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''Xanthomonas'' (from greek: ''xanthos'' – “yellow”; ''monas'' – “entity”) is a genus of
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
, many of which cause
plant diseases Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomy ...
. There are at least 27 plant associated ''Xanthomonas spp.'', that all together infect at least 400 plant species. Different species typically have specific host and/or tissue range and colonization strategies.


Taxonomy

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 In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...
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. ''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.


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 In microbiology and bacteriology, Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish b ...
– negative *Obligate aerobes *Catalase positive *Oxidase negative


''Xanthomonas'' plant pathogens

''Xanthomonas'' species can cause bacterial spots and blights of leaves, stems, and fruits on a wide variety of plant species. Pathogenic species show high degrees of specificity and some are split into multiple
pathovar A pathovar is a bacterial strain or set of strains with the same or similar characteristics, that is differentiated at infrasubspecific level from other strains of the same species or subspecies on the basis of distinctive pathogenicity to one o ...
s, a species designation based on host specificity.
Citrus canker Citrus canker is a disease affecting ''Citrus'' species caused by the bacterium '' Xanthomonas'' (''X.axonopodis; X. campestris''). Infection causes lesions on the leaves, stems, and fruit of citrus trees, including lime, oranges, and grapefrui ...
, caused by ''Xanthomonas citri'' subsp. ''citri'' is an economically important disease of many citrus species (lime, orange, lemon, pamelo, etc.) Bacterial leaf spot has caused significant crop losses over the years. Causes of this disease include ''Xanthomonas euvesicatoria'' and ''Xanthomonas perforans'' = 'Xanthomonas axonopodis'' (syn. ''campestris'') pv. ''vesicatoria'' ''Xanthomonas vesicatoria'', and ''Xanthomonas gardneri''. In some areas where infection begins soon after transplanting, the total crop can be lost as a result of this disease. ''Xanthomonas campestris'' pv. ''punicae'' cause bacterial blight of pomogranate. Bacterial blight of rice, caused by ''
Xanthomonas oryzae ''Xanthomonas oryzae'' is a species of bacteria. The major host of the bacterium is rice. The species contains two pathovars, neither of which is native to Europe: ''X. o.'' pv. ''oryzae'' and ''X. o.'' pv. ''oryzicola''. ''Xanthomonas oryzae ...
'' pv. ''oryzae'', is a disease found worldwide and particularly destructive in the rice-producing regions in Asia.


Plant pathogenesis and disease control

Contaminated seeds, weeds, infected plant debris are the main route of transmission. Infection starts with epiphytic stage – i.e. bacteria grow on the aerial tissues of plant host (leaf, fruit, etc) followed by endophytic stage when bacteria enter and colonise host tissues through wounds or natural openings. When population of bacteria increases it re-emerges to the surface and is transmitted mainly by wind, rain or through seeds or agricultural machinery, while animal and insect vectors seems to play minor role. ''Xanthomonas'' uses surface polysacharides, adhesion proteins and type IV pili to attach to the surface and can form
biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...
s to sustain abiotic stresses (UV, drought, etc). ''Xanthomonas'' produce xanthomonadins - yellow pigments that protect from radiation caused from natural light. Resistance to UV is mostly conferred by genes related to oxidative stress and DNA repair. Response to light is important in pathogenicity of these bacteria and regulates surface attachment and production of biofilm. Xanthomonas possess almost all known secretion systems (types I to VI) that play different roles in the life and disease cycle, with type III secretion system ( T3SS) being the key factor of pathogenicity. Typically, ''Xanthomonas'' T3SS injects a cocktail of 20-30 effector proteins that interfere with plant immune system and various host cellular processes. Many of the effectors are presumably redundant as individual deletions of effector genes does not impair virulence, however mutations in T3SS apparatus has strong effect. Secretion of the effectors is coordinated with expression of other virulence factors via shared regulatory networks. The effector repertoire has been proposed to be a determinant of host specificity. ''Xanthomonas'' actively kill other bacterial using type IV secretion system and defend itself from amoeba using type VI secretion system. To prevent infections, limiting the introduction of the bacteria is key. Some resistant cultivars of certain plant species are available as this may be the most economical means for controlling this disease. For chemical control, preventative applications are best to reduce the potential for bacterial development. Copper-containing products offer some protection along with field-grade antibiotics such as
oxytetracycline Oxytetracycline is a broad-spectrum tetracycline antibiotic, the second of the group to be discovered. Oxytetracycline works by interfering with the ability of bacteria to produce essential proteins. Without these proteins, the bacteria cannot g ...
, which is labeled for use on some food crops in the United States. Curative applications of chemical pesticides may slow or reduce the spread of the bacterium, but will not cure already diseased plants. 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 Xanthan gum () is a polysaccharide with many industrial uses, including as a common food additive. It is an effective thickening agent, emulsifier, and stabilizer that prevents ingredients from separating. It can be produced from simple sugars ...
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 Fera Science, formerly the Food and Environment Research Agency, is a UK research organisation. It is a joint private/public sector venture between Capita plc and the UK Government (Defra). History The Food and Environment Research Agency (FER ...
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.


References

{{Taxonbar, from=Q140652 * Bacteria genera