Intracellular survival of Brucella abortus, Mycobacterium bovis BCG, Salmonella dublin, and Salmonella typhimurium in macrophages from cattle genetically resistant to Brucella abortus.

Peripheral blood monocyte-derived macrophages were obtained from a herd of cows selected, bred, and confirmed as resistant or susceptible by in vivo challenge of Brucella abortus Strain 2308. The ability to control in vitro intracellular bacterial replication of B. abortus Strain 2308, Mycobacterium bovis Bacillus Calmette-Guerin (BCG) Montreal Strain 9003, Salmonella dublin Strain 5631, and Salmonella typhimurium Strain 14028 was evaluated in a bactericidal assay. The macrophages from resistant cattle were significantly superior (P < 0.05) in controlling intracellular growth of B. abortus, M. bovis BCG, S. dublin but not of S, typhimurium than macrophages from susceptible animals. Controls of all four pathogens correlated strongly with each other in resistant or susceptible macrophages. Data from resistant cattle had a tighter grouping than that of susceptible cattle, while data from susceptible cattle overlapped considerably with data from resistant animals. Therefore, this assay was considered a phenotypic marker of the resistant trait. For each bacterial species a percent bacterial survival value was used as a cut-off point to designate animals as resistant or susceptible. These data were compared with the in vivo challenged resistant or susceptible classification by using the Chi-square analyses. A cut-off point of 70 percent bacterial survival for B. abortus designated 14 cattle as susceptible and seven as resistant and this correlated 100 percent with the number of animals designated as to the relevant category by in vivo challenge. A value of 65 percent bacterial survival for M. bovis BCG, and 100 percent bacterial survival for S. dublin correlated highly with actual numbers of animals designated as susceptible or resistant.