Incidence of lysogeny within temperate and extreme soil environments

A companion study indicated that approximately 30% of cultivable soil bacteria may contain inducible prophage; however, the degree to which this cultivation-based estimate applies to autochthonous communities of soil bacteria is unknown. To estimate the prevalence of lysogeny within soil bacterial communities, induction assays were carried out by extracting bacteria from soil and subsequently exposing extracts to mitomycin C (MC; 0.5 microg ml(-1)), or by exposing bacteria to MC (1.0 microg ml(-1)) through direct addition to soil slurries. Induction was assessed as an increase in viral direct counts relative to those obtained in controls, as detected by epifluorescence microscopy. Extracting bacteria from soils followed by 18 h MC exposure generated significantly higher prophage induction than all other treatments (P < 0.05). For three Antarctic soil samples, estimates of inducible fraction (IF) were statistically indistinguishable across two independent assays (P = 0.82), indicating that this approach is highly reproducible. Although IF was lower in Antarctic soils (4-20%) and higher in temperate Delaware soils (22-68%), no clear correlations were found between lysogeny and soil physical properties. For Delaware soils, IF estimates were similar between whole soil assays (44%) and cultivation-based approaches (30%). While these data suggest that lysogeny is common among soil bacteria, the specific factors which promote temperate interactions remain unclear.