv. campestris confirmed that xopAC functions as both a virulence and an avirulence gene in Arabidopsis and that xopAM functions as a second avirulence gene on plants of the Col-0 ecotype. However, we did not detect the effect of any other effector in the X. campestris pv. campestris 8004 strain, likely due to other genetic background effects. These results highlight the complex genetic basis of pathogenicity at the pathovar level and encourage us to challenge the agronomical relevance of some virulence determinants identified solely in model strains. IMPORTANCE The identification and understanding of the genetic determinants of bacterial virulence are essential to be able to design efficient protection strategies for infected plants. The recent availability of genomic resources for a limited number of pathogen isolates and host genotypes has strongly biased our research toward genotype-specific approaches. Indeed, these do not consider the natural variation in both pathogens and hosts, so their applied relevance should be challenged. In our study, we exploited the genetic diversity of Xanthomonas campestris pv. campestris, the causal agent of black rot on Brassicaceae (e.g., cabbage), to mine for pathogenicity determinants. This work evidenced the contribution of known and unknown loci to pathogenicity relevant at the pathovar level and identified these virulence determinants as prime targets for breeding resistance to X. campestris pv. campestris in Brassicaceae.

Xanthomonas campestris pv. campestris (cause of black rot of crucifers) in the genomic era is still a worldwide threat to brassica crops.

BACKGROUND: Xanthomonas campestris pv. campestris (Xcc) (Pammel) Dowson is a Gram-negative bacterium that causes black rot, the most important disease of vegetable brassica crops worldwide. Intensive molecular investigation of Xcc is gaining momentum and several whole genome sequences are available. TAXONOMY: Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadacea; Genus Xanthomonas; Species X. campestris. HOST RANGE AND SYMPTOMS: Xcc can cause disease in a large number of species of Brassicaceae (ex-Cruciferae), including economically important vegetable Brassica crops and a number of other cruciferous crops, ornamentals and weeds, including the model plant Arabidopsis thaliana. Black rot is a systemic vascular disease. Typical disease symptoms include V-shaped yellow lesions starting from the leaf margins and blackening of the veins. RACE STRUCTURE, PATHOGENESIS AND EPIDEMIOLOGY: Collections of Xcc isolates have been differentiated into physiological races based on the response of several brassica species lines. Black rot is a seed-borne disease. The disease is favoured by warm, humid conditions and can spread rapidly from rain dispersal and irrigation water. DISEASE CONTROL: The control of black rot is difficult and relies on the use of pathogen-free planting material and the elimination of other potential inoculum sources (infected crop debris and cruciferous weeds). Major gene resistance is very rare in B. oleracea (brassica C genome). Resistance is more readily available in other species, including potentially useful sources of broad-spectrum resistance in B. rapa and B. carinata (A and BC genomes, respectively) and in the wild relative A. thaliana. GENOME: The reference genomes of three isolates have been released. The genome consists of a single chromosome of approximately 5 100 000 bp, with a GC content of approximately 65% and an average predicted number of coding DNA sequences (CDS) of 4308. IMPORTANT GENES IDENTIFIED: Three different secretion systems have been identified and studied in Xcc. The gene clusters xps and xcs encode a type II secretion system and xps genes have been linked to pathogenicity. The role of the type IV secretion system in pathogenicity is still uncertain. The hrp gene cluster encodes a type III secretion system that is associated with pathogenicity. An inventory of candidate effector genes has been assembled based on homology with known effectors. A range of other genes have been associated with virulence and pathogenicity, including the rpf, gum and wxc genes involved in the regulation of the synthesis of extracellular degrading enzymes, xanthan gum and lipopolysaccharides. USEFUL WEBSITE: http://www.xanthomonas.org/

Sensitive and specific detection of Xanthomonas campestris pv. vesicatoria by PCR using pathovar-specific primers based on rhs family gene sequences.

The present study describes PCR assay to detect bacterial spot caused by Xanthomonas campestris pv. vesicatoria in pepper and tomato. One set of PCR primer was developed to amplify gene required for an rhs family gene homologous to rhsA, cell envelope biogenesis, outer membrane. Only a PCR product of a 517bp was produced in PCR reaction with the Xanthomonas campestris pv. vesicatoria (XCVF/XCVR) primer set. A specific, and highly sensitive and rapid PCR assay for the detection of X. campestris pv. vesicatoria was achieved. The protocol can be used as a reliable diagnostic tool for specific detection of X. campestris pv. vesicatoria in pepper or tomato.

A novel pepper membrane-located receptor-like protein gene CaMRP1 is required for disease susceptibility, methyl jasmonate insensitivity and salt tolerance.

Plant receptor proteins are involved in the signaling networks required for defense against pathogens. The novel pepper pathogen-induced gene CaMRP1 was isolated from pepper leaves infected with Xanthomonas campestris pv. vesicatoria (Xcv). This gene is predicted to encode a membrane-located receptor-like protein that has an N-terminal signal peptide and a C-terminal transmembrane helix. A CaMRP1-GFP fusion protein localized primarily to the plasma membrane of plant cells. Strong and early induction of CaMRP1 expression occurred following exposure of pepper plants to Xcv, Colletotricum coccodes, methyl jasmonate (MeJA) and wounding stress. Virus-induced gene silencing (VIGS) of CaMRP1 in pepper conferred enhanced basal resistance to Xcv infection, accompanied by induction of genes encoding basic PR1 (CaBPR1), defensin (CaDEF1) and SAR8.2 (CaSAR82A). In contrast, CaMRP1 overexpression (OX) in transgenic Arabidopsis plants resulted in increased disease susceptibility to Hyaloperonospora parasitica infection. Arabidopsis plants overexpressing CaMRP1 exhibited insensitivity to MeJA by causing reduced expression of MeJA-responsive genes. Overexpression also resulted in tolerance to NaCl and during salt stress, the expression of several abscisic acid-responsive genes was induced. Together, these results suggest that pepper CaMRP1 may belong to a new subfamily of membrane-located receptor-like proteins that regulate disease susceptibility, MeJA-insensitivity and salt tolerance.

Investigation of the chemical structure and biological activity of oligosaccharides isolated from rough-type Xanthomonas campestris pv. campestris B100 lipopolysaccharide.

The rough-type lipopolysaccharide (LPS) of the phytopathogenic bacterium Xanthomonas campestris pv. campestris B 100 was isolated utilizing the hot phenol-water method and successively de-acylated by treatment with hydrazine and hot potassium hydroxide. Four compounds were separated by preparative high-performance anion-exchange chromatography and studied by sugar analysis and by 1D and 2D homonuclear and heteronuclear (1)H-, (13)C- and (31)P-NMR spectroscopy as well as ESI FT-MS. The two main products were a heptasaccharide and a pentasaccharide of the structures alpha-D-Manp-(1-->3)-alpha-D-Man p-(1-->4)-beta-D-Glcp-(1-->4)-alpha-D-Manp-3P -(1-->5)-alpha-Kdo-(2-->6)-beta-D-GlcpN-4P-(1-->6)-alpha-D-Glc pN-1P (1) and beta-D-Glcp-(1-->4)-alpha-D-Man p-3P-(1-->5)-alpha-Kdo-(2-->6)-beta-D-GlcpN-4 P-(1-->6)-alpha-D-GlcpN-1P (2), respectively. The products in smaller amounts were a heptasaccharide and pentasaccharide possessing the above structures plus a phosphate group at C-4 of the Kdo residue (compounds 3 and 4). Both, heptasaccharide 1 and pentasaccharide 2 were able to induce an oxidative burst in cell cultures of the non-host plant tobacco.

Isolation and characterization of a xanthan-degrading Microbacterium sp. strain XT11 from garden soil.

AIMS: Isolation and characterization of the xanthan-degrading Microbacterium sp. XT11. METHODS AND RESULTS: The bacterial isolate XT11, capable of fragmenting xanthan, has been isolated from soil sample. Morphological and biochemical analyses, as well as 16S rRNA gene sequence comparisons, demonstrated that strain XT11 should be grouped in the genus Microbacterium, and represented a new member in this family. Xanthan could be degraded by the xanthan-degrading enzyme released from strain XT11. It has been shown that xantho-oligosaccharides fragmented from xanthan had both elicitor activity and antibacterial effect against Xanthomonas campestris pv. campestris. CONCLUSIONS: The xanthan-degrading enzyme produced by the newly isolated XT11 could fragment xanthan to form oligosaccharides. SIGNIFICANCE AND IMPACT OF THE STUDY: Xanthan-degrading products would be useful for potential application in the control of black rot of cruciferous plants caused by X. campestris pv. campestris and, as an oligosaccharide elicitor, in making these plants resistant to disease.

A multiplex real-time PCR assay for detection of Xanthomonas campestris from brassicas.

AIMS: To develop a sensitive real-time PCR-based protocol for the detection of Xanthomonas campestris pathovars from Brassica seed. METHODS AND RESULTS: A 5' nuclease real-time PCR assay was developed to screen Brassica spp. seed for the presence of X. campestris pathovars that cause black rot. The assay amplifies a 78-bp segment of the X. campestris hrpF gene and a 100-bp segment of the Brassica spp. 18S-25S internal transcribed spacer region. The Brassica spp. target provides an internal control for the amplification process to prevent false negatives that may arise from inhibitors that are often present in extracts from plant material. Whilst the primers were compatible with SYBR Green I assays, the use of fluorescently labelled probes in a 5' nuclease assay afforded greatest sensitivity and specificity. Seed batches carrying one artificially infected seed among 10,000 were readily detected using the assay. The multiplex real-time PCR assay permitted the rapid detection of pathogenic strains of X. campestris from bacterial colonies, Brassica seed and plants. CONCLUSIONS: Strains of X. campestris pathogenic to brassicas were readily detected from seed via a multiplex 5' nuclease real-time PCR assay. The real-time assay offers an improvement in sensitivity and a reduced turn-around time over the conventional multiplex PCR. SIGNIFICANCE AND IMPACT OF THE STUDY: Real-time PCR can be used to rapidly screen Brassica spp. seed batches for the presence of X. campestris pathovars. This assay provides a means for growers and the seed industry to be aware of the black rot status of their planting material, so that they may more effectively employ disease control measures or seed disinfection.

New media for the semiselective isolation and enumeration of Xanthomonas campestris pv. mangiferaeindicae, the causal agent of mango bacterial black spot.

AIMS: Mango bacterial black spot, caused by Xanthomonas campestris pv. mangiferaeindicae, is a potentially severe disease in several tropical and subtropical areas. Data describing the life cycle of the pathogen are needed for improving integrated pest management strategies. Because of the important bacterial microflora associated with mango leaves, isolation of the pathogen is often difficult using nonselective agar media. METHODS AND RESULTS: A previously developed medium, BVGA, failed to inhibit several Gram-negative saprophytic bacteria, especially those belonging to Enterobacteriaceae. Two new semiselective media were developed. The selectivity of KC and NCTM3 media was achieved using cephalexin 40 mg l(-1), kasugamycin 20 mg l(-1) and neomycin 1 mg l(-1), cephalexin 100 mg l(-1), trimethoprime 5 mg l(-1), pivmecillinam 100 mg l(-1) respectively. Plating efficiencies ranged from 76 to 104% and from 78 to 132% for KC and NCTM3 respectively. CONCLUSIONS: The new media allowed the growth of X. campestris pv. mangiferaeindicae whatever its country of isolation. The pathogen was repeatedly isolated with these media from asymptomatic leaves sampled in growth chamber experiments. SIGNIFICANCE AND IMPACT OF THE STUDY: This work provides a description of new semiselective media, which should be valuable tools to study the ecology and epidemiology of X. campestris pv. mangiferaeindicae.

Sensitive and specific detection of Xanthomonas campestris pv campestris by PCR using species-specific primers based on hrpF gene sequences.

A sensitive and specific assay was developed to detect bacterial black rot of crucifers caused by Xanthomonas campestris pv. campestris (X. c. pv. campestris), in cabbage seed and plant. Primers XCF and XCR from hrpF homologous to nolX, host recognition protein, were used to amplify a 525 bp DNA fragment. PCR technique was applied to detect the pathogen in naturally infected seed and plant of cabbage. The PCR product was only produced from X. c. pv. campestris among 40 isolates of Xanthomonas strains, Escherichia coli (O157:H7), Pectobacterium carotovorum subsp. carotovorum, and other reference bacteria.

Isolation of a Xanthomonas campestris strain with elevated beta-galactosidase activity for direct use of lactose in xanthan gum production.

AIMS: To isolate a Xanthomonas campestris strain that can use lactose directly for xanthan gum production. METHODS AND RESULTS: The presence of indigenous beta-galactosidase gene in the wild-type Xc17 was detected by PCR and Southern hybridization. Treatment of Xc17 with nitrous acid resulted in the isolation of Xc17L with a 3.5-fold elevation of beta-galactosidase activity capable of growing in lactose-based medium. Xc17L is stable for at least 100 generations in terms of beta-galactosidase expression. The amounts of xanthan produced by Xc17L in lactose-based medium are comparable to those in glucose-based medium. CONCLUSIONS: Xc17L is potentially useful for xanthan production from whey, a waste containing lactose. SIGNIFICANCE AND IMPACT OF THE STUDY: A lactose-utilizing strain of X. campestris strain can be constructed without incorporation of any exotic DNA or antibiotic resistance gene and therefore concern of a gene-modified organism and fear of a spread of an antibiotic-resistant gene are avoided.

Detection of the plant pathogenic bacterium Xanthomonas campestris pv. Campestris in seed extracts of Brassica sp. Applying fluorescent antibodies and flow cytometry.

BACKGROUND: Xanthomonas campestris pv. campestris (Xcc) is a seed-transmitted plant pathogenic bacterium that causes black rot of crucifers. Seed lots and plants are screened for contamination with this pathogen using plating or serological assays. These methods, however, are time consuming and not very sensitive, respectively. Therefore, flow cytometry (FCM) was evaluated as a tool for the rapid detection and quantification of Xcc cells labeled with a mixture of specific fluorescein isothicyanate (FITC)-monoclonal antibodies (mAb) in pure culture, in mixed cultures of Xcc with either the common saprophyte Pseudomonas fluorescens (Psf) or a nonpathogenic X. campestris isolate (Xc), and in crude seed extracts. METHODS: The mAb 18G12, conjugated with FITC, was tested at dilutions of 1:50, 1:100, 1:200, and 1:400. For mixed suspensions of Xcc and Psf, mAb 18G12 was used at a dilution of 1:100. The combination of mAbs 18G12, 2F4, and 20H6, all conjugated with FITC, was used at a dilution of 1:100 for the detection and quantification of Xcc cells in mixed suspensions containing Xcc and Xc and in crude seed extracts. The analyses were performed with a Coulter EPICS XL-MCL flow cytometer, at low flow rate during 2 min. RESULTS: Using FCM, Xcc cells labeled with FITC-conjugated mAbs (18G12, 2F4, and 20H6) were detected and quantified rapidly at low numbers, i.e., 10(3) colony-forming units per milliliter in pure and in mixed cultures with Psf. The presence of the nonpathogenic Xc in the seed extracts did not interfere with the FCM results. Xcc cells were distinguished from the cells of other organisms and from small particles present in the seed extract based on the high-intensity fluorescence of the labeled cells. CONCLUSION: The application of FCM in combination with FITC-conjugated mAbs appears to be a promising technique for the detection and quantification of Xcc cells in seed extracts of crucifers.