Survival and effects of wild-type, mutant, and recombinant Streptomyces in a soil ecosystem

Abstract

In a laboratory simulation, selected wild-type, mutant, and recombinant Streptomyces were released into a silt loam soil. Strains included genetically enhanced lignin decomposers and those expressing recombinant plasmids. Their survival and effects on soil organic carbon mineralization were monitored in sterile and nonsterile soil, with and without lignocellulose supplementation. Survival was followed by viable plate counts on selective media. CO2 evolution was monitored in respiration cabinets. All strains, whether released as spores or mycelia, survived in nonsterile soil for up to 30 days. Selected strains released as spores survived for at least 10 months. With all strains, the numbers of colony-forming units per gram of soil slowly declined until relatively similar, stable population levels were achieved. Spores were more stable than mycelia. Only one recombinant survived significantly better in nonsterile soil than did its corresponding nonrecombinant parent, but only during the 1st to 2nd week after release. With two exceptions, there were no statistically significant short-term effects of release on the rates of carbon mineralization in unamended or lignocellulose-amended sterile and nonsterile soils. One recombinant, Streptomyces lividans TK23-3651, significantly affected the short-term rate of soil organic carbon turnover. After its release, the rate of soil organic carbon mineralization increased, particularly in nonsterile soil amended with lignocellulose. The cumulative amount of CO2 evolved over a 30-day period was significantly higher than for control soils or those inoculated with other Streptomyces. Another recombinant, S. lividans TK23/pSE1, temporarily reduced carbon mineralization rates, but only in nonsterile, unamended soil during the first few days after release. This is the first report of released, genetically altered Streptomyces having a measurable effect on a natural ecosystem. The significant enhancing effect of strain TK23-3651 was transient, and additional studies showed that this strain was genetically unstable in soil.Key words: Streptomyces, recombinant, soil, environment, release.