Invasion of bovine PBMC by M. bovis Mb1. Bovine PBMC from 12 beef cattle were incubated for various times (1, 2, 3, and 24 h) with M. bovis Mb1, and the percentage of M. bovis-infected PBMC was determined by a gentamicin resistance assay. (A) Viable M. bovis recovered. At 1, 2, and 3 h, M. bovis demonstrates a low-level invasion/infection of PBMC (between 5 and 10% of PBMC are infected). At 24 h, all PBMC are infected, and those with >100% infection are representative of more than 1 M. bovis organism per PBMC. The bars show the median of the values. After 24 h of incubation, the number of intracellular M. bovis was significantly higher (***, P < 0.001) than the 1-, 2-, and 3-h time points. (B) Viable bovine PBMC after incubation with M. bovis. C, control PBMC; I, PBMC infected with M. bovis. The bars show the median of the values. After 24 h of incubation with M. bovis, there was a significant decrease (***, P < 0.001) in PBMC viability.