Bacterial Growth Rates and Competition Affect Nodulation and Root Colonization by Rhizobium meliloti

The addition of streptomycin to nonsterile soil suppressed the numbers of bacterial cells in the rhizosphere of alfalfa (Medicago sativa L.) for several days, resulted in the enhanced growth of a streptomycin-resistant strain of Rhizobium meliloti, and increased the numbers of nodules on the alfalfa roots. A bacterial mixture inoculated into sterile soil inhibited the colonization of alfalfa roots by R. meliloti, caused a diminution in the number of nodules, and reduced plant growth. Enterobacter aerogenes, Pseudomonas marginalis, Acinetobacter sp., and Klebsiella pneumoniae suppressed the colonization by R. meliloti of roots grown on agar and reduced nodulation by R. meliloti, the suppression of nodulation being statistically significant for the first three species. Bradyrhizobium sp. and "Sarcina lutea" did not suppress root colonization nor nodulation by R. meliloti. The doubling times in the rhizosphere for E. aerogenes, P. marginalis, Acinetobacter sp., and K. pneumoniae were less and the doubling times for Bradyrhizobium sp. and "S. lutea" were greater than the doubling time of R. meliloti. Under the same conditions, Arthrobacter citreus injured alfalfa roots. We suggest that competition by soil bacteria reduces nodulation by rhizobia in soil and that the extent of inhibition is related to the growth rates of the rhizosphere bacteria.