A snap-shot of Mannheimia hemolyticaA1 gene expression during infection in the bovine host

It is expected that Mannheimia hemolytica A1 expresses a particular collection of genes during infection in the host. The bacterial gene products are produced in the in vivo environment to facilitate growth and survival. Here, we examined gene expression by M. hemolytica A1 in the bovine host after 6 days of infection. Total RNA from M. hemolytica A1 recovered from pneumonic lungs of two animals was used to produce cDNA to screen a custom M. hemolytica A1 microarray. The expression profile was compared to a RNA sample from an in vitro grown culture. The data showed that 44 genes were differentially expressed by more than eightfold when compared with the in vitro sample. Seventeen genes were found to have higher expression in vivo and 27 genes had lower expression. Several virulence-associated genes including those encoding leukotoxin, a capsule biosynthetic enzyme and the serotype-specific antigen, Ssa, had reduced expression, suggesting that their products may not be important during the later stages of infection. Most of the genes up-regulated in vivo encoded hypothetical or conserved hypothetical proteins. Three Mu-like bacteriophage-related genes were up-regulated in the in vivo sample, suggesting that the prophage may be transcriptionally active. The results provide a glimpse of gene expression by the bacterium in the host after pulmonary infection has been established.