Abstract
BackgroundRahnella is a widely distributed genus belonging to the Enterobacteriaceae and frequently present on vegetables. Although Rahnella has interesting agro-economical and industrial properties and several strains possess antibiotic resistances and toxin genes which might spread within microbial communities, little is known about plasmids of this genus. Thus, we isolated a number of Rahnella strains and investigated their complements of small plasmids.ResultsIn total 53 strains were investigated and 11 plasmids observed. Seven belonged to the ColE1 family; one was ColE2-like and three shared homology to rolling circle plasmids. One of them belonged to the pC194/pUB110 family and two showed similarity to poorly characterised plasmid groups. The G+C content of two rolling circle plasmids deviated considerably from that of Rahnella, indicating that their usual hosts might belong to other genera. Most ColE1-like plasmids formed a subgroup within the ColE1 family that seems to be fairly specific for Rahnella. Intriguingly, the multimer resolution sites of all ColE1-like plasmids had the same orientation with respect to the origin of replication. This arrangement might be necessary to prevent inappropriate synthesis of a small regulatory RNA that regulates cell division. Although the ColE1-like plasmids did not possess any mobilisation system, they shared large parts with high sequence identity in coding and non-coding regions. In addition, highly homologous regions of plasmids isolated from Rahnella and the chromosomes of Erwinia tasmaniensis and Photorhabdus luminescens could be identified.ConclusionsFor the genus Rahnella we observed plasmid-containing isolates at a frequency of 19%, which is in the average range for Enterobacteriaceae. These plasmids belonged to diffent groups with members of the ColE1-family most frequently found. Regions of striking sequence homology of plasmids and bacterial chromosomes highlight the importance of plasmids for lateral gene transfer (including chromosomal sequences) to distinct genera.
Highlights
Rahnella is a widely distributed genus belonging to the Enterobacteriaceae and frequently present on vegetables
10 novel plasmids were detected in addition to pHW15. Their sizes ranged from 2.9 to 7.0 kb, which is typical for small plasmids from enterobacteria [23]
They are extremely rare in Citrobacter freundii while 42% of Escherichia coli isolates possess at least one plasmid [23]
Summary
Rahnella is a widely distributed genus belonging to the Enterobacteriaceae and frequently present on vegetables. Rahnella has interesting agro-economical and industrial properties and several strains possess antibiotic resistances and toxin genes which might spread within microbial communities, little is known about plasmids of this genus. We isolated a number of Rahnella strains and investigated their complements of small plasmids. An improved understanding of mobile genetic elements of Rahnella is crucial to assess the potential of lateral gene transfer to other species including human pathogens. Rahnella is widely distributed and frequent on vegetables and likely to be routinely present in the human diet, little is known about plasmids of this genus. To gain insights into the frequency, diversity and evolution of small (less than 15 kb) Rahnella plasmids, we isolated strains from different geographic origins and sample materials. Sequence analysis provided evidence for lateral gene transfer within Rahnella as well as between Rahnella and other genera
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