**1. Introduction**

The well know endophytic plant growth promoting bacterium *Xanthobacter autotrophicus*, was described as *Corynebacterium autotrophicus* due its specific genetic qualities to grow under chemolithoautotrophically and for being able to fix molecular nitrogen (N2) as nitrogen source [1]: *X. autotrophicus* are rods, according to growth condition show pleomorphism depends on the species and the carbon and nitrogen source on which they are grown. *X. autotrophicus* is a Gram-type negative rods with high concentrations of polyphosphate granule belongs phylogenetically to the family Hyphomicrobiaceae in the class Alphaproteobacterial, grow heterotrophically under aerobic or microaerophilic conditions with acids, alcohols, and selectively with some carbohydrates as energy and carbon source like: fructose, galactose, mannose and sucrose [2–7].

The endophyte plant growth promoting bacteria: *X. autotrophicus* can fix dinitrogen under heterotrophic and thioautotrophic conditions is able to grow with


*Symbols and abbreviations: +, positive; (+), positive except for some unusual strains;* −*, negative; (*−*), negative except for some unusual strains; +/*− *not determined; TCA = tricarboxylic acid.a 1 = X. autotrophicus in the reference; 2 = X. autotrophicus; 3 = X. autotrophicus repetitions. s Lime production in glucose.*

*c Pale yellow indicating low concentration [6, 22–26].*

#### **Table 1.**

*Main biochemical characteristics among some species of the genus Xanthobacter.a*

H2 plus O2 or H2 + Na2S2O3 as energy source and with CO2 as only inorganic carbon source [8–10] at reduced O2 tension [11, 12] in the absence of organic or inorganic nitrogen (N) compounds as aminoamides, peptides, proteins, nucleotides, well known sources as NH4 + (ammonia) or NO3 − (nitrate) at the soil [13] and culture artificial media [14]. On the basis of their numbers, *X. autotrophicus* should be regarded as an associative symbiosis diazotroph due although entering roots of wheat (*Triticum aestivum*), bean (*Phaseolus vulgaris*), root beet (*Beta vulgaris*), rice (*Oriza sativa*) [15] tomato (*Solanum lycopersicum*), lettuce (*Lactuca sativa*) [16, 17] does not form nodules the way do symbiotic N2-fixing bacteria in legume as *Bradyrhizobium japonicum* does. The special position of *X. autotrophicus* among the chemolithoautotrophic and other the N2-fixing aerobic bacteria [1, 11], *X. autotrophicus* is able to grow with H2/CO2/O2 or to have high hydrogenase activity [10, 18], beside reaction of nitrogenase as the other well-known genera: *Azotobacter*, *Derxia*, *Bradyrhizobium* and *Rhizobium* [19, 20]. Originally, one key taxonomic property for discriminating *X. autotrophicus* from other genera yellow pigmented zeaxanthin dirhamnoside bacteria, including diazotrophs [2]. *Xanthobacter* strains can be isolated easily if certain conditions are applied: no other or very limiting sources of nitrogen other than N2 or H2/CO2/O2/N2 [9, 11] as gas phase providing an electron donor, a carbon source, electron acceptors, in liquid media; yellow colonies are showed on nutrient agar plates [1, 6]. Because its metabolic diversity *Xanthobacter* species are widespread in natural habitats [21] as is showed in **Table 1**.

Xanthobacter autotrophicus *an Endophytic Beneficial Bacterium for Wheat and Other Plants… DOI: http://dx.doi.org/10.5772/intechopen.102066*
