PLASMIDS ISOLATED FROM MARINE SEDIMENT MICROBIAL COMMUNITIES CONTAIN REPLICATION AND INCOMPATIBILITY REGIONS UNRELATED TO THOSE OF KNOWN PLASMID GROUPS

Two hundred ninety-seven bacteria carrying plasmids that range in size from 5 to 250 kb were identified from more than 1,000 aerobic heterot rophic bacteria isolated from coastal California marine sediments, Whi le some isolates contained numerous (three to five) small (5- to 10-kb ) plasmids, the majority of the natural isolates typically contained o ne large (40- to 100-kb) plasmid, By the method of plasmid isolation u sed in this study, the frequency of plasmid incidence ranged from 21 t o 28% depending on the samples examined, Diversity of the plasmids occ urring in the marine sediment bacterial populations was examined at th e molecular level by hybridization with 14 different DNA probes specif ic for the incompatibility and replication (inc/rep) regions of a numb er 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 culturabl e bacterial population of marine sediments and the replicon probes spe cific 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 isol ates, may not be representative of the plasmid diversity occurring in some marine environments, Since the vast majority of marine bacteria a re not culturable under laboratory conditions, we also screened microb ial community DNA for the presence of broad- and narrow-host-range pla smid replication sequences, Although the replication origin of the con jugally promiscuous broad-host-range plasmid RK2 (incP) was not detect able 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 th e signal suggests the presence of a low level of the incP replicon wit hin the marine microbial community, In contrast, replication sequences specific for the narrow-host-range plasmid F were not detectable in D NA extracted from marine sediment microbial communities. With the poss ible exception of mercuric chloride, phenotypic analysis of the 297 pl asmid-bearing isolates did not demonstrate a correlation between plasm id content and antibiotic or heavy metal resistance traits.