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Appl. Environ. Microbiol., 03 1997, 888-895, Vol 63, No. 3
PA Sobecky, TJ Mincer, MC Chang and DR Helinski
Two hundred ninety-seven bacteria carrying plasmids that range in size from
5 to 250 kb were identified from more than 1,000 aerobic heterotrophic
bacteria isolated from coastal California marine sediments. While some
isolates contained numerous (three to five) small (5- to 10-kb) plasmids,
the majority of the natural isolates typically contained one large (40- to
100-kb) plasmid. By the method of plasmid isolation used in this study, the
frequency of plasmid incidence ranged from 24 to 28% depending on the
samples examined. Diversity of the plasmids occurring in the marine
sediment bacterial populations was examined at the molecular level by
hybridization with 14 different DNA probes specific for the incompatibility
and replication (inc/rep) regions of a number of well-characterized plasmid
incompatibility groups (repB/O, FIA, FII, FIB, HI1, HI2, I1, L/M, X, N, P,
Q, W, and U). Interestingly, we found no DNA homology between the plasmids
isolated from the culturable bacterial population of marine sediments and
the replicon probes specific for numerous incompatibility groups developed
by Couturier et al. (M. F. Couturier, F. Bex, P. L. Bergquist, and W. K.
Maas, Microbiol. Rev. 52:375-395, 1988). Our findings suggest that plasmids
in marine sediment microbial communities contain novel,
as-yet-uncharacterized, incompatibility and replication regions and that
the present replicon typing system, based primarily on plasmids derived
from clinical isolates, may not be representative of the plasmid diversity
occurring in some marine environments. Since the vast majority of marine
bacteria are not culturable under laboratory conditions, we also screened
microbial community DNA for the presence of broad- and narrow-host-range
plasmid replication sequences. Although the replication origin of the
conjugally promiscuous broad-host-range plasmid RK2 (incP) was not
detectable in any of the plasmid-containing culturable marine isolates, DNA
extracted from the microbial community and amplified by PCR yielded a
positive signal for RK2 oriV replication sequences. The strength of the
signal suggests the presence of a low level of the incP replicon within the
marine microbial community. In contrast, replication sequences specific for
the narrow-host-range plasmid F were not detectable in DNA extracted from
marine sediment microbial communities. With the possible exception of
mercuric chloride, phenotypic analysis of the 297 plasmid-bearing isolates
did not demonstrate a correlation between plasmid content and antibiotic or
heavy metal resistance traits.
Copyright © 1997, American Society for Microbiology
Plasmids isolated from marine sediment microbial communities contain replication and incompatibility regions unrelated to those of known plasmid groups
Department of Biology, University of California, San Diego, La Jolla 92093-0634, USA.
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