Phages are viruses infecting specific host bacteria, thus influencing the mortality and structure of the bacterial community. They show several life cycles of which lysogeny seems to be the most common in the heterogeneous soil environment. During lysogeny the phage genome is temporarily integrated into the host genome as a prophage and replicated within the host until the production of new phage particles is induced by environmental signals. In this study, we aimed to isolate and characterize these temperate phages from soil and groundwater bacteria.

Bacteria were isolated on solid agar from soil and groundwater samples from the Hainich National Park (Thuringia, Germany) and identified by 16S rRNA-gene sequencing. In a high-throughput approach all isolates were screened for inducible phages using Mitomycin C, which causes mutagenic stress and triggers the release of the phages. The supposedly prophage carrying isolates were grown and induced in a larger volume for subsequent purification of the phages, and characterization via transmission electron microscopy and genome sequencing.

In total we isolated 227 bacteria. Most of the soil isolates belonged to the class Actinobacteria, whereas most of the isolates from water belonged to Beta- and Gammaproteobacteria. Thirty-nine percent of all isolates showed the typical growth curve of bacteria carrying inducible prophages. Another forty-five percent showed a non-typical growth curve, which may be indicative for the presence of defective prophages, and only ten percent of the isolates were not affected by the exposure to Mitomycin C. Transmission electron microscopy allowed a first separation of the phages into distinct families. First results of the genome sequencing showed that the same putative circular phage genome with 59,363 bp was isolated from different bacterial hosts. Analyses of other putative phage genomes ranging between 47,500 bp and 100,000 bp will be also presented and discussed.