Virulence attenuation of a Mycobacterium avium subspecies paratuberculosis S-type strain prepared from intestinal mucosa after bacterial culture. Evaluation in an experimental ovine model

• An experimental infection of lambs was used for the evaluation of Map strains virulence.
• An inoculum from gut tissue homogenate was compared with the Map strain isolated from it.
• Inoculum pathogenicity was attenuated after culture of Map present in the gut homogenate.
• Intestinal LT plays a key role in the establishment of Map infection in the host.
• Triggering of both peripheral immune responses is related to lesion severity.

The differences in pathogenicity between an inoculum derived directly from an intestinal tissue homogenate from a paratuberculosis affected sheep and the S-type Mycobacterium avium subsp. partuberculosis (Map) strain isolated in laboratory media from the mentioned homogenate were assessed in two experiments in lambs. Specific peripheral immune responses were significantly lower in animals inoculated with the cultured organisms that showed only granulomatous lesions in the intestinal lymphoid tissue. However, in the homogenate group, more abundant granulomata also occurred in the lamina propria. Map was isolated only in lambs infected with the culture strain. Map DNA was demonstrated by nested-PCR in all the lambs but in a lower proportion (57.1% vs 100%) in those from the culture group. Under these particular experimental conditions, the results suggest that an attenuation of Map virulence has occurred in the cultured strain compared to the initial tissue homogenate, even after a low number of passages.