R-046. Plasmid Diversity Is High in E. coli and Salmonella and Limited in Marine Symbiotic Vibrio

L. E. Williams, J. Kramer, D. Adin, E. Stabb, A. O. Summers;
Univ. of Georgia, Athens, GA.

A more complete picture of plasmid distribution and diversity is needed to assess their roles in bacterial evolution. We examined plasmids in three genera of proteobacteria: 303 strains of Escherichia coli and Salmonella in the reference collections ECOR, SARA, SARB, and SARC (with additional non-subspecies I strains) and 63 strains of marine symbionts Vibrio fischeri and V. logei from research collections. We isolated plasmids with the CosMCPrep kit (Williams et al. 2006) and did RFLP analysis of individual plasmids with AccI. More E. coli and Salmonella strains had plasmids (89% for ECOR, 60% for subspecies I Salmonella, and 46% for non-subspecies I Salmonella), than did the Vibrio strains (35%). ECOR and SAR had 213 large plasmids (range = 30 - 120 kb, avg = 78 kb), as estimated from plasmids of known size, whereas Vibrio had only 18 large plasmids. In E. coli, plasmid DNA comprised up to 8% of total genomic DNA in a strain (assuming a 5 Mb chromosome), compared to up to 4% for Salmonella. RFLP analysis of 175 of the large ECOR and SAR plasmids revealed high diversity. Most plasmids’ RFLP patterns were unique, although a few common patterns were seen, such as the virulence plasmid pSLT in 14 sv Typhimurium strains and a 40 kb plasmid in three sv Heidelberg strains. In contrast, of 11 squid-associated Vibrio large plasmids tested, four had similar patterns, differing by one fragment, and two ~190 kb plasmids had identical RFLP patterns. Plasmids <30 kb occurred 2-fold more often with large plasmids than alone, possibly due to mobilization. The same 2 kb plasmid occurred in 11 ECOR strains, nine from Swedish women, one from a U.S. woman and one from a U.S. zoo gorilla. Co-occurring large plasmids varied in size and RFLP pattern, discounting dependence on a particular large plasmid to explain the 2 kb plasmid’s presence. This is the first analysis, to our knowledge, of the prevalence and diversity of plasmids in the widely used ECOR and SAR reference collections. The abundance and high variation of E. coli and Salmonella plasmids contrasts with the low prevalence and limited diversity of Vibrio plasmids. This trend may reflect the different lifestyles of their host bacteria.