The conjugative properties of an indigenous 85 MDa plasmid (designated pCH1) from were studied. To determine if pCH1 was transmissible by conjugation, mating experiments were performed between legionellae that harboured pCH1 and several plasmid-less recipients. Plasmid transfer was monitored by colony hybridization, using a cloned 21.0 kb I restriction fragment from pCH1 as a probe. The results from these experiments showed that pCH1 could be conjugatively transferred into several strains of serogroup 1 but not into strain Bloomington-2 (serogroup 3) or . Southern hybridization experiments in which pCH1 DNA was used as a probe showed that pCH1 does not share homology with other indigenous plasmids. There was no detectable DNA homology between pCH1 and chromosomal DNA. Additional mating experiments revealed that pCH1 was unable to mobilize the chromosome. The conjugative transfer of pCH1 into plasmid-less avirulent or virulent serogroup 1 strains did not alter the intracellular growth characteristics of these strains in U937 cells, a human-monocyte-like cell line, or in the amoeba . These results suggest that pCH1 does not contribute to the ability of to enter or grow within eukaryotic cells.


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