|Xanthomonas translucens growing on sucrose peptone agar showing yellow pigment|
The Xanthomonas genus 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 2 groups, A and B. Later work using DNA:DNA hybridization has served as a framework for the general Xanthomonas spp. classification. Other tools including Multi-locus Sequence Analysis (MLSA) and Amplified Fragment Length Polymorphism (AFLP) have been utilized for classification within clades. So while previous research has illustrated the complexity of the Xanthomonas genus, it appears that recent research has resulted in a clearer picture. More recently, genome wide analysis of multiple Xanthomonas strains mostly support the previous phylogenies.
Morphology and Growth
Individual cell characteristics
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
mucoid, convex, and yellow on YDC media 
Yellow pigment from xanthomonadin, which contains Bromine
Most produce large amounts of extracellular polysaccharide
Biochemical and Physiological Tests
Gram stain - negative
Catalase positive 
Oxidase negative 
Temperature Range - 4 to 37 C
Infected plant tissue should be surface disinfested using a dilute Clorox solution (10%) and rinse thoroughly. If unsure of bacterial presence, examine symptomatic tissue by cutting along the infected area, placing cut tissue on glass microscope slide and cover with a cover-slip, and examine for bacterial streaming at high magnification. At 200X magnification, bacterial streaming can be observed as an oozing mass from the cut area. Once bacterial streaming is confirmed, place surface disinfested tissue in a few drops of sterile water and allow bacteria to stream out into the water droplet. After bacteria has colonized the water droplet (30–60 seconds), use a sterile loop to streak the bacteria on to solid agar media. Allow the bacteria to grow for at least 48 hours at room temperature and examine periodically for colony growth.
Xanthomonas Plant Pathogens
Xanthomonas spp. 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 pathovars, a species designation based on host specificity.
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.
Plant Pathogenesis and Disease Control
Xanthomonas species can be easily spread in water, movement of infected material like seed or propagation plants, and by mechanical means like infected pruning tools. Upon contact with a susceptible host, bacteria will enter through wounds or natural plant openings as a means to infect. They inject a number of effector proteins, including TAL effectors, into the plant via their secretion systems (i.e., Type III 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 bacteria development. Copper containing products offer some protection along with field grade antibiotics like 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.
Industrial Use of Xanthomonas
Xanthomonas produces an extrapolysaccharide called xanthan gum that has a wide range of industrial uses including foods, petroleum products, and cosmetics.
Isolates of most species of Xanthomonas are available from the National Collection of Plant Pathogenic Bacteria (NCPPB) in the United Kingdom and other international culture collections such as ICMP in New Zealand, CFBP in France, and VKM in Russia. It is also taken out from M.T.C.C. India.
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