Xanthomonas campestris

Xanthomonas campestris
The bacterial genus Xanthomonas contains organisms that resemble Pseudomonas (small, motile, Gram-negative rods), are aerobic, produce yellow pigments and typically parasitise plants.
X. campestris is the most important species of the genus. It is subdivided into pathovers (pvs) based on host range, geographical distribution and other factors. For example: 
X. campestris pv. campestris causes brown rotting lesions on cabbage and cauliflower leaves. 
X. campestris pv. oryzae causes leaf blight of rice, leading to serious crop losses in Asia (e.g. India), China, and N. and S. America. 
X. campestris pv. citri causes citrus canker in Asia and N. and S. America, again with serious economic consequences.

However, X. campestris is perhaps even more significant as a source of commercial polysaccharide, termed xanthan or xanthan gum.

Streaked colonies of Xanthomonas campestris on an agar plate. The colonies have produced copious extracellular slime - a complex polysaccharide composed of more than one type of sugar (a heteropolymer). It is termed xanthan and has important commercial uses as a gel that is stable at relatively high temperatures. About 20,000 tonnes of xanthan are produced industrially from X. campestris each year. It is used as a gelling and stabilising agent in salad dressings, ice creams, toothpastes, cosmetics, water-based paints, etc., and also as a drilling lubricant in oil wells.
The gum itself is colourless. The bacteria have a yellow pigment in the wall, but it is extractable only with organic solvents so it does not interfere with the commercial processing of xanthan.