Xanthomonadales

Xanthomonadales is an order of Gram-negative, chemoorganotrophic bacteria within the class Gammaproteobacteria of the phylum Proteobacteria. The order encompasses several families, most notably Xanthomonadaceae and Pseudoxanthomonadaceae, and contains a variety of genera including Xanthomonas, Xylella, Stenotrophomonas, and Pseudoxanthomonas. Members are predominantly aerobic or facultatively anaerobic, and most are rod-shaped or ovoid. They are usually motile, frequently by a single polar flagellum, though some species are nonmotile. Biochemical traits commonly include catalase positivity and oxidase negativity, and many strains are capable of utilizing a range of organic substrates.

A distinctive feature of many Xanthomonadales is pigment production; several genera, especially Xanthomonas, synthesize yellow pigments called xanthomonadins, contributing to colony color. Some members produce extracellular polysaccharides; xanthan gum from Xanthomonas is widely used industrially as a thickener and stabilizer.

Ecologically, Xanthomonadales includes important plant-associated bacteria. Numerous Xanthomonas species are plant pathogens causing diseases such as citrus canker, bacterial leaf spot, and black rot of crucifers. Other members inhabit soil, water, and plant surfaces as saprophytes or endophytes. Some species, notably Stenotrophomonas maltophilia, are opportunistic human pathogens, particularly in immunocompromised individuals.

Taxonomically, Xanthomonadales has undergone revisions with molecular data (16S rRNA gene sequencing and whole-genome analyses) refining relationships among genera and families. The group remains of interest for plant pathology, biopolymer production, and environmental microbiology.