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United States Department of Agriculture

Agricultural Research Service


item Costa, Jose
item Loper, Joyce

Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Erwinia carotovora subsp. carotovora causes soft rot of potato and other plant hosts. The related bacterium Erwinia carotovora subsp. betavasculorum strain Ecb168 suppresses populations of E. c. carotovora in potato tubers due to the production of an antibiotic. In this study, we showed that Ecb168 produces an autoinducer, a compound that diffuses out of the cell into the environment, where it is detected by neighboring bacterial cells. When adequate concentrations of the autoinducer accumulate in the environment, bacterial cells are induced to alter their metabolism. We also showed that antibiotic and extracellular enzyme production by Ecb168 requires the presence of an autoinducer. A gene required for autoinducer production by Ecb168 was cloned and sequenced; the DNA sequence was similar to those of genes responsible for autoinducer production by other bacteria. Therefore, the capacity of Ecb168 to serve as a biological control agent of potato soft rot is dependent on a diffusible compound called an autoinducer, which appears to be an important means by which bacterial cells communicate with their neighbors in nature.

Technical Abstract: Erwinia carotovora subsp. betavasculorum strain Ecb168 causes vascular necrosis and root rot of sugarbeet and produces an antibiotic(s) that is antagonistic against other Erwinia spp. EcbI- mutants of Ecb168, each containing a single transposon insertion in the ecbI gene (for E. carotovora subsp. betavasculorum inducer), do not produce detectable levels of extracellular protease or antibiotic, and express less pectate lyase activity and virulence than the wildtype strain. A plasmid containing the cloned ecbI gene complemented the EcbI- mutants for these phenotypes. Protease production by EcbI- mutants was restored on plates by neighboring cells of Escherichia coli containing ecbI. Production of a diffusible homoserine lactone-autoinducer by wildtype Ecb168 was detected with indicator strains of Escherichia coli and Agrobacterium tumefaciens. EcbI- mutant strains did not produce an autoinducer detected by the indicator strains. Antibiotic production by EcbI- mutants was restored by cell-free culture supernatants of Ecb168 or E. coli containing a cloned ecbI gene. The nucleotide sequence of ecbI is similar to those of carI (89%) and expI (74%), two luxI homologs that encode for production of a diffusible homoserine lactone autoinducer in Erwinia carotovora subsp. carotovora. A luxR homolog, termed ecbR for E. carotovora subsp. betavasculorum regulator, is convergently transcribed and overlaps with ecbI by 17 bp at their 3' ends. These results are consistent with the hypothesis that a quorum-sensing system related to the prototypic luxI/luxR gene pair controls antibiotic and exoenzyme production in E. carotovora subsp. betavasculorum.

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