High and low-virulent bovine Pasteurella multocida capsular type A isolates exhibit different virulence gene expression patterns in vitro and in vivo

- We illuminate the virulence gene expression patterns difference in high and low virulent Pasteurella multocida isolates during in vitro and in vivo growth.
- In vitro, virulence genes in low virulent isolate are more highly expressed than in high virulent isolate, but in vivo, the expression of most virulence genes is converse in both isolates.
- Magnitudes of virulence gene expression differ among bacterial samples isolated from different organs.
- Virulence gene expression in both isolates appears no correlation to bacterial loads in individual organs.

Pasteurella multocida capsular type A causes respiratory disease in cattle. P. multocida virulence gene expression patterns, especially among different virulent isolates, during in vitro and in vivo growth are poorly understood. Here we show that the highly virulent bovine P. multocida capsular type A isolate PmCQ2 exhibits a significantly higher growth rate in mice, as compared with a strain of lower virulence, P. multocida capsular type A isolate PmCQ6. Among the six known and potential virulence genes (ompA, ompH, pfhB2, hasR, pm0979, and pm0442) investigated, most genes were expressed more highly in both isolates when grown in vivo as compared with in vitro, with ompH and pm0442 having the highest magnitude of expression. Virulence gene expression was higher in PmCQ6 than in PmCQ2 during in vitro growth. However, in mice, most virulence genes were expressed more highly in PmCQ2 as compared with PmCQ6. Virulence gene expression was highest in the liver and lowest in the lung, but was uncorrelated to bacterial loads. This study indicates that individual pathogenic capacity of P. multocida isolates is associated with the virulence gene expression patterns in vivo growth but not in vitro, and the investigation of virulence gene expression in pathogen should be performed in vivo.