Summary: From a representative sample of bacteria, isolated from mature sugar beet leaves () grown at three separate locations in the UK, 79 (18%) were shown to contain plasmids ranging in size from 10 kb to 200 kb. A sensitive colony blot method was developed to facilitate the screening of both Gram-negative and Gram-positive isolates to determine the distribution of known plasmid incompatibility groups among plasmids isolated from the natural environment using the collection of inc/rep probes derived from basic replicons [rep FIA, FIIA, FIB, HI1, HI2, I1, B/O, L/M, N, P, Q, U, W and X, as described by Couturier . (1988) 52, 375-395]. After hybridization with each of the radiolabeled replicon probes, 54 of these 79 plasmid-containing natural isolates, which included spp., spp. and Gram-positive bacteria, failed to react. Reactivity was observed with 25 of the 29 and isolates investigated. Of the plasmid-containing examined, 18 reacted with the repFIB probe, six with the repFIIA probe and one isolate, SBN169, hybridized to both. Southern hybridization demonstrated that the different isolates which shared homology with the repFIB probe contained a common 1 kb fragment. By comparing the restriction fragment patterns of total plasmid DNA, the and isolates were divided into 10 distinct groups and the other non-reactive isolates divided into a further 24 groups. Plasmid type appeared to be restricted to the geographical location from which the host bacteria were isolated. This study illustrates that sugar beet phyllosphere bacteria support a diverse array of plasmids which cannot be readily classified at the molecular level into the recognized incompatibility groups commonly described for clinical isolates.


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