Colorimetric assay using XTT for assessing virulence of avian Pasteurella multocida strains

A colorimetric assay using sodium 3,3'-[1[(phenylamino)carbonyl]3,4- tetrazolium]-bis(4-methoxy-6-nitro) benzene sulfonic acid hydrate (XTT) was adapted to quantitate bactericidal activity of chicken macrophage HD 11 cell line against five Pasteurella multocida strains and an avirulent transposon insertion mutant. The strains used were virulent P1059, and D92, and four avirulent strains including a streptomycin resistant mutant of P1059 (P1059 SmR), two live vaccine strains namely, the Clemson University (CU) and M9, and a transposon insertion mutant PmTn-294. Percentage of bacteria killed by chicken macrophage (HD 11) cells was determined by extrapolation from a standard formazan curve derived by incubating XTT with known bacterial cell numbers of each strain. The amount of formazan as measured by absorption at 450 nm was directly related to the number of viable bacterial cells. The percentages of P1059 SmR, CU, M9 and PmTn-294 killed by HD 11 cells were approximately 50%, 61%, 25% and 34%, respectively. By contrast, the virulent P1059 and D92 strains were resistant to killing, and were able to replicate inside the HD 11 cells. Association of virulence with resistance to phagocytic killing by HD 11 cells as assessed by the colorimetric bactericidal assay, was validated with resistance to complement (C')-mediated killing and a turkey mortality test. Strains P1059 and D92 were resistant to C'-mediated killing, whereas strains P1059 SmR, CU, M9 and PmTn-294 strains were susceptible. All turkeys challenged with P1059 or D92 were dead within 18 hrs. Mortality did not occur in turkeys challenged with strains of P1059 SmR, M9 and PmTn-294. The mortality among CU challenged turkeys ranged from 0 to 40%. The results suggest that the colorimetric bactericidal assay using XTT can be used to quantitate chicken macrophage phagocytic killing of P. multocida strains, and may be a valuable assay to differentiate virulent from avirulent strains of avian P. multocida.