In microbiology the bacterium Xanthomonas campestris can be a gift from nature or a scourge of plants. Tiny but mighty, these micro-organisms cause plant blight but are also significant in making xanthan gum, a ubiquitous emulsifier in food, art, cosmetics and medicine.
What is Xanthomonas campestris?
Xanthomonas campestris is a gram-negative bacterium of the family Xanthomonadaceae. It is rod-shaped, motile, and known for its pathogenicity in plants. This bacterium is notorious for causing blight and disease. It's been isolated from various plants world-wide.
Extremely adaptable, Xanthomonas campestris is a plague to gardeners, invading plants especially of the Brassicaceae or Cruciferae (mustard and cabbage) families. Its discovery dates back to the early 20th century.
Xanthomonas campestris lives in soil and water, where it survives in a saprophytic form, obtaining food by absorbing dissolved organic material. This bacterium can be found in diverse environments, including agricultural fields, gardens, and natural ecosystems.
While acting an agent of chaos on one side, Xanthomonas campestris is also the source of creation. Its activities lead to production of xanthan gum, a polysaccharide widely used in the food and pharmaceutical industries.
Xanthomonas comes from the Greek ξανθός xanthós meaning "blond". The primary route of transmission for Xanthomonas campestris is through seeds, which are also the source of inoculum. Inoculum is the group of microorganisms released into the fermentation medium.
Creation of Xanthan Gum
A fermentation medium is a nutrient rich broth beloved by the bacterium. It usually contains sources of carbon, nitrogen, minerals, and some growth factors, such as essential amino acids and vitamins. This raises happy healthy bacteria.
Xanthan gum is prized for its thickening, stabilizing, and emulsifying properties. The fermentation process using Xanthomonas campestris has made it a commercially valuable microorganism.
Steps in Xanthan Gum Production
The production of xanthan gum can be broken down into the following steps:
Preparation and Inoculation: A pure culture of Xanthomonas campestris is grown in a nutrient-rich broth that contains carbohydrates (usually glucose or other sugars) as the primary carbon source. Sterile conditions are maintained to prevent contamination.
Fermentation: The inoculated broth is transferred to a fermenter, where it is kept under controlled conditions of temperature, pH, and aeration. During fermentation, the bacteria metabolize the sugars, leading to the production of xanthan gum.
Polysaccharide Synthesis: Inside the bacterial cells, during the growth phase, Xanthomonas campestris synthesizes xanthan gum from activated sugar substrates such as glucose through the action of specific enzymes (e.g., glucosyltransferases). The structure consists of a repeating unit composed of D-glucose, D-mannose, and D-glucuronic acid.
Extraction and Purification: Once fermentation is complete, xanthan gum is precipitated from the broth by adding isopropanol or ethanol. The precipitated gum is then washed, dried, and ground into a powder to obtain the final product.
Quality Control: The xanthan gum undergoes quality testing to ensure it meets the required standards for its intended applications.
Black Rot of Crucifers
Xanthomonas campestris is responsible for several diseases in plants, the most notable being black rot of crucifers, which affects crops like cabbage, broccoli, and cauliflower. The bacterium infects plants through natural openings, such as stomata and wounds.
Symptoms include yellowing of leaves, blackening of veins, lesions, and ultimately plant death. This little opportunists waste no time. The spread of Xanthomonas campestris can be quick, especially under warm and humid conditions.
Disease Development in Plants
The disease process caused by Xanthomonas campestris can be outlined as follows:
Entry: The bacteria enter the host plant through natural openings like stomata or through mechanical injuries. They can also be transmitted via contaminated seeds or tools.
Colonization: Once inside, Xanthomonas campestris colonizes the plant tissues, particularly the vascular system, where it multiplies rapidly. The bacteria produce cell-wall-degrading enzymes that help in breaching the plant's defense mechanisms.
Toxin Production: The bacterium secretes various phytotoxins and polysaccharides that contribute to pathogenesis. These substances enable the bacteria to thrive within the plant and induce disease symptoms.
Symptom Development: As the bacteria proliferate, they cause disturbances in normal physiological functions. Symptoms such as chlorosis (yellowing of leaves), necrosis (death of tissue), and wilting appear as a result of bacterial spread and the resultant plant stress.
Spread: The bacteria can spread from infected plants to healthy ones through wind, rain, and insect activity. They can also persist in the environment, reinfecting crops in subsequent seasons.
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