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 increasingly 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 types of soil samples were supplemented with Escherichia coli or Alcaligenes eutrophus suspensions and incubated at 30 degrees C in the presence or absence of dehydrating silica gel. Alternatively, seeded soil samples were desiccated by freeze-drying. At regular time points cells and total DNA were extracted and colony forming units and plasmid DNA were determined, respectively. These analyses showed that the decrease of the number of colony forming units was faster in desiccating than in control soil. Both in desiccating and in control soil, plasmid DNA levels were more stable than culturable counts. Long-term incubation experiments showed that in desiccating soil but not in control soil E. coli plasmid DNA remained intact and biologically active for at least 17 days after disappearance of E. coli culturable counts.