Survival of Inoculants in Polluted Sediments: Effect of Strain Origin and Carbon Source Competition

Abstract

Inoculation of polluted environments with specialized strains is an important strategy in bioremediation. Especially the efficacy and safety of genetically engineered biodegradative microorganisms could be improved further, if host organisms with predictable survival in specific target ecosystems were identified and could be engineered to carry catabolic gene cassettes. Here we empirically tested whether strains originally isolated from the target ecosystem (indigenous inoculants) had superior survival rates compared to strains derived from geographically and ecologically different locations (nonindigenous inoculants). We experimentally tested this approach by isolating various strains of Pseudomonas and related genera from a highly polluted river sediment (Spittelwasser River) and following their survival in sediment slurries from the same river in comparison with that of nonindigenous strains. Both indigenous and nonindigenous inoculants grew and maintained cell densities of between 107 and 109 cfu ml−1 in autoclaved sediment slurries for up to 70 days, indicating that they were not negatively affected by abiotic factors, especially potentially toxic pollutants. In live sediment slurries, however, all inoculants died out at varying rates. If live sediment slurries were amended with a carbon source that the inoculant could degrade, most strains were able to grow, regardless of their original source. In some cases, high cell densities of the inoculant were maintained for up to 70 days, even after exhaustion of the added carbon source. Reisolating these strains and monitoring their survival in sediment slurries from a different sampling date did not reveal increased survival rates compared to the original strains. It is concluded that the use of indigenous isolates for bioremediation and as hosts for constructing genetically engineered organisms does not provide any advantage in dynamic, highly competitive environments. For safety reasons, therefore, a limited number of well-characterized strains whose pathogenic potential is known and for which ecological information can be obtained, should be used. To ensure survival of the inoculant, either the site must be engineered to provide a temporary advantage for the introduced strain, or the inoculant must be able to exploit a specific niche better than the indigenous microbial community.