Isolation of Bdellovibrio spp. that prey on Azospirillum brasilense in soil

Abstract

Bdellovibrios that prey on Azospirillum brasilense were isolated from a Latosol and a Podzolic soil from Brazil which were stored air-dried for 2 years. The addition of A. brasilense strain Cd or Sp 7 cells and nutrients to these soils stimulated growth of indigenous bdellovibrios; direct assay of these soils did not yield bdellovibrios. Two other Podzolic soils from Brazil and three Chernozemic soils from Canada did not contain detectable bdellovibrios. After enrichment with strain Cd cells, the Podzolic soil yielded 1340 bdellovibrios per g of soil, whereas enrichment with strain Sp 7 cells yielded only 50. Escherichia coli and Enterobacter aerogenes cells did not stimulate growth of bdellovibrios in this soil, but did stimulate growth of bdellovibrios in the Latosol as did strains Cd and Sp 7. The morphology of an azospirilla-attacking Bdellovibrio, isolated from the Podzolic soil, was typical of the genus; attack-phase cells were curved rods, 0.2–0.4 by 1.0–1.3 μm, motile by means of a single polar flagellum. In broth culture this Bdellovibrio isolate preyed on several different gram-negative bacteria, although the apparent growth rate on prey cells was A. brasilense strain Cd > strain Sp 7 > A. lipoferum strain Sp Br 17 = E. coli = E. aerogenes > A. brasilense strain Sp 35. Pseudomonas fluorescens, Ensifer adhaerens, and 20 unidentified bacterial isolates from the Podzolic soil were not suitable prey. These results indicate that Bdellovibrio survive in some air-dry soils at undetectable levels but respond quickly to the presence of a large number of prey cells. In addition, the presence in soil of bdellovibrios that exhibit a faster growth rate on A. brasilense strain Cd than on strain Sp 7 cells and other azospirilla indicates potential problems when using strain Cd as a crop inoculant in certain soils.