Acidophilic chemoorganotrophic bacteria of the genus Acidocella, Acidiphilium, and Acidobacterium were tolerant to concentrations of monomeric aluminum ion (Al 3+ ) of up to 100 mM in an acid medium of pH 3.5 or less. Furthermore, their growth was remarkably enhanced by the presence of aluminum. The maximum amount of Al absorbed was 2.02 mg·g dry cell wt -1 for Acidocella facilis. Aluminum phosphate as well as aluminum sulfate among aluminum chemicals tested provided for the most luxuriant growth of this bacterium, whereas aluminum acetate and aluminum lactate had inhibitory effects on growth. The enhanced growth seemed to be caused by a chemical buffering action of the aluminum, but could not be explained merely by an alleviation of H + stress or by the scavenging of growth-inhibitory organic acids excreted. The reasons why the acidophiles are genuinely aluminophilic as well as aluminotolerant still remain to be elucidated. Considering that monomeric Al 3+ is dominant under strongly acid conditions, the possible cultivation of acidophiles at a pH below 4 affords a useful in vitro model system to probe the aluminum toxicity of plants as well as aluminum-microorganism interactions.

The colony-forming process of bacteria in a paddy field soil on 100-fold diluted nutrient broth (DNB) plates containing As(III) (250-1,000 ppm) was analyzed by the first-order reaction (FOR) model. The process on plates with 250 ppm As(III) was simulated by a superimposition of three FOR model curves, which are respectively referred to as component colony-forming curve (cCFC) I, II, or III. Bacterial isolates from the 250 ppm As(III) plates were divided into three groups according to cCFC, along which each respective bacterium produced its colony. The three cCFC groups showed different trends of physiological properties: (1) the range of tr, which is the waiting time for the appearance of the first colony of each single population in the presence or absence of As(III), (2) the range of the maximal tolerant As(III) concentration for each isolate and some of its taxonomic properties. Twelve out of 72 strains examined harbored one or more plasmids, suggesting location of As(III)-tolerant genes on the chromosomal DNA.A variety of bacteria which are tolerant to biologically inhibitory elements, including both heavy metals and metals, inhabit soils. The bacterial tolerance to the many inhibitory elements results often from the presence of plasmids in cells (20). The plasmids, a series of extrachromosomal tolerance factors, are thought to be distributed widely in natural environments such as soils, water, plants, and animals and to be propagated among a wide range of microorganisms (17). The grouping of soil bacteria tolerant to toxic elements and the detection of plasmids responsible