Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2003
Publication Date: 6/13/2003
Citation: Bailey, V.L., Smith, J.L., Bolton, Jr. H., 2003. Novel antibiotics as inhibitors for the selective respiratory inhibition method of measuring fungal:bacterial ratios in soil. Biology and Fertility of Soils 38:154-160. Interpretive Summary: In soil management the ratio of bacteria to fungi in soil can be important for the nutrition of the plants or crop. These ratios have been used for determining fertilizer and nutrient needs of plants and for estimating disease potential of certain soils. These ratios have been determined in the past by inhibiting the respiration of one organism group and then the other. This is accomplished using one antibiotic that inhibits fungi and another that inhibits bacteria. Unfortunately, some inhibitors are more efficient than others. Since we are working with several different ecosystems we investigated some novel pairs of antibiotics for their efficiency in inhibiting target organisms. Our findings suggest that for this method to work we need to optimize both the inhibitor concentration and the antibiotic type.
Technical Abstract: The use of the selective inhibition method for measuring fungal-to-bacterial ratios may be limited due to biocide selectivity and the overlap of antibiotic activity. This study was conducted to evaluate novel pairs of antibiotics for their specificity in soils of different origins and their potential reduction in inhibition of non-target organisms. Four soil selected for this study were from a semi-arid shrub-steppe, a loblolly pine forest soil, and two grassland soils (restored and farmed prairie soils). Three bactericides were tested: oxytetracycline hydrochloride, streptomycin sulphate, and bronopol. Three fungicides were tested: captan, ketoconazole, and nystatin. The inhibitor additivity ratio and fungal to bacterial ratios were calculated from control and treated soils where inhibition was measured as CO2 respiration reduction with biocides. We were best able to minimize non-target inhibition by the antibiotics to less than 5% and thus calculate reliable fungal-to-bacterial ratios using captan to inhibit fungi in all four soils, and bronopol to inhibit bacteria in three of the four soils. The most successful bactericide in the restored prairie was oxytetracycline-HCl. Our paper demonstrates that application of novel antibiotics is not uniformly successful in soils of different origin and that the SIR selective inhibition technique requires more than just optimization of antibiotic concentration; it also requires optimization of antibiotic selection.