ANALYSIS OF ARSENIC-TOLERANT BACTERIAL-POPULATIONS IN A PADDY FIELD SOIL ACCORDING TO RATES OF DEVELOPMENT ON SOLID MEDIA

The colony-forming process of bacteria in a paddy field soil on 100-fo ld diluted nutrient broth (DNB) plates containing As(III) (250-1,000 p pm) was analyzed by the first-order reaction (FOR) model, The process on plates with 250 ppm As(III) was simulated by a superimposition of t hree FOR model curves, which are respectively referred to as component colony-forming curve (cCFC) I, II, or III, Bacterial isolates from th e 250 ppm As(III) plates were divided into three groups according to c CFC, along which each respective bacterium produced its colony, The th ree 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) concentratio n for each isolate and some of its taxonomic properties. Twelve out of 72 strains examined harbored one or more plasmids, suggesting locatio n of As(III)-tolerant genes on the chromosomal DNA.