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16S rRNA gene sequence analysis provided evidence for two different mycobacterial species, Mycobacterium lepraemurium and a potentially novel species, as causative agents of 'feline leprosy'. Comparison of 16S rRNA gene sequence data obtained for M. lepraemurium and the potentially novel species indicated 12 nucleotide differences over a 446 bp region encompassing the V2 and V3 hypervariable regions. From available 16S rRNA gene sequence data, M. lepraemurium shared greatest nucleotide identity with M. avium subsp paratuberculosis and M. avium. The novel species had a long helix 18 in the V3 region and shared greatest nucleotide identity with M. leprae, M. haemophilum and M. malmoense. The novel species had an additional 'A' nucleotide at position 105 of the aligned 16S rRNA gene sequence, the only other mycobacterial database sequence having this same extra nucleotide being M. leprae. This nucleotide variation was exploited to develop specific PCR assays for the two species. These were found to be effective and specific when tested against a panel of mycobacteria including species found in feline leprosy lesions and closely related mycobacteria and also when applied directly to formalin-fixed, paraffin-embedded tissues from feline leprosy cases.

Cell-free extracts of Mycobacterium lepraemurium from mouse liver and M. leprae from armadillo liver were analysed for the presence of any mycobacterial catalase by using the specific inhibitor 3-amino-1,2,4-triazole and seroprecipitation titrations. These studies clearly demonstrated the presence of a "T" type of mycobacterial catalase in M. lepraemurium and placed it, in terms of immunological distance, in a position between M. tuberculosis and M. avium. The results did not reveal any detectable "T" catalase activity in the M. leprae preparations. The "M" type catalase activity which was observed did not bind to antisera against "M" catalase of M. kansasii, but was bound to the extent of 80% to antisera against normal armadillo liver catalase. The significance of the component of the "M" catalase in M. leprae preparations which did not react against antibodies to normal liver remains to be determined.