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Xanthomonas (from greek: xanthos – "yellow"; monas – "entity") is a genus of bacteria, many of which cause plant pathology. 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.
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 pathotypes along the lines of the most recent phylogenetic data.
There is a proposal to unify Xylella into Xanthomonas on the basis of paraphyly ( Xylella branches within Xanthomonas). The same study has a more radical proposal to unify Stenotrophomonas and Pseudoxanthomonas into Xanthomonas on the basis of insufficient divergence. 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.
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.
Colony growth characteristics include:
Biochemical and physiological test results are:
Xanthomonas uses surface polysacharides, adhesion proteins and type IV pili to attach to the surface and can form 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, 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.
Multiple genomes of Xanthomonas have been sequenced and additional data sets/tools are available at The Xanthomonas Resource and at PhytoBacExplorer.
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