A probe based on 16S ribosomal RNA (rRNA) sequences was developed to detect , the causative agent of Johne's disease in cattle. Three universal primers were used to sequence the amplified fragments of the 16S rRNA gene of various species of mycobacteria. When the nucleotide sequences were analysed, a deletion was detected in the sequence of the fast-growing species. An oligonucleotide probe (P) directed to this region was synthesised and hybridised directly with total RNA of various mycobacterial strains in a dot-spot assay. The probe detected , some other slow-growing mycobacteria of the (MAI) complex, and one atypical strain, . To increase the sensitivity of the probe, a 413-bp fragment of the 16S rRNA gene of between P and a second oligonucleotide primer was amplified and hybridised with a -specific probe. When faecal samples of cattle were tested, all culture-positive samples were positive in the PCR assay.


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