STABILITY OF PLASMID DNA OF ESCHERICHIA-COLI C600 AND ALCALIGENES-EUTROPHUS CH34 INOCULATED IN DESICCATING SOIL

Molecular methods based on detection of specific DNA sequences are inc reasingly used to monitor microbial strains and communities in soils. Here, we report that desiccation of soil, a condition that frequently occurs in nature, may contribute considerably to dissimilarity between DNA levels and colony forming units of introduced bacteria. Three typ es of soil samples were supplemented with Escherichia coli or Alcalige nes eutrophus suspensions and incubated at 30 degrees C in the presenc e or absence of dehydrating silica gel. Alternatively, seeded soil sam ples were desiccated by freeze-drying. At regular time points cells an d total DNA were extracted and colony forming units and plasmid DNA we re determined, respectively. These analyses showed that the decrease o f the number of colony forming units was faster in desiccating than in control soil. Both in desiccating and in control soil, plasmid DNA le vels were more stable than culturable counts. Long-term incubation exp eriments showed that in desiccating soil but not in control soil E. co li plasmid DNA remained intact and biologically active for at least 17 days after disappearance of E. coli culturable counts.