Gene Escape Model: Transfer of Heavy Metal Resistance Genes

Conjugal transfer from Escherichia coli to Alcaligenes eutrophus of the A. eutrophus genes coding for plasmid-borne resistance to cadmium, cobalt, and zinc (czc genes) was investigated on agar plates and in soil samples. This czc fragment is not expressed in the donor strain, E. coli, but it is expressed in the recipient strain, A. eutrophus. Hence, expression of heavy metal resistance by cells plated on a medium containing heavy metals represents escape of the czc genes. The two plasmids into which this DNA fragment has been cloned previously and which were used in these experiments are the nonconjugative, mobilizable plasmid pDN705 and the nonconjugative, nonmobilizable plasmid pMOL149. In plate matings at 28 to 30°C, the direct mobilization of pDN705 occurred at a frequency of 2.4 x 10-2 per recipient, and the mobilization of the same plasmid by means of the IncPl conjugative plasmids RP4 or pULB113 (present either in a third cell [triparental cross] or in the recipient strain itself [retromobilization]) occurred at average frequencies of 8 x i0and 2 x 10-5 per recipient, respectively. The czc genes cloned into the TraMobplasmid pMOL149 were transferred at a frequency of 10-7 to 10-8 and only by means of plasmid pULB113. The direct mobilization of pDN705 was further investigated in sandy, sandy-loam, and clay soils. In sterile soils, transfer frequencies at 20°C were highest in the sandy-loam soil (10-5 per recipient) and were enhanced in all soils by the addition of easily metabolizable nutrients. For nonsterile soils, transfer of pDN705 at 28 and 20°C was observed in the sandy-loam soil, only when the soil was amended with nutrients. Frequencies varied between 1.5 x 10-8 and 1.5 x 10-6 per recipient. The results demonstrate that even genes incorporated into nonmobilizable plasmids can be exchanged between two different genera and that the presence of broad-host-range plasmids in putative recipients among soil bacteria could increase the risk of gene dissemination in case of release of genetically engineered microorganisms. The results also reveal that in certain soils, environmental conditions and particularly nutrient levels are conducive to gene transfer.