Monitoring survival and impact of bacteria introduced into the field, and their potential for gene exchange with native soil bacteria, has been the subject of a long-term study at Rothamsted.  A genetically marked strain of Rhizobium leguminosarum, which forms nitrogen-fixing root nodules in symbiotic association with leguminous plants, was introduced to soil as a seed inoculant in a field experiment at Rothamsted in 1987 and its population monitored in the presence and absence of the host plant, pea.  Most rhizobia carry symbiotic genes on large plasmids and in 1994 we introduced a second strain that had lost its symbiotic plasmid, to examine gene transfer between rhizobia in soil.  Both strains survived in the presence and absence of the host plant but surprisingly the strain lacking a symbiotic plasmid was at higher numbers in bulk soil and difficult to detect in the rhizosphere whilst the converse was true for the strain carrying a symbiotic plasmid (Hirsch, 1966; Clark et al., 2002).

Survival of GM rhizobia in field release site

No gene transfer was detected in the field during these experiments although it was observed in the lab, and there was no obvious impact of the introduced inoculant strains on other soil and rhizosphere bacteria. (Penny Hirsch and Ian Clark)

RSM2004 and CT0370 in soil and roots

References:

Clark, I. M., Mendum, T. A. & Hirsch, P. R. (2002). The influence of the symbiotic plasmid pRL1J1 on the distribution of GM rhizobia in soil and crop rhizospheres, and implications for gene flow. Antonie van Leeuwenhoek 81, 607-616.

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