The Genus Pelobacter

Abstract

The genus Pelobacter was proposed as a taxonomic entity consisting of strictly anaerobic, Gram-negative, nonsporeforming, rod-shaped bacteria that use only a very limited number of substrates. The members of the genus are all unable to ferment sugars and therefore cannot be grouped with any other genus in the family Bacteroidaceae (Krieg and Holt, 1984). The genus comprises five different species, P. acidigallici (Schink and Pfennig, 1982), P. venetianus (Schink and Stieb, 1983), P. carbinolicus (Schink, 1984), P. propionicus (Schink, 1984), and P. acetylenicus (Schink, 1985), which all are based on 3 5 described strains.Comparisons of the various Pelobacter species by DNADNA hybridization experiments revealed that the genus is rather inhomogenous; therefore, a reorganization may perhaps be necessary in the future (J. P. Touzel and B. Schink, unpublished observations). Whereas the species P. venetianus, P. carbinolicus, and P. acetylenicus form a homogenous cluster, P. acidigallici and P. propionicus appear to be only distantly related to the others. These findings are consistent to some extent with the fermentation patterns of these species (see below). Comparison of three Pelobacter species with other anaerobes on the basis of 16S rRNA structure analysis supports this view. Whereas P. venetianus and P. carbinolicus exhibit a rather high similarity, with an SAB of 0.70, P. acidigallici is related to both at an SAB of only 0.53 (Stackebrandt et al., 1989). It is interesting to note that these three Pelobacter strains did not show any resemblance to other fermenting Gram-negative strict anaerobes; instead, they appeared to be highly related to several strains of sulfurreducing anaerobes, namely, Desulfuromonas succinoxidans, D. acetexigens, and D. acetoxidans, to which they are even more closely related than P. acidigallici is to the other two Pelobacter species. Since Pelobacter species and the obligately sulfur-respiring bacteria are quite diverse metabolically and the latter are supposed to have derived directly from phototrophic ancestors, it has been suggested that the genus Pelobacter represents a group of fermenting bacteria that developed a fermentative metabolism as a secondary evolutionary event and that they are separate from the first fermentative bacteria (Stackebrandt et al., 1989).