Xanthomonas

The genus Xanthomonas has been subject of numerous taxonomic and phylogenetic studies and was first described as Bacterium vesicatorium as a pathogen of pepper and tomato in 1921. Dowson later reclassified the bacterium as Xanthomonas campestris and proposed the genus Xanthomonas.Xanthomonas was first described as a monotypic genus and further research resulted in the division into two groups, A and B. Later work using DNA:DNA hybridization has served as a framework for the general Xanthomonas species classification. Other tools, including multilocus sequence analysis and amplified fragment-length polymorphism, have been used for classification within clades. While previous research has illustrated the complexity of the genus Xanthomonas, recent research appears to have resulted in a clearer picture. More recently, genome-wide analysis of multiple Xanthomonas strains mostly supports the previous phylogenies. Plant-pathogenic Xanthomonas spp. are evolutionary linked to opportunistic human pathogen Stenotrophomonas maltophilia, that was previously called Xanthomonas maltophilia. There is a proposal to reorganize Xanthomonas banana and maize/corn pathotypes along the lines of the most recent phylogenetic data. Although GTDB acknowledges the paraphyly of Xanthomonas and also uses a divergence-based genus assignment method, it does not endorse a "lumper" view. It instead splits Xanthomonas into three genera (defined around X. campestris, X. albilineans, and "X. massiliensis" respectively) without merging. Reclassification hotspots Xanthomonas axonopodis is at the center of a species complex that includes X. citri, X. euvesicatoria (including X. perforans and X. alfalfae), and X. phaseoli. Each of these species have been relatively recently split out and numerous strains or pathovars may still be incorrectly associated with X. axonopodis. Pathovars that should be X. citri have been mislabeled X. axonopodis, X. campestris, and X. cissicola. Pathovars that should be X. euvesicatoria or X. varsicola have been mislabeled X. campestris. == Morphology and growth ==

Individual cell characteristics include: • Cell type – straight rods • Size – 0.4 – 1.0 μm wide by 1.2 – 3.0 μm long • Motility – motile by a single polar flagellum Colony growth characteristics include: • Mucoid, convex, and yellow colonies on YDC medium • Yellow pigment from xanthomonadin, which contains bromine • Most produce large amounts of extracellular polysaccharide • Temperature range – 4 to 37 °C, optimal growth 25–30 °C Biochemical and physiological test results are: • Gram stain – negative • Obligate aerobes • Catalase positive • Oxidase negative == Xanthomonas plant pathogens ==

s, a species designation based on host specificity. • Citrus canker, caused by Xanthomonas citri subsp. citri is an economically important disease of many citrus species (lime, orange, lemon, pamelo, etc.) • Xanthomonas campestris pv. punicae cause bacterial blight of pomogranate. • Bacterial blight of rice, caused by Xanthomonas oryzae pv. oryzae, is a disease found worldwide and particularly destructive in the rice-producing regions in Asia. == Plant pathogenesis and disease control ==

File:Xanthomonas-cycle.jpg|thumb|Xanthomonas spp. life cycle Xanthomonas actively kill other bacterial using type IV secretion system and defend itself from amoeba using type VI secretion system. It is important to consult chemical pesticide labels when attempting to control bacterial diseases, as different Xanthomonas species can have different responses to these applications. Over-reliance on chemical control methods can also result in the selection of resistant isolates, so these applications should be considered a last resort. Potential use of bacteriophages is also considered, however major limiting factors are their sensitivity to environmental conditions and in particular to UV radiation. Plant beneficial microorganisms or attenuated strains of Xanthomonas are being tested as a biocontrol reasoning that they could compete by occupying the same niche and even eradicate pathogenic strain. Generation of plant species resistant to Xanthomonas is another potential strategy. ==Industrial use==

Xanthomonas species produce an edible polysaccharide called xanthan gum that has a wide range of industrial uses, including foods, petroleum products, and cosmetics. Xanthan also plays role in the disease cycle of Xanthomonas. In particular, xanthan gum is one of the main components of biofilm matrix. Biofilms help these bacteria sustain abiotic stresses on the leaf surface. Genes for Xanthan gum biosynthesis comprise the gum operon (gumB-gymM) coding for 12 enzymes. Xanthan production by Xanthomonas spp. that thrive in vascular plant systems might block the water flow of the plant and as a result cause wilting. == Xanthomonas resources ==

Isolates of most species of Xanthomonas are available from the National Collection of Plant Pathogenic Bacteria in the United Kingdom and other international culture collections such as ICMP in New Zealand, CFBP in France, and VKM in Russia. It also can be taken out from MTCC India. Multiple genomes of Xanthomonas have been sequenced and additional data sets/tools are available at The Xanthomonas Resource and at PhytoBacExplorer. ==See also==