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Bacterial kidney disease in salmonid fish is caused by the slow-growing Gram-positive rod, Renibacterium salmoninarum. The partial sequence of 16S rRNA from R. salmoninarum was determined and compared with published bacterial 16S rRNA sequences. From this sequence information, a 30-bases-long oligonucleotide was designed and used as a specific probe for identification of R. salmoninarum in filter hybridization experiments. Strong specific hybridization signals were observed for all strains of R. salmoninarum tested. Furthermore, no cross-hybridization could be seen against 22 other bacterial species, among them other salmonid fish pathogens. The detection limit for the probe in direct filter hybridization by the dot-blot technique was 2.5 x 10(4) bacteria. It was also possible to detect R. salmoninarum in clinical samples by direct filter hybridization.

A hybridization assay for direct detection and identification of Mobiluncus species has been developed and tested. A [32P]-labelled synthetic oligonucleotide probe, complementary to a nucleotide sequence in the variable region V8 of Mobiluncus 16S ribosomal RNA, was utilized. One of the advantages of using rRNA as target molecule for the hybridization assays is the copy number of rRNA, which can be as high as 10(4), and that additionally three to six sites on the minus strand of the DNA gene can be utilized. This probe was found to be sensitive and to react with 62 of 68 tested typical or atypical Mobiluncus isolates. It was also specific, and was shown not to react with 96 tested unrelated bacterial species and isolates, including taxonomically closely related species like Actinomyces or Bifidobacterium spp., or with bacteria isolated from the vagina of both healthy persons with an undisturbed flora, as well as from patients suffering from the bacterial vaginosis syndrome (BV).