''Xanthomonas campestris'' is a

gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

, obligate aerobic bacterium that is a member of the

Xanthomonadaceae Xanthomonadaceae is a family of Pseudomonadota within the order Xanthomonadales The Xanthomonadales are a bacterial order within the Gammaproteobacteria. They are one of the largest groups of bacterial phytopathogens, harbouring species such ...

, a family of bacteria that are commonly known for their association with plant disease. This species includes ''Xanthomonas campestris'' pv. ''campestris'', the cause of black rot in brassicas (

cruciferous vegetables Cruciferous vegetables are vegetables of the family Brassicaceae (also called Cruciferae) with many genera, species, and cultivars being raised for food production such as cauliflower, cabbage, kale, garden cress, bok choy, broccoli, Brusse ...

), one of the most important diseases of brassicas worldwide. These bacteria are facultative saprophytes, meaning that they are typically

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

while also having the ability to live on dead or decaying organic matter under the proper conditions. Upon initial infection, the bacteria remain in the

epiphytic An epiphyte is a plant or plant-like organism that grows on the surface of another plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphyt ...

stage; however, the harmful endophytic stage is reached when the bacteria actually enter the plant host through natural openings. In general, the genes that contribute significantly to the plant-bacteria relationship are the avirulence (''avr'') genes, the hypersensitivity response and

pathogenicity In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term ...

(''hrp'') genes, and the

pathogenicity factor Virulence factors (preferably known as pathogenicity factors or effectors in botany) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and protozoa) to achieve the following: * co ...

s (''rpf'') genes. Additionally, the virulence determinants associated with the seedborne diseases that result from this bacterium include extracellular enzymes,

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

lipopolysaccharide Lipopolysaccharide (LPS), now more commonly known as endotoxin, is a collective term for components of the outermost membrane of the cell envelope of gram-negative bacteria, such as '' E. coli'' and ''Salmonella'' with a common structural archit ...

s, etc. Several strains of ''Xanthomonas campestris'' produce an exopolysaccharide called xanthan or

xanthan gum Xanthan gum () is a polysaccharide with many industrial uses, including as a common food additive. It is an effective thickening agent and stabilizer that prevents ingredients from separating. It can be produced from simple sugars by fermentat ...

, which has important uses as a thickener in the food, oil, agricultural, and pharmaceutical industries.

Pathovars

Over 140

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 of ''Xanthomonas campestris'' have been described initially and typically named according to the plant that they were first found to infect. However, several studies have subsequently proposed the reclassification of many of these pathovars in different species within the genus ''Xanthomonas.'' This left six pathovars of ''X. campestris'' remaining in this species, which included pathogens of ''Brassicaceae'' plants ''X. campestris'' pv. ''aberrans, X. campestris'' pv. ''armoraciae, X. campestris'' pv. ''barbareae, X. campestris'' pv. ''campestris'''', X. campestris'' pv. ''incanae, and X. campestris'' pv. ''raphani'', but still included a small number of other pathovars like ''X. campestris'' pv. ''plantaginis'' and ''X. campestris'' pv. ''papavericola''. Further investigation of pathogenicity profiles and multilocus sequencing typing suggested that the list could be narrowed down to just three main pathovars with the different symptoms being black rot, leaf spot, and bacterial blight. Both ''X. campestris'' pv. ''campestris'' (known for causing black rot of crucifers) and ''X. campestris'' pv. ''incanae'' (known for causing bacterial blight of garden stocks) are vascular pathogens, and they have been found to invade the plant host through wounds or

hydathode A hydathode is a type of pore, commonly found in vascular plants, that secretes water through pores in the epidermis or leaf margin, typically at the tip of a marginal tooth or serration. Hydathodes occur in the leaves of submerged aquatic plant ...

s. ''Xanthomonas campestris'' pv. ''campestris'' also has some limited ability to infect the plant host through the stomata. ''Xanthomonas'' pv. ''raphani'' has been found to enter the plant through its

stoma In botany, a stoma (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...

ta to cause infection of the tissue, or rather

parenchyma upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...

. This results in bacterial spot on a wider range of hosts, which includes both crucifers and certain solanaceous plants. 12 pathovars (''vitiswoodrowii'', ''vitiscarnosae'', ''vitistrifoliae'', ''bilvae'', ''azadirachtae'', ''durantae'', ''centellae'', ''thespesiae'', ''leeana'', ''merremiae'', ''thirumalacharii'', and ''trichodesmae'') were moved to ''

Xanthomonas citri ''Xanthomonas citri'' is a Gram-negative bacteria, Gram-negative, Bacillus (shape), rod-shaped bacterium. Although it is harmless for humans, it is a phytopathology, phytopathogen, known for being the causing agent of citrus canker. Many pathova ...

'' based on phylogenomic evidence in 2022. Pathovars transferred earlier to ''X. citri'' include ''viticola''. 20 pathovars were moved to '' Xanthomonas euvesicatoria'' in 2023 based on phylogenomic evidence.

Phytopathology

Relationships between ''Xanthomonas campestris'' bacteria and plants can be both compatible and incompatible. It is in the compatible relationships, where the bacteria are able to overcome the host's defenses, rather than experience attenuated growth, that disease symptoms will be seen in the plants. This is due to toxins, extracellular enzymes (exported by the

type II secretion system The type 2 secretion system (often referred to as the type II secretion system or by the initials T2SS) is a type of protein secretion machinery found in various species of Gram-negative bacteria, including many human pathogens such as ''Pseudomo ...

s, a fatty acid-dependent cell-cell communication system, and proteins (secreted by the

type III secretion system The type III secretion system (T3SS or TTSS) is one of the bacterial secretion systems used by bacteria to secrete their effector proteins into the host's cells to promote virulence and colonisation. While the type III secretion system has been ...

), for example. The genes in the bacterial genome that are responsible for such interactions include avirulence (''avr'') and hypersensitivity response and

(''hrp'') genes. Gene-for-gene patterns control the interactions between the ''Xanthomonas campestris'', a bacterial pathogen, and plants. ''Avr'' genes are a group of genes that impact the specificity of the interaction between the bacteria and the plant host. When either these bacterial genes or a plant's resistance genes to the pathogen are not present, the interaction will result in disease. Alternatively, when the genes are present, the plant's resistance genes will produce a product that is able to recognize the ''avr'' genes of the bacteria, which allows for the plant host to have resistance. ''Hrp'' genes are responsible for the determination of the outcome of the interaction between the plant and bacteria. When mutation occurs within these genes, there is impact on both compatible and incompatible interactions. This is the case because there may be an impact on pathogenicity and hypersensitivity response, respectively. Therefore, the plant's ability to limit the spread of the microbial pathogen may be inhibited. The regulation of

s (''rpf'') gene cluster also plays an important role in the plant-bacteria interactions by encoding for a cell-cell signaling system involving diffusible signal factor that is necessary for full

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...

. This regulation system is involved in the regulation of both the formation and dispersal of ''Xanthomonas campestris'' biofilms, which is specifically related to the ''rpfB'' and ''rpfF'' genes within the cluster. With a mutation in the ''rpf'' genes, the synthesis of various extracellular enzymes will be downregulated. This includes endoglucanase,

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

, and extracellular polysaccharide (EPS) xanthan, for example, which are important to the virulence of the bacteria. Xanthomonas-cycle

Like with ''Xanthomonas'' species in general, the primary route of transmission for ''Xanthomonas campestris'' is through seeds, which act as the source of inoculum. Therefore, the bacteria are initially in the

stage of the infection cycle, which is when they grow on the surface of the plant, such as on the aerial tissues of leaves and fruit. Various adhesion strategies are utilized by the microbes in order to remain attached to the plant surfaces, and this includes bacterial surface polysaccharides, adhesion proteins, and type IV pili. Then,

biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

matrices composed primarily of xanthan will form, which will help the bacteria to survive in the harsh

abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them und ...

conditions of the plant surfaces. These biofilms, along with pigments, also help the bacteria to survive the potential damages to DNA and membrane that result from radiation and light. Eventually, the bacteria enter the host tissue through natural openings such as pores and

ta, or wounds, which is hypothesized to be a result of

chemotaxis Chemotaxis (from ''chemical substance, chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell organism, single-cell or multicellular organisms direct thei ...

. When this happens, the endophytic stage has been reached and colonization in the

vascular system In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart a ...

takes place. This is when the development of symptoms, such as lesions of leaves, will occur. Progression into the stem and roots can eventually happen as well, which is when there is systematic infection of the plant. Furthermore, ''Xanthomonas campestris'' can be spread to other plants when the population of bacteria has increased enough that they emerge on the plant surfaces again. This dispersal includes both environmental and mechanical routes, such as through wind, rain, people, non-specific vectors, seed dispersal, etc.

Industrial applications

''Xanthomonas campestris'' is commonly used industrially to produce a water-soluble exo-polysaccharide, known as

, from

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

of carbon sources like glucose. In this process, a preserved culture of the gram-negative bacterium is expanded through growth and then used as an inoculum in

bioreactor A bioreactor is any manufactured device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical reaction, chemical process is carried out which involves organisms or biochemistry, biochem ...

s with liquid

growth media A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss ''Physcomitrella patens''. Differe ...

. Under select modes of operation, such as batch fermentation, and proper growth conditions, fermentation then takes place. Therefore, as the microorganism grows, there is production of xanthan, which is secreted from the bacterial cells and ultimately recovered from the media and purified using alcohol precipitation techniques. This product is particularly applicable to the food industry as a suspending, thickening, and stabilizing agent. However, it also has applications pertaining to the

agricultural industry Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created food ...

, the

oil industry The petroleum industry, also known as the oil industry, includes the global processes of exploration, extraction, refining, transportation (often by oil tankers and pipelines), and marketing of petroleum products. The largest volume products ...

, the

pharmaceutical industry The pharmaceutical industry is a medical industry that discovers, develops, produces, and markets pharmaceutical goods such as medications and medical devices. Medications are then administered to (or self-administered by) patients for curing ...

, etc. Because of that, work is being done to investigate advancements that can be made to the current xanthan gum production processes.

References

External links

Type strain of ''Xanthomonas campestris'' at Bac''Dive'' - the Bacterial Diversity Metadatabase
{{Authority control campestris Bacterial plant pathogens and diseases Soybean diseases

Pathovars

Phytopathology

Industrial applications

References

Further reading

External links