The Genus Herbaspirillum MICHAEL SCHMID, JOSE IVO BALDANI AND ANTON HARTMANN

Owing to its cell form, growth behavior and habitat within grass roots, the first isolates of the later defined genus Herbaspirillum were initially thought to be a new Azospirillum species (Baldani et al., 1984). However, RNA-RNA hybridization experiments showed no close relatedness with Azospirillum spp. or Aquaspirillum itersonii (Falk et al., 1986). The first species of the newly defined genus Herbaspirillum, Herbaspirillum seropedicae (Baldani et al., 1986), was named after the location of the EMBRAPA National Center for Agrobiology (CNPAB) in Seropedica, Rio de Janeiro, Brazil. The genus Herbaspirillum was extended with [Pseudomonas] rubrisubalbicans, causative agent of “mottled strip disease” in some susceptible sugar-cane varieties, because DNA-rDNA and DNA-DNA reassociation hybridization studies showed a high degree of DNA similarity (Gillis et al., 1990; Baldani et al., 1992). Additional physiological and biochemical features, including the ability to fix nitrogen, confirmed the reclassification as Herbaspirillum rubrisubalbicans (Baldani et al., 1996). A group of clinical isolates (EF group 1) had to be included in the genus Herbaspirillum as “species 3” because of its molecular and overall physiological relatedness. However, members of Herbaspirillum species 3 do not exhibit nitrogen-fixing ability. More recently, several new species of Herbaspirillum were isolated from diverse plants like Miscanthus sinensis and Pennisetum purpureum (H. frisingense; Kirchhof et al., 2001) and nodules of Phaseolus (H. lusitanum; Valverde et al., 2003). On the basis of molecular relatedness, a group of bacteria having the ability to efficiently degrade chlorophenols was also included in the genus Herbaspirillum as Herbaspirillum chlorophenolicum (Im et al., 2004). Although most of the bacteria in the genus Herbaspirillum are N2-fixing bacteria colonizing diverse plants endophytically (Döbereiner, 1992; Döbereiner et al., 1993), clinical and environmental isolates belong to this genus, too. This resembles the situation in other species of the Betaproteobacteria, where plantassociated or even symbiotic diazotrophs, opportunistic pathogens, and potent degraders of pollutants belong to the same genera like Burkholderia (Coenye and Vandamme, 2003), Ralstonia (Chen et al., 2001) and Azoarcus (Reinhold-Hurek and Hurek, 2000).