The Xanthobacteraceae are a family of bacteria that includes Azorhizobium, a genus of rhizobia. Xanthobacteraceae bacteria are diverse and Gram-negative, rod-shaped, and may be motile or non-motile depending on the specific bacteria. Their cells range in size from 0.4–1.0 × 0.8–6 µm, but when grown in the presence of alcohol as the sole carbon source, they can reach up to 10 µm in length. These bacteria do not form spores and have opaque, slimy colonies that appear slightly yellow due to the presence of zeaxanthin dirhamnoside.

The genus Xanthobacter was established in 1978 by Wiegel et al. based on numerical taxonomic comparisons of microorganisms that were then classified in the genus Corynebacterium. In 2005, Lee et al. proposed the family Xanthobacteraceae based on a comparison of 16S rRNA of the members of Alphaproteobacteria. The family includes five genera, namely Xanthobacter, Azorhizobium, Ancylobacter, Labrys, and Starkeya.

Phenotypic properties 
The Xanthobacteraceae family is highly diverse, with some cells being polymorphic in shape. Their cells have a Gram-negative type cell wall and contain ubiquinone Q-10 as their major respiratory quinone. Refractile (phosphate) and lipid bodies are evenly distributed throughout the cells. However, as cells also contain polyphosphate granules, sometimes the Gram reaction can give false positive results.

Most chemolithoautotrophic strains of Xanthobacteraceae require H2, O2 and CO2 in mineral media, while chemoorganoheterotrophic strains utilize various carbon sources such as methanol, ethanol, n-propanol, n-butanol, and different organic acids. Some genera within the family demonstrate the ability to fix nitrogen under reduced oxygen pressure.

Genotype of Xanthobacteraceae 
On average the chromosomes are 4.77–5.37 Mbp in length. In Azorhizobium caulinodans, Starkeya novella, and X. autotrophicus, there are 4417–4847 predicted genes presents in the genome. A 316-kb plasmid containing 308 genes present in X. autotrophicus Py2 strains.

Ecology 
Members of the genus can be found in freshwater, wet soil that contains decaying organic materials and in the sediments. Rice paddies, soils environments, freshwater habitats such as ponds, creeks and lakes contain Ancylobacter aquaticus. Study showed that there is a relationship between the watershed urbanization and the alteration of bacterial community composition. Xanthobacteraceae consistently showed decreased abundance on increasing watershed urbanization.

Phages 
There are three known phages that can infect Xanthobacter autotrophicus strain GZ29. There are two lytic phages named CA1 and CA2. Both have head of 61-68 nm in diameter. CA1 has a 98-100 nm tails while the length of tail for CA2 is 166-175 nm. The third phage called CA3 is lysogenic in nature and contain head of 37-43 nm and a tail of 43-50 nm in length. CA3 also contains a small DNA molecule of 3.3 kDa.

In vitro growth condition and maintenance 
Most of the strains can grow chemolithoautotrophically in the mineral media in the presence of H2, O2 and CO2. Other strains can grow chemoorganoheterotrophically on methanol, ethanol, propanol, n-butanol and organic acids. Temperature for optimal growth varies from 25–42 °C. Generally, they can grow at pH 6.5-8 with optimum growth at pH 7.5. Some strains decrease the pH of the medium during growth. Therefore, addition of buffer is recommended to maintain the optimal growth. Cultures can be maintained for 10 months at 2-5 °C in liquid medium and for up to 15 months in sealed agar slants. At -20 °C, the culture can be stored for 3 years in the presence of 60% (v/v) glycerol. Lyophilization is recommended for long term storage.

Pathogenicity and antibiotic sensitivity 
There is no known pathogenic strain found in the Xanthobacteraceae.  Some species of genus Azorhizobium are associated with plant such as Sesbania and some other leguminous plants that live in symbiosis. Some species of Xanthobacteraceae are sensitive to penicillin, novobiocin and polymyxin B. X. autotrophicus and X. flavus  are resistant to erythromycin and bacitracin.

Application 
Xanthobacteraceae species, including X. viscosus and X. aminoxidans, are commonly found in activated sludge from water treatment plants, indicating their potential role in organic compound degradation within polluted environments. Recent studies have uncovered the biotechnological applications of Xanthobacter species. Some bacteria within this family can degrade toxic compounds, such as polycyclic aromatic compounds (PAHs), into CO2 and water. Furthermore, certain Xanthobacteraceae strains, such as Starkeya sp. strain N1B, can use toxic aromatic hydrocarbons like naphthalene as their sole carbon source for bacterial cellulose production. This process results in the production of cellulosic biofilm using toxic compounds, such as naphthalene Christal.

Phylogeny
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN). The phylogeny is based on whole-genome analysis.

References 

Hyphomicrobiales