The Holophaga/Acidobacterium division was the most abundant
and diverse group among analyzed clones obtained from
the clay fraction of the AM treatment. This bacterial group was
also dominant in other soils (4, 10, 39). In this study, members
of the Holophaga/Acidobacterium division mainly colonized silt
and clay fractions. Because this phylum includes Holophaga
foetida, Geothrix fermetans, and Acidobacterium capsulatum as
the only cultivated species so far, very little information on the
role of these microbes in natural environments is available. In
addition, members of the Verrucomicrobiales, showing highest
similarity to the genus Prosthecobacter, were found mainly in
silt and clay fractions. One characteristic of this genus is the
formation of prosthecae, which consist of narrow, cytoplasmcontaining
extensions of the cell wall. They confer several
advantages to aerobic heterotrophic bacteria, such as enhanced
respiration and nutrient uptake, as well as improved
attachment to solid substrates (25). Furthermore, the known
members of the genus Prosthecobacter utilize only a narrow
range of carbohydrates as their sole carbon source (25), which
may explain their location in smaller size fractions.