A Microbial Biorealm page on the genus Xanthomonas campestris
Higher order taxa
[Class] Gamma Proteobacteria
[Species] Xanthomonas campestris
Description and significance
Xanthomonas campestris is an aerobic, Gram-negative rod known to cause the black rot in crucifers by darkening the vascular tissues. Host associated, over 20 different pathovars of X. campestris have been identified by their distinctive pathogenicity on a wide range of plants including crops and wild plants. This bacterium is mesophilic with optimal temperature at 25-30 degrees Celsius (77-85 degrees Fahrenheit) and inactive at temperatures below 10 degrees Celsius (50 degrees Fahrenheit) . X. campestris have Hypersensitive response and pathogenicity (Hrp) pili that help transfer effector proteins to decrease the host’s defense and glide through water [2, 3]. They can live in a soil for over a year and spread through any movement of water including rain, irrigation and surface water. Spraying healthy plants with copper fungicides may reduce the spread of the bacteria in the field . However, once the plant has been infected, X. campestris will eventually spread to the seed stalk inhibiting the growth of a healthy offspring.
By pure culture fermentation, X. campestris can produce an extracellular polysaccharide known as xanthan gum that is commercially manufactured as a stabilizing agent used in many everyday products including salad dressing or toothpaste . X. campestris is a model organism for studying interactions between plant and bacteria. Due to the deficit in crops, further research of this bactera is in progress in hopes of learning how to make plants resistant to this pathogen.
The genome structures of X. campestris contain variation depending on the pathovars. However, the different strains of X. campestris exhibit similar characteristics like circular chromosomes, the mobile genetic elements and protein coding sequences . In 2002, the complete genome of X. campestris pv campestris (Xcc) str. ATCC 33913 was completed containing over 5,076,188 nucleotides that encode for over 4200 protein coding and 61 structural RNAs . With over 548 unique coding sequences, X. campestris pv. vesicatoria (Xcv) is composed of a 5.17-Mb circular chromosome, four plasmids, and an essential type III protein secretion system for pathogenicity . Using Recominbase-based In Vivo Expression Technology to target tomato, Xcv has been found to have 61 genes that are involved in the interaction between pathogen and host including a necessary virulence transporter, citH homologue gene .
Cell structure and metabolism
X. campestris is a rod-shaped Gram-negative bacteria characterized by its two cell walls and yellow pigment. It has a filamentous structure called hypersensitive response and pathogenicity (Hrp) pili that is attached to type III protein secretion system implementing the ability to transfer bacterial proteins to the plant and also motility in water .
This aerobic bacterium performs a number of metabolic pathways that are uniquely dependent on the pathovar. Entire genome sequencing of Xcv exhibit carbohydrate metabolisms that include: Glycolysis/ Gluconeogenesis, Citrate cycle (TCA cycle), Pentose phosphate pathway, and more. Xcv gains its energy source through oxidative phosphorylation, carbon fixation, methane, nitrogen, and sulfur metabolism .
X. campestris acquire carbon from the host converting it to glucose through gluconeogenesis. Further research has shown that in gluconeogenesis, Xcc contain only the malic enzyme-PpsA route which is essential for virulence . In addition, this plant pathogen contain a type III secretion system (TTSS) that is necessary in order to attack the host . TTSS is important in pathogenesis because it transfers effector proteins in order to lower the host’s defense. Creating biofilm on plant surfaces, X. campestris exemplifies cell-cell communication through diffusible signal factor (DSF). .
Possessing the ability to ferment, X. campestris creates an extracellular polysaccharide, xanthan, which is commercially produced and used in a variety of everyday products as a stabilizing agent . See Application to Biotechnology for more details.
X. campestris causes great loss in agriculture due to their habitat in plants. It can live in the soil for over a year and spread through overhead irrigation and surface water . This bacterium thrive particularly during wet and warm weathers with optimal temperatures at 25-30 degrees Celsius (77-85 degrees Fahrenheit) . X. campestris depend on water for survival and movement to the next host. Due to contamination during cultural operations, affected plants usually occur in the same rows when farmed . See pathology for more details.
X. campestris can be spotted by the black lesions that develop on plant surfaces when contaminated. The pathogen first interacts with the host by secreting an array of effector proteins including hypersensitive reaction using type III secretion system (TTSS) . These effectors may behave avirulently by disguising itself to secrete several hypersensitive reaction and outer proteins in order for interaction to occur with the host cells . X. campestris then targets the vascular tissue causing darkening and marginal leaf chlorosis . The bacteria seeps into the leaf towards the stomates, hydathodes, and to the epidermal cells initiating new lesion . Severe infection occurs in a young seedling. Since the disease advances throughout the plant, the main stem cannot form, stunting development and blackening the veins. Eventually, the bacteria proliferates throughout the vascular system and to the seed stalk causing the seed to become infected of future diseases .
Appearing more during the wet seasons, X. campestris possess pili that accommodate a gliding movement through wet leaves. Covering an affected area in numerous numbers, once a plant is diseased, the pathogen will spread in any form of water movement including splashes of rain drop to a new host .
Virulence factors consist of lytic enzymes that attack the plant's cell wall, excretion of proteases, amylases, cellulases and lipases that help lower the plant’s defense mechanisms . In addition, the Rpf gene cluster is also crucial for pathogenesis in order for X. campestris to moderate the production of these virulent factors .
Application to Biotechnology
X. campestris ferments a stabilizing agent called xanthan gum that is used in many everyday products. It was first commercially produced at Kelco Company, a major pharmaceutical company. This polysaccharide is an ingredient in products like Kraft French dressing, Weight Watchers food, Wonder Bread products, and more . From carbohydrate fermentation by X. campestris, xanthan gum’s pseudoplastic, easily blended characteristic allows it to be used as a thickener by increasing viscosity of a liquid . In addition, xanthan gum also prolongs oil and gas wells even after production. Either pumped into the ground or using high pressure sandblasting, mixing water and xanthan gum into the wells will help thicken the liquid to release crude products of oil and cut through rocks in gas and oil wells. Xanthan gum costs $7 per pound compared to cornstarch for 89 cents per pound .
8.1 Biological Control with strains of Bacillus
Genome sequencing is being done in search of the essential genes needed in order to develop resistant plants. An experiment was done using Bacillus strains including B. cereus, B. lentimorbus, and B. pumilus as a rival for pathogen Xcc in cabbage. Evidence has shown some hope for biological control when the Bacillus strain was added in the roots .
8.2 Regulatory network for cell-cell communication
Recent studies demonstrate X. campestris use the diffusible signal factor (DSF) for cell-cell communication. In order for microcolonies to form structured biofilm, synthesis of DSF is required from the regulation of pathogenicity factor (Rpf) cluster. The Rpf cluster contains necessary genes for pathogenesis to occur. Without the critical DSF signaling, Xcc will create an unstructured biofilm of bacteria. Further research is being conducted to understand how the regulatory network is being monitored .
8.3 Genetic Variation in wild crucifers
Researchers have found genetic diversity in Xcc on wild crucifers. With the most diverse and abundant wild cruciferous plants in the world, studies was done in California to find any differences in genetic strains on Xcc in infected wild weeds. From both non-cultivated and cultivated areas, Xcc was isolated from different regions of California. Using Amplified fragment length polymorphism PCR (AFLP) to identify genetic variation in strains, over 72 strains were sequenced to show 7 unique genotypes that were limited to their respective sites. Non-cultivated wild weeds near the coast had strains of Xcc that were specific to the region and different from the weeds grown near produced crop areas. 
 Averre, Charles W. "Black Rot of Cabbage and Related Crops" Accessed on August 16, 2007.
 Ritchie David F, Averre Charles W. "Bacterial Spot of Pepper and Tomato". North Carolina State University College of Agriculture and Life Sciences. Accessed on August 20, 2007.
 Weber E, Ojanen-Reuhs T, Huguet E, Hause G, Romantschuk M, Korhonen TK, Bonas U, Koebnik R. "The type III-dependent Hrp pilus is required for productive interaction of Xanthomonas campestris pv. vesicatoria with pepper host plants". Journal of Bacteriology. 2005. Volume 187(7) p.2458-69.
 Kuntz, L. “X is for Xanthan Gum”. Food Product Design. Accessed on August 26, 2007.
 Vorhölter FJ, Thias T, Meyer F, Bekel T, Kaiser O, Pühler A, Niehaus K. "Comparison of two Xanthomonas campestris pv. campestris genomes revealed differences in their gene composition". Journal Biotechnology. 2003. Volume 106(203). p.193-202.
 "Xanthomonas campestris pv. campestris str. ATCC 33913, complete genome”. Public Library of Science Biology. 2002. Accessed on August 25, 2007.
 Thieme F, Koebnik R, Bekel T, Berger C, Boch J, Büttner D, Caldana C, Gaigalat L, Goesmann A, Kay S, Kirchner O, Lanz C, Linke B, McHardy AC, Meyer F, Mittenhuber G, Nies DH, Niesbach-Klösgen U, Patschkowski T, Rückert C, Rupp O, Schneiker S, Schuster SC, Vorhölter FJ, Weber E, Pühler A, Bonas U, Bartels D, Kaiser O. "Insights into genome plasticity and pathogenicity of the plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria revleaed by the complete genome sequence". Journal of Bacteriology. 2005. Volume 187(21). p 7254-66.
Tamir Ariel D., Navon N, Burdman S. "Identification of Xanthomonas campestris pv. vesicatoria genes induced in its interaction with tomato". Journal of Bacteriology. 2007. Volume 189(17). P.6359-71.
 Weber E, Koebnik R. "Domain structure of HrpE, the Hrp pilus subunit of Xanthomonas campestris pv. vesicatoria". Journal of Bacteriology. 2005. Volume 187(17). p. 6175-86.
 "Xanthomonas campestris pv. vesicatoria." Kyoto Encyclopedia of Genes and Genomes. Accessed on August 24, 2007.
 Tang DJ, He YQ, Feng JX, He BR, Jiang BL, Lu GT, Chen B, Tang JL. "Xanthomonas campestris pv. campestris posesses a single gluconeogenic pathway that is required for virulence." Journal of Bacteriology. 2005. Volume 187(17). p 6231-7.
 Torres PS, Malamud F, Rigano LA, Russo DM, Marano MR, Castagnaro AP, Zorreguieta A, Bouarab K, Dow JM, Vojnov AA. "Controlled synthesis of the DSF cell-cell signal is required for biofilm formation and virulence in Xanthomonas campestris." Environmental Microbiology. 2007. Volume 9(8). p. 2101-9.
 Wang L, Tang X, He C. "The bifunctional effector AvrXccC of Xanthomonas campestris pv. campestris requires plasma membrane-anchoring for host recognition." Molecular Plant Pathology. 2007. Volume 8(4). p 491–501.
 Niehaus Karsten. "The Xanthomonas campestris pv. campestris genome project." Accessed on August 20, 2007.
 Dow JM, Crossman L, Findlay K, He YQ, Feng JX, Tang JL. "Biofilm dispersal in Xanthomonas campestris is controlled by cell-cell signaling and is required for full virulence to plants". Proceedings of the National Academy of Sciences of the United States of America. 2003. Volume 100(19). p 10995-1000.
 "Technologies in the Marketplace." United States Department of Agriculture, Agricultural Research Service (ARS). Accessed on August 21, 2007.
 Massomo S, Mortensen C, Mabagala R, Newman M, Hockenhull J. "Biological control of black rot (Xanthomonas campestris pv. campestris) of Cabbage in Tanzania with Bacillus strains". Journal of Phytopathology. 2004. Volume 152(2). p. 98-105.
 Torres PS, Malamud F, Rigano LA, Russo DM, Marano MR, Castagnaro AP, Zorreguieta A, Bouarab K, Dow JM, Vojnov AA. "Controlled synthesis of the DSF cell-cell signal is required for biofilm formation and virulence in Xanthomonas campestris". Environmental microbiology. 2007. Volume 9(8). p 2101-9.
 Ignatov, A., Sechler, A.J., Schuenzel, E., Agarkova, I.V., Vidaver, A.K., Oliver, B., Schaad, N.W. 2007. "Genetic diversity in populations of Xanthomonas campestris pv. campestris in cruciferous weeds in central coastal California". Phytopathology. 2007. Volume 97. p. 803-812.
Edited by Tammie Chau, student of Rachel Larsen