Pseudomonas fluorescens Biotype G, the Dominant Fluorescent Pseudomonad in South Australian Soils and Wheat Rhizospheres

Summary. Estimates of the numbers of fluorescent pseudomonads in 15 different soils and in rhizospheres of wheat growing in these soils showed that they constituted only 0·06% and 0·27% of the bacterial population in the soil and rhizospheres, respectively. The counts showed that the variability between replicate samples was 6‐fold greater for the fluorescent pseudomonads than for general aerobic bacteria. This high variability supports the hypothesis that there is a heterogeneous distribution of fluorescent pseudomonads through soil where they probably occur as discrete colonies associated with particulate organic matter. One hundred and eightysix cultures of fluorescent pseudomonads were tested for their reactions on 27 biochemical and nutritional tests and sensitivity to 8 antibiotics. The cultures were grouped according to 6 tests (gelatin hydrolysis, levan production, denitrification and ethanol, trehalose and sorbitol utilization) which would place them into recognized biotypes. On the basis of these GLDETS tests 65% and 59% of the isolates from soil and rhizosphere, respectively, fell into biotype G of Pseudomonas fluorescens. Only in 4 of the 15 soils were other biotypes more abundant than biotype G. Amongst the isolates outside this dominant biotype G those from the wheat rhizosphere had a higher proportion giving positive GLDETS tests than did those from soil.