Results presented in this paper have shown that a widely distributed LM-resistance determinant is present on at least 3 distinct Bacteroides R-plasmids. In fact these plasmids bear no homology outside of the defined regions implicated in the LM resistance. This resistance determinant on pBF4, pBFTM10, and pBI136 is located within DNA segments bounded on each side by a directly repeated sequence of more than 500 bp. The intervening sequences of these 3 elements are variable, and range in size from about 3.7 kb to 7.2 kb (Fig. 7). Apart from the EcoRI/AvaI restriction sites which characterize the repeated sequence, there is a notable lack of common restriction sites within these elements. These results suggest that the elements do possess a certain degree of structural similarity but significant evolutionary divergence has occurred. The presence and location of the directly repeated sequences, their association with specific deletions, and their association with an antibiotic-resistance determinant, are features common to many antibiotic-resistance transposons described for other prokaryotes. In addition, these elements are highly mobile, being found on a number of R-plasmids. The unique relationship between pBF4, pBI106, and pBI136 described here is a clear indication of the potential for these DNA sequences to move from one molecule to another. Given the extensive dissemination and the genetic and structural characteristics described above, it seems likely that the LM-resistance determinant is carried on transposon-like elements present in pBF4, pBFTM10, and pBI136. However, further experimentation will be necessary to document the transposition event. Bacteroides strains such as B. fragilis V503, possess a transmissible LM-resistance determinant which does not appear to be associated with detectable extrachromosomal elements (5,9,10). Presently, a number of strains of this type have been found over a wide geographic area. The LM-resistance genes associated with these strains are apparently similar to the one carried on the Bacteroides R-plasmids because homology between the 2 has been observed. However, it is important to note that within the limits of Southern filter blot hybridization, neither V503 nor its transconjugants possess the directly repeated sequence found on the LM-resistance plasmids (Fig. 8). The elusive nature of the V503 LM-resistance elements presents an intriguing problem. One model that has been proposed is that these resistance determinants reside on a conjugative transposon similar to Tn916 of Streptococcus faecalis (3).(ABSTRACT TRUNCATED AT 400 WORDS)