Radiolabeled substrate assay to measure inhibition of growth of Haemophilus somnus by normal bovine serum.

An assay was developed to measure the inhibition of cell division of Haemophilus somnus by normal bovine serum. Dilution plate counts, turbidometric measurements, and release of radiolabeled cell constituents were found to be time-consuming or difficult to perform and reproduce. This assay determined the inhibitory effect of normal serum on population growth, by measuring the relative rate of macromolecular (DNA) synthesis in exponential cultures. This method was simple to perform and gave reliable results in 1 day. When this assay was used, a significant difference (P less than 0.025) in serum susceptibility was demonstrated between a H somnus isolate recovered from the trachea of a healthy calf and an isolate recovered at necropsy from a brain lesion of a cow. The implication of this finding, in relation to the pathogenicity of the organism, it briefly discussed.